Plant Science curriculum

A cell wall alone does not make a plant, because fungi have walls too, built from different polymers. Photosynthesis alone does not either, since plastids first arose when a photosynthetic prokaryote entered the ancestors of algae. The broad group of green plants even includes chlorophyte algae, so where you draw the line depends on the group you mean.

Charophytes are green algae of fresh water and damp surfaces, and no living group sits closer to land plants. Which class sits nearest has shifted: earlier studies pointed to Charophyceae or Coleochaetophyceae, while recent well-supported trees favor the Zygnematophyceae, a group that has lost apical growth and plasmodesmata. So a close relative need not resemble the ancestor.

Early land plants also met falling supplies of water and nutrients, wide swings in air temperature, and much more visible and ultraviolet light. Even for a single trait the driver is argued. A standard hypothesis credits rising ultraviolet with the rise of flavonoid chemistry, but one 2022 analysis proposes that nutrient and water scarcity drove it instead.

A surface that reduces water loss also restricts carbon dioxide, and stomata are the controllable way around that. Guard cells regulate carbon dioxide entry and water loss together, and drought signals close the pore. The barrier is ancient: moss makes a phenol-rich cuticle although it has no lignin, and deleting one gene in that pathway leaves the moss cuticle leaky and blocks its normal development.

Lignin marks a real boundary, because early-diverging land plants such as mosses do not form it. Wood shows the double duty: the same cells conduct water and carry the mechanical load of the plant body. The group's reach shows in the numbers, with about 375,000 species of vascular plant now generating over 90 percent of terrestrial productivity.

The useful contrast is a moss or liverwort sporophyte, a single axis that ends in a spore capsule and stops. Indeterminate growth arose in vascular plant shoots, along with branching, before leaves had evolved. A meristem keeps this possible because it is a tissue of continuously dividing cells, a standing source of new tissue rather than a fixed body plan.

The key split is which life stage is the plant you see. Bryophytes (liverworts, hornworts and mosses, about 20,000 species) live mainly as a haploid photosynthetic body, while vascular plants built their shoots and leaves in the diploid sporophyte stage. How these groups relate is still debated, though there is significant support for bryophytes as one group. Later traits such as roots, flowers and seeds opened progressively drier ground.

Cellulose supplies tensile strength, while the surrounding matrix changes by layer: typically pectin in primary walls and brittle lignin in secondary walls. Reading outward from the cell interior, you pass the secondary wall, then the primary wall, then the middle lamella shared with the next cell. Most wood cells are dead at maturity, so the wall itself gives wood most of its properties.

A 1905 botany text describes a swollen cell as an equilibrium: water enters the watery cell sap until the stretched elastic wall pushes back. Its beet-slice demonstration shows the idea, with salt solution turning tissue limp and fresh water making it rigid again. Modern work on the reverse process finds that the vacuole mainly drives it.

Plastids can change identity. Colorless proplastids, found mainly in meristems and reproductive tissue, can become green chloroplasts when triggered by light. Chromoplasts accumulate carotenoids, as seen in ripening fruit. Colorless leucoplasts include starch-storing amyloplasts, oil-storing elaioplasts and protein-storing proteinoplasts. So a plastid tells you what a cell is doing now, not a fixed type.

A plasmodesma carries a continuous plasma membrane through the wall, which is why the cytoplasm of many cells can act as one connected space. The channels are not always open: microbial signals such as chitin trigger callose deposits that close them. The apoplast is the other world, made of the free spaces and cell walls outside the membranes.

The same outer layer does opposite jobs by location. On a shoot, a cuticle over the epidermis reduces water loss, while tiny trichome hairs add defense. On a young root, some epidermal cells grow out as tubular root hairs that raise the absorbing surface. One layer can seal or absorb, so match each feature to the organ.

Whether the cell is alive at maturity separates the extremes. Parenchyma stays alive, which is why starch grains in wood sit in these cells. A 1905 botany text describes sclerenchyma as usually without living contents at maturity and common in the hard parts of nuts, and collenchyma as a strengthening tissue of the cortex with walls thickened at the angles, as in tomato.

Tracheids are often more than 100 times longer than wide and make up over 90 percent of softwood volume, doing both conduction and support. Vessel elements are the single most important feature separating hardwoods from softwoods. In phloem, sieve elements lose their nucleus as they develop, so an adjoining companion cell connected through many plasmodesmata works with each one as a unit.

Ask where the new cells are added. A root apical meristem forms in the embryo and lengthens the primary root from its tip. The vascular cambium is a thin lateral layer that widens a stem by adding wood inside and bark outside. Intercalary meristems sit along the stem itself: in maize, forming them sets node number, and elongation of intercalary cells sets internode length.

Roots were a major innovation of the Early Devonian, giving large plants better anchorage and better capture of water and nutrients. The three jobs usually listed are uptake, anchoring and storage. Roots also talk to the rest of the plant and to neighbors: they exude compounds called strigolactones, which parasitic weeds use as a cue to germinate.

A 1905 botany text says hickory and walnut have strong taproots reaching deep into the soil and suggests this may help in dry seasons on dry, gravelly ground. An 1887 text describes fibrous roots, as in corn and most annuals, as slender, thread-like absorbers. Do not assume the old reading settles it: one 2023 study notes that how the two forms differ under drought remains largely unexplored.

Read a root tip as a timeline written in space. The cap sits at the very tip, protecting the root and sensing gravity. Behind the meristem, cells lengthen in the elongation zone, and only on entering the maturation zone do epidermal cells activate the factors that start root hairs. Position along the root therefore tells you what stage a cell has reached.

Each hair is one epidermal cell that grows out as a tube, expanding only at its tip, so absorbing area climbs with little added tissue. Hairs also help host beneficial microbes. Older botany tied them to handling: an 1887 text says they form near young root tips and names the dormant season as the best time to transplant trees and shrubs.

Three routes run across the root, not two. Solutes can diffuse through free spaces and cell walls, the apoplastic path; move cell to cell through plasmodesmata, the symplastic path; or take a coupled transcellular route using carriers in membranes. The carriers of that third route are polarized, moving solutes in on one side and out on the other, while the wall route can be fully blocked before the stele.

Do not confuse two barriers. The Casparian strip is built by CASP proteins recruiting lignin-making enzymes, and it can fully seal the path through the walls before the stele. Suberin lamellae coat the whole endodermal cell and do something different: they leave the wall route open and instead limit transport across the membrane. Rice also shows suberin deposited at the strip itself.

An 1887 botany text separates fleshy roots that store food from fibrous ones that absorb, and describes aerial roots that fix epiphytes to a surface, often without reaching the ground. Mangroves show two more forms today: red mangroves grow arching prop roots, and black mangroves send up pneumatophores that rise perpendicular to the soil surface.

Wood shows one stem running several systems at once: it conducts water, supports the plant body and stores biochemicals. Storage is the easiest to overlook. The carbon needed for a new flush of leaves has to be kept somewhere, often as starch and lipids in sapwood parenchyma, so a bare winter trunk is still a working food reserve.

In the terms of an 1887 botany text, a node is the point where leaves attach, and the bare interval between two nodes is an internode. The same text says branches properly start in the axil, the upper angle between leaf and stem, from an axillary bud. That gives you a field test: leaves and branches both come from nodes, and the bud position tells you which is which.

"Auxin blocks the buds" is too simple. Auxin was long treated as the master regulator, but it appears to act indirectly, and the dose needed to restore inhibition after cutting the tip was very high. A century of work has added cytokinin, the branch-inhibiting strigolactones, and a renewed role for sugar supply, so outgrowth reads as a network decision.

A 1905 botany text has you stand a cut stem in dye: impatiens and sunflower color in a concentric ring, while a corn stem shows irregularly scattered strands. The difference that matters for growth is separate. Wood comes from a vascular cambium making secondary xylem, and monocots lack secondary xylem, so their stems do not build wood the way a softwood or hardwood tree does.

The cambium is a thin cylinder that has to widen as the trunk grows, so some of its cell divisions increase its own surface rather than add wood or bark. Two kinds of initial cells do this work: fusiform initials make the up-and-down axial cells, and ray initials make the radial cells. Rays stay continuous through the cambium, linking wood to the inner bark.

A ring is one growth increment, and annual only where the seasons are regular: in many tropical woods rings are not evident at all, which is why specialists prefer "growth ring" to "annual ring". Within a ring, the change from earlywood to latewood can be gradual or abrupt. Ring-porous hardwoods such as white ash make large earlywood vessels, while maple is diffuse-porous.

Bark has two working layers. Outer bark protects and limits water loss, while inner bark carries the sugars made in leaves to the roots and growing tissue. Heartwood at the center is non-conductive, so losing it does not cut the supply lines. Strip the inner bark in a ring, though, and the sugar route is gone even if most of the wood remains.

An 1887 botany text gives the classic test: a stem shows itself by its buds. A potato tuber has eyes, which are buds, so the text counts it as a thickened underground stem and not a root. It also has stolons rooting where they touch down, strawberry runners rooting from the tip, and corms and rootstocks storing food in stem tissue, whereas a bulb stores it mainly in thick leaf bases.

Think of it as a compromise between light and gas. A 1905 botany text describes palisade cells packed close in parallel, which it says slows evaporation, above a loose spongy layer with large spaces between cells, and puts stomata usually on the underside. Modern optics adds a role: palisade cells take most of the strongly absorbed light, so green reaches chloroplasts in the spongy layer.

The opening is powered by osmosis, not muscle. Guard cells gain ions and organic solutes, water follows, and the swelling cell is shaped by its wall into a wider pore. Closing runs the same motor in reverse, with water leaving as turgor falls. Grasses build a distinct version, with dumbbell-shaped guard cells and specialized subsidiary cells beside them.

Sort the inputs by what they push. Blue light, red light, heat and low carbon dioxide inside the leaf favor opening, while abscisic acid, elevated carbon dioxide and dry air favor closing. Not every signal starts in the guard cell. Mesophyll cells produce messengers for the red-light response, and vascular tissue takes part in making abscisic acid during drought.

An 1887 botany text defines a compound leaf as one with its blade in entirely separate parts, called leaflets, usually each on its own stalklet, and treats a jointed stalklet as settling the question. It admits that deeply cut simple leaves blur the line. When in doubt, look for the bud: by the same text, a branch normally starts in a leaf axil, so the axil marks where the whole leaf joins the stem.

An 1887 botany text sets out the two patterns: in a netted leaf, veins branch off the main ribs, and in a parallel-veined leaf, as in any grass or lily of the valley, slender veins run side by side from base to tip, joined only by minute cross-veinlets. A 1905 text already warned that the link to plant group holds generally, though not always, so treat this as a first clue.

Count leaves per node. In the terms of an 1887 botany text, alternate means one leaf at each node, opposite means a pair on opposite sides of the stem, and whorled means more than two, dividing the circle equally. The same text says each species keeps a uniform order, that even alternate leaves follow a spiral, and that the arrangement gives leaves good exposure to light.

Leaves can differ because each forms under its own light. In Arabidopsis, sun-leaf thickening happens in stages, ending with cell expansion that sets final thickness, and sucrose can promote that expansion even in low light. A 1905 botany text already noted that strong light may add palisade layers and that leaves grown in deep shade may have none at all.

An 1887 botany text sets out a bud test for look-alikes: a barberry spine is a reduced leaf, revealed by the axillary bud above it, while a hawthorn thorn is a stunted branch and a rose prickle only an outgrowth of the bark. It also has a tendril replacing the uppermost leaflets, bulb scales as thick leaf bases full of stored food, and pitcher leaves as hollow tubes where insects drown.

The one-line equation hides two linked stages. Light energy pulls electrons from water and passes them to NADP+, releasing oxygen and making ATP. The Calvin-Benson cycle then spends that ATP and NADPH to fix carbon dioxide. Sugar is not the only outcome either, because the cycle's products also feed starch, sucrose, isoprenoids and nucleotides.

A leaf does not rely on one pigment. Chlorophylls and carotenoids bound in antenna proteins both feed photosynthesis, and the proteins hold them at set orientations, which creates an energy funnel toward the reaction centers. Carotenoids carry a risk: their short-lived excited state could drain that energy, so their placement has to be finely controlled.

Weak absorption by extracted chlorophyll is misleading, because a whole leaf behaves differently. Palisade cells take most of the strongly absorbed light, so green penetrates deeper and drives reactions in chloroplasts lower down. Added to bright white light, green can drive photosynthesis more effectively than other added colors. Green quanta still drive electron transport about 20 percent less well than blue quanta.

Inside the chloroplast, thylakoid membranes enclose an inner space and are surrounded by the stroma. Stacks called grana concentrate photosystem II and its antenna, while photosystem I and ATP synthase sit in the grana end membranes and the stroma membranes that connect the stacks. "Dark" reactions is a misleading name: several stromal enzymes have low activity in the dark and switch on in the light.

Light drives two separate steps, at photosystem II and photosystem I, which is why the energy diagram is drawn as a Z. In between, electrons travel from water through the cytochrome b6f complex and plastocyanin, then on from photosystem I to ferredoxin and NADPH. A side route, cyclic flow, recycles electrons around photosystem I, so the cell gets extra ATP and no extra NADPH.

Each charge separation moves one electron, but one oxygen molecule needs four. Early flash experiments showed it directly: photosystem II releases oxygen after four consecutive flashes. A cluster of four manganese ions and one calcium ion solves the mismatch by storing oxidizing equivalents between flashes. Oxygen leaves as the by-product, and the reaction also helps build the proton gradient.

Electron flow through photosystem II and the Q-cycle at cytochrome b6f builds the gradient, which stores energy in two forms, an electric field and a pH difference. ATP synthase couples proton flow to the rotation of a ring of c subunits, and each full turn makes three ATP. Spinach's ring has 14 subunits, which works out to about 4.67 protons per ATP.

The handoff runs through ferredoxin, the soluble carrier that photosystem I reduces. An enzyme called FNR then uses it to turn NADP+ into NADPH. Supply and demand do not quite match: linear flow likely yields about 1.28 ATP per NADPH, while fixing carbon and regenerating the cycle needs 1.5. Cyclic flow is one way to raise the ratio, since it adds ATP without adding NADPH.

The bookkeeping explains the cost. Each carbon dioxide fixed, along with regenerating the cycle's intermediates, takes 3 ATP and 2 NADPH, so the three turns behind one exported three-carbon sugar cost 9 ATP and 6 NADPH. The three phases are carboxylation, reduction and regeneration, and products leave for starch, sucrose and other pathways.

Rubisco is abundant because it is poor at its job. Its median turnover across species is about 3 reactions per second, much slower than most central metabolic enzymes, and it can react with oxygen instead of carbon dioxide. A plant compensates with sheer quantity, about 30 to 50 percent of the soluble protein in C3 leaves, an enormous investment of energy and nitrogen.

When rubisco fixes oxygen instead of carbon dioxide, the product has to be recycled through several cell compartments, at an energy cost. Heat shifts the balance further toward oxygen, because rubisco's specificity for carbon dioxide falls as temperature rises. It is generally considered a futile cycle, but that is contested: one review argues that its waste has been overstated.

Heat hurts a C3 leaf twice. A steeper humidity gradient drives water loss and triggers stomatal closure, which limits the carbon dioxide reaching the leaf interior. At the same time, warmth lowers rubisco's specificity for carbon dioxide over oxygen. The two compound each other, since more carbon dioxide inside the leaf is what would ease photorespiration at high temperature.

C4 plants do not replace rubisco; they feed it. In mesophyll cells PEP carboxylase fixes carbon dioxide into oxaloacetate, which becomes malate or aspartate and moves to the bundle sheath. Decarboxylation there raises carbon dioxide around rubisco, and in the NADP-ME type pyruvate returns to the mesophyll, where ATP remakes PEP. The wreath of cells around the vein is called Kranz anatomy.

CAM uses the same PEP carboxylase idea as C4, but it splits the work by time inside one cell type instead of between two. At night, PEP carboxylase in the cytosol fixes carbon dioxide into malate, stored in the vacuole. Stomata stay closed in the day to save water, and rubisco activates just after dawn. It suits hot, arid regions, where malic acid banked at night funds fixation by day.

The extra machinery costs energy, and low light exposes it. One account of the dim forest interior has ATP running short for the C4 carbon pump, carbon dioxide leaking from the bundle sheath, and carbon gain falling. That is one reason offered for why C4 trees are rare, alongside reduced quantum yield and higher energy demand. These costs limit a C4 tree without ruling one out.

"The one scarcest factor" is a useful first idea, but a leaf can be more complicated. Where carbon dioxide supply is what runs short, rubisco and PEP carboxylase are limited by that supply rather than by electron transport. A leaf may also be limited in two ways at once: simulations of C3-C4 leaves pair a light-saturated mesophyll with a light-limited bundle sheath.

Respiration is not a night shift that replaces photosynthesis. Gross photosynthesis is estimated as net assimilation plus mitochondrial respiration, and that respiration spends carbon on growth and upkeep. A 1905 botany text detected the carbon dioxide from germinating peas. The rate is not constant either: across 31 species at constant temperature, night respiration fell an average of 25 percent after 8 hours of darkness.

A gas analyzer on a leaf sees only the balance, which is net photosynthesis. Estimating gross fixation takes an extra trick, such as feeding the leaf labeled carbon dioxide for a short time and tracking its uptake. The gap matters because crop source strength depends on the net rate, along with how well stored reserves can be remobilized from source tissues.

At that light level a leaf takes in just enough to pay for its own respiration, so there is nothing left to build with. A low point is a key trait for seedlings surviving under a canopy, and it is not fixed for a species. In one experiment on Pinus thunbergii seedlings, partnering with ectomycorrhizal fungi lowered it, helping them cope with shade.

Different problems need different forms. Starch granules form in chloroplasts for temporary storage, amyloplasts hold dense starch elsewhere, and sapwood parenchyma keeps reserves in wood. Sucrose travels: it is the main sugar in cereal phloem sap, where its concentration helps maintain the pressure gradient that drives flow. A 1905 botany text reports leaves kept dark for a day or two holding no starch.

The labels describe a job at a moment, not an organ type. Fully expanded leaves supply growing and storage organs, but carbon stored in wood has to be drawn on for the next flush of leaves. Demand also feeds back on supply: when sink demand drops, export slows, sucrose builds up in the leaf, and loading capacity falls.

In rice, high night temperature increases carbon lost through night respiration and also reduces source-sink enzyme activity, which leads to lower biomass and poor grain filling. Yield loss is therefore not only a respiration story. Temperature also explains less of the pattern than usually assumed: in a 31-species study it controlled under half of the variation in night respiration.

Flooding and drought can look alike from above. Excess water fills soil pores and disrupts oxygen diffusion to plant tissues, which limits root metabolism and so limits water uptake, sometimes leaving a flooded plant short of water. Oxygen deficiency can end in root death. Some plants respond by forming aerenchyma-rich adventitious roots, a process taking about 3 to 7 days.

Water crosses a membrane by simple diffusion or, more often, through aquaporin pores, and the direction depends on which side holds more dissolved material. Hyperosmotic stress reverses the flow and can detach the protoplast from the wall. The membrane sets what can pass, and a 1905 botany text gives the wall's part: its elasticity resists further swelling, so the cell can take on water safely.

Water potential folds several effects into one figure: hydrostatic pressure, dissolved solutes, attraction to solid surfaces, and gravity. That lets you compare very different places, such as a salty soil and a root. Salt lowers soil water potential and makes uptake harder, so a soil can be wet and still be hard for a plant to draw from.

Wilting has two meanings that are easy to confuse. A 1905 botany text shows a leaf in salt solution drooping as if wilted, and limp beet slices stiffening again in water. Permanent wilting is a soil limit: the moisture level where plants wilt and do not recover even with enough water supplied. Crops also differ in the leaf water potential at which they lose turgor.

Under hyperosmotic stress the living protoplast detaches from the wall, a process driven mainly by the vacuole. It is reversible and characteristic of living cells, so it is a feature of a live cell rather than a sign of a dead one. Salt injures plants in more than one way: alongside this osmotic water deficit, there is a separate ionic stress from the salt itself.

Cohesion is water sticking to water; adhesion is water sticking to something else. Both come from the opposite charges on hydrogen and oxygen atoms in the molecule. A paper towel shows them working together: molecules climb the fibers and drag others along. In a plant, transport relies on the same attractions to keep water columns continuous from root to leaf.

The pull starts as a physical effect in very fine spaces. Water in the leaf sits as thin films along cell walls bordering the air spaces. As it evaporates, air-water menisci recede into wall pores and curve more tightly, which generates strong capillary tension and lowers the local water potential. Surface tension itself comes from water's cohesive forces.

These channels are not plain holes. Their activity can be changed through gating, controlled by phosphorylation, pH, calcium and interactions with other proteins, and they can also carry gases and metalloids as well as water. They matter along the whole route, including the phloem, where they help water enter sieve elements. Permeability is therefore something a cell regulates.

The theory dates to the 1890s and says the transpirational pull from continuous water columns creates tension gradients of several megapascals, enough to beat gravity and friction. It is widely taught, but it is not unchallenged. One review argues that plants also use other routes, including uptake by inverse transpiration and water secretion across membranes, under a multi-force theory.

The scale is easy to underestimate: in a growing season a leaf transpires many times its own weight in water, and a large oak can release about 40,000 gallons a year. Xylem carries water and essential minerals from root to leaf, and the water leaves mostly as vapor through stomata. One exception to the water-only picture is deciduous trees, whose xylem carries remobilized carbon at the winter-spring transition.

Guttation droplets are fluid secreted through pores called hydathodes at the margins and tips of leaves, and they contain xylem and phloem sap, so insects feed on them. Root pressure controls the process, and temperature, humidity and sunlight affect it. One study across bamboo species found plant height rising linearly with basal root pressure.

A break starts when sap tension reaches a critical threshold: air is pulled in through pit membranes between conduits, or gas bubbles nucleate from dissolved gas. Lost transport capacity can ultimately kill the plant. Plants answer in three ways, with resistant anatomy, new conduits from new growth, and active repair by living cells around the embolized conduit.

The difference starts with who has them: softwoods have only tracheids for conduction, while hardwoods add vessel elements. Vessel size is itself a strategy. A ring-porous hardwood can have earlywood vessels an order of magnitude wider than those in its latewood, and mixing sizes is thought to help because large, cavitation-prone vessels sit close to smaller, safer ones.

Humidity and wind act on the air next to the leaf: water evaporates more easily into drier air, so rising humidity slows the rate, and wind replaces saturated air near the leaf with drier air. Warmth raises the rate too, in part by opening the pores, while cold closes them. Dry air is not a simple accelerator, because a steep humidity gradient can also trigger stomatal closure.

At the leaf, one common measure is carbon dioxide assimilated divided by stomatal conductance, called intrinsic transpiration efficiency. Scale changes the answer, since gains at the plant level are hard to translate to a whole farm or landscape. C4 metabolism offers one route, but inserting it into C3 crops such as rice is proving a challenging task.

In softwoods and hardwoods alike, most wood cells have died by maturity, sapwood included, so a conduit has no pump of its own. Dead does not mean the living plant plays no part. Parenchyma cells stay alive between the conduits, handle storage and synthesis, and are implicated in repairing embolized conduits, a metabolically active process.

Loading comes first: in apoplasmic loaders, proton-coupled sucrose transporters concentrate sugar into the sieve element complex. That creates an osmotically derived pressure that drives bulk flow, with aquaporins letting water in. Sugar unloads at the far end. Not all plants load actively. Some are classed as passive symplasmic loaders, and whether the proposed mechanism holds across all of them is unclear.

A sieve element gets its open channel through partial self-digestion, keeping only a thin cytoplasm along the wall, with no nucleus. Its companion cell does the opposite, with a dense cytoplasm, and extensive plasmodesmata join the two into a single metabolic and genetic unit. In many plants, sugar transporters load the sugar directly into this complex.

Direction follows the source and the sink, wherever they happen to sit. Inner bark moves sugars from the leaves to the roots or to growing portions of the tree, so the same trunk can serve targets below and above the leaves. Xylem is different, running generally from roots to transpiring leaves. Ask where the sugar is needed before you draw an arrow.

A tidy contrast, but do not read it as absolute. Phloem also carries water, mineral elements, amino nitrogen compounds and signaling molecules along with sugar. Xylem can carry remobilized carbon in deciduous trees between winter and spring. A sturdier split is how the flow is driven: pressure from sugar loading in phloem, and, on the standard account, tension from evaporation in xylem.

Girdling is also a research tool, because it physically blocks phloem transport in the stem while water keeps moving in the wood. In one study of Norway spruce saplings, it caused a significant drop in fine root biomass. Coarse roots behaved differently: mobilized starch reserves actually drove a striking increase in their radial growth, especially under drought.

Researchers use this directly. Cut the stylet of a feeding aphid, and a droplet of phloem sap forms at the severed tip, which can be measured or collected as a sample of the sap itself. Flow is not constant: one wheat study found collection nearly impossible at about 10:00 in the morning because flow from severed stylets was so low.

For a seed crop, yield is the cumulative result of source and sink strength through seed development. Breeders track this partly through harvest index, one of the most studied traits in plant breeding. It is not purely genetic. Much of the variation comes from climate and soil, and extreme heat or cold during reproductive development also moves it.

Pore space decides how the other parts behave. Pores hold water and air, so saturation drives out air and starves roots of oxygen, while compaction reduces total pore volume and crushes large pores into smaller ones. Organic matter does not just sit on top: in an A horizon, humified organic matter is closely mixed with the mineral fraction.

Texture is the weight share of sand, silt and clay particles under 2 millimeters, so it can be felt: sand is gritty and visible, silt feels smooth, and clay can be sticky. Finer is not always wetter in the useful sense. Available water rises from sands to silt loams, then falls in clay soils, because clay holds so much water at the wilting point.

Texture is the particles; structure is how they clump. A unit of soil structure holds together more strongly internally than it sticks to its neighbors, so under stress soil ruptures along set planes into forms such as platy or prismatic units. Unlike texture, structure responds to management. Organic matter improves it and increases pore size, while compaction crushes the larger pores.

Not every soil has every layer. Most soils have an A surface horizon, B subsoil and C substratum, while some have an O horizon of organic material, which can even be buried. An E horizon is defined by what it lost: clay, iron or aluminum wash out and leave sand and silt behind. C layers are little changed by soil formation, and R is bedrock, which is not soil.

The scale is logarithmic, so a soil at pH 5 is 10 times more acidic than one at pH 6 and 100 times more than pH 7. Different elements fail at each end. Acid soils tend to run short of calcium, magnesium, phosphorus and molybdenum, while aluminum and manganese can reach toxic levels. Very alkaline soils often lack phosphorus, iron, copper, zinc and boron.

Weathering releases cations such as calcium, magnesium, potassium and sodium, and some are held at the surfaces of clay minerals and humus. Coarse soils retain them poorly and let water percolate quickly, so they can lose cations and acidify easily. As cations leave, hydrogen ions take their place, and clay and organic matter act as buffers against the swing in pH.

Not all water in a soil is plant water. Field capacity is what remains after free drainage, usually over one to two days, and the wilting point is where plants fail to recover. Plant-available water is the difference between them. So a clay can offer less than a loam, salts can trap water at the low end, and organic matter improves storage directly and through structure.

Soil life does chemistry and construction. Organisms decompose organic matter, cycle nutrients, degrade pollutants, and form and stabilize soil structure. Management changes the community: populations of bacteria and fungi are higher where residues are returned, and acidification hampers most bacteria while fungi adapt across a wide pH range. Earthworms are the one group staff can count in the field without special training.

The classic test, from Arnon and Stout, has three parts: lacking the element, the life cycle of a plant cannot finish; the deficiency is specific to that element; and the element acts directly in nutrition. Being useful is not enough. Silicon is not classed as essential despite many reported benefits, and the list can grow: other essential elements may yet be found.

Carbon, hydrogen and oxygen are macronutrients too, taken from air and water rather than soil. Of the rest, nitrogen, phosphorus and potassium are primary, and sulfur, calcium and magnesium are secondary. Roles differ: nitrogen builds amino acids and chlorophyll, phosphorus handles energy storage and transfer, potassium helps regulate stomata, and magnesium is the central atom of chlorophyll.

Small amounts do not mean small roles, because some of these elements sit at key steps of a single process. Molybdenum is tied to the enzyme nitrate reductase, and manganese is part of the cluster that splits water in photosynthesis. Essentiality was shown element by element, with iron in 1860 and chlorine not until 1954. Nickel and cobalt are special cases, tied to particular metabolism.

The three numbers are percentages, but not of the same kind of thing. The first is the element itself, so 100 pounds of 10-34-0 holds 10 pounds of it. The other two are listed as oxides, P2O5 and K2O. To convert those to the elements, multiply P2O5 by 0.44 and K2O by 0.83, which is why the label overstates the actual phosphorus and potassium.

Location is the clue because a plant can move some nutrients and not others. Nitrogen, phosphorus, potassium and magnesium are mobile in the plant, so shortage shows first on older leaves as reserves move to new growth. Calcium, iron, boron, manganese and zinc are not, so symptoms tend to show in new tissue. Mobility in the plant and in the soil are separate questions.

Two questions narrow it down: which leaves, and where on the leaf. Nitrogen shortage starts as pale yellow on older leaves, from the tips, and lower leaves may die or drop. Iron shortage shows as interveinal chlorosis on the youngest, apical leaves. A pattern is a lead rather than a verdict, since iron shortage can itself come from waterlogging or overfertilizing, not just low soil iron.

Oversupply has its own symptoms. Too much zinc inhibits root growth and causes chlorosis in young leaves, and excess boron causes dark speckles or dead tissue on older leaf edges. Excess can also upset balance rather than poison directly: iron competes with other vital metals, so extra iron can lead to metal imbalance. In very acid soils, aluminum and manganese can reach toxic levels.

Water-culture work grew seedlings of peas, corn or beans with their roots in a defined solution, using recipes such as one by Sachs, and by 1905 a textbook could list calcium, potassium, magnesium, phosphorus, sulfur and iron as indispensable. Newer solution techniques may yet reveal more essential elements. Soilless systems also show that soil is not required, though the root microbiome changes.

Prokaryotes carry nitrogenase, the enzyme that turns nitrogen gas into ammonia at ordinary temperature and pressure, and it works only when oxygen is scarce. A plant can house these microbes and feed them, but the chemistry is theirs. That is why a legume does not fix nitrogen by itself, and why a field missing the right rhizobium may need its seed treated with an inoculum.

Inside a nodule the plant supplies carbohydrates from photosynthesis and the bacteria return fixed nitrogen. Leghemoglobin keeps the local oxygen low, because oxygen would inactivate nitrogenase. Nodules are not exclusive to legumes: Sitka alders host nitrogen-fixing bacteria in root nodules too, and their nitrogen-rich leaf litter builds fertile soil after a glacier retreats.

The fungal threads act as extra absorbing surface, taking up water and minerals such as phosphorus, while the fungus takes carbohydrates in return. The partnership can be exploited. Ghost plant and other mycotrophic wildflowers have no chlorophyll and tap the fungus for carbon that came from a nearby tree, and the fungus receives nothing back.

Arbuscular fungi are obligate partners, widespread among land plants. They pass through a root cell's wall but not its membrane and form highly branched arbuscules, where phosphorus is handed over. Ectomycorrhizal fungi, usually partnered with a tree, build a dense mantle around fine root tips plus a Hartig net between root cells, without entering them. Some trees, such as poplar, respond to both.

Carnivorous plants grow in nutrient-poor habitats. Their enzymes break prey down and the plant absorbs what is released, with ammonium the likely preferred form of nitrogen. Most keep using their trap leaves for photosynthesis as well, and a pitcherplant's roots still take up nutrients, so prey adds to the plant's income rather than replacing the rest of it.

A haustorium is the structure that taps the host. Holoparasites such as dodder and broomrape cannot photosynthesize and rely on the host for water and nutrients, while hemiparasites such as witchweed stay photosynthetically active. Do not confuse them with mycotrophic plants, which also lack chlorophyll but get their carbon through a fungus instead of directly from a green plant.

An epiphyte grows on another plant but does not parasitize it. Its nutrients come from the atmosphere, from water running down the tree, and from trapped litter. Ferns high in a rainforest can root in a mat of moss and lichen that behaves like soil and holds water. A tree seedling that starts on a branch and later sends roots to the ground is not truly epiphytic.

Auxin from young leaves at the shoot tip is thought to hold side buds back, which is why cutting the tip can release them, though sugar signals can start that release before auxin falls. Together with gibberellins it sets how far shoots elongate. Applied forms of auxin are also the active ingredients in rooting powders used on cuttings.

Gibberellins strongly influence stem elongation and promote germination, where they work against abscisic acid. Short crops can arise in two ways. Many semi-dwarf rice varieties lack a working gibberellin-making enzyme, while the common wheat dwarfing alleles leave the hormone in place and impede its signaling. The wheat alleles also shorten the emerging coleoptile, which can hurt seedling establishment.

Cytokinins stimulate cell division, and in tissue culture their effect depends on auxin. A high auxin-to-cytokinin ratio favors root formation and a low ratio favors shoots, a pattern traced to Skoog and Miller in 1957. That is why callus is moved to a high-cytokinin medium to raise shoots. Read the ratio as a balance, because the two hormones can also work against each other in development.

Drought triggers abscisic acid synthesis, and the hormone makes guard cells lose ions and water so the stomata close. In seeds it supports dormancy: its level is low in developing seeds and highest at maturity, which may stop embryos germinating on the tree. For tree seeds that link is still a correlation, so ABA levels alone do not prove what causes dormancy.

Because ethylene is a gas, its signal can travel beyond the tissue that made it. In climacteric fruits such as apples and peaches its production is autocatalytic, so a little ethylene triggers more and ripening speeds up. Grapes and strawberries lack that surge, so the same logic fails for them. In an abscission zone, ethylene activates enzymes that break down cell walls and let fruit detach.

Abscisic acid and gibberellin act antagonistically in seeds, so the balance between them, not either level alone, tilts a seed toward dormancy or germination. Location matters as well: auxin raises expression of certain cytokinin-response genes in the root but lowers it in the shoot meristem. Before predicting an effect, ask which organ you mean and which other hormones are present.

Rooting powders rely mainly on IAA, IBA, and NAA. The herbicide 2,4-D, on the market since the mid-1940s, is a synthetic auxin most often used to control broadleaf weeds selectively in grass crops, and weeds have evolved resistance to it. Ripening can be held back by AVG, which suppresses a step in ethylene synthesis, or by 1-MCP, which blocks the ethylene receptor.

Auxin that helps at one dose can harm at another. In a greenhouse trial on Photinia stem cuttings, IBA promoted rooting, but 8000 ppm reduced rooting success and increased callusing, and high NAA or IAA gave stunted or malformed roots. That is one study on one shrub, so treat it as the shape of the response rather than a number to reuse.

Blue light is the trigger, sensed by the phototropins phot1 and phot2. An auxin maximum then forms on the shaded side of the shoot, and uneven growth bends the stem toward the light. Do not assume a root simply mirrors this: reports disagree about which side of the root tip accumulates auxin.

Statocyte cells, in the root cap for roots, hold starch-filled plastids that sediment under gravity. When a plant tips, their new position relocates PIN auxin transporters, sending auxin toward the lower side of the organ. In a root, that buildup is thought to slow elongation on the lower side, so the root bends. Statolith position is the likely signal, and how it becomes a chemical signal is still unresolved.

These are two different responses. A tendril reacts to contact with a support: Darwin's 19th-century account describes tendrils that are sensitive to a touch and then contract into a spire. Thigmomorphogenesis is a change in body form: repeated mechanical stimulation usually means less height and leaf area and thicker stems. Do not treat wind as identical to touch, since one study found opposite effects from wind and brushing.

Phytochrome flips between a red-absorbing form, Pr, and a far-red-absorbing form, Pfr, as light quality changes. A neighbor's canopy lowers the red to far-red ratio, and plants respond by elongating stems and petioles. Seeds use the same switch: red light converts Pr to Pfr, which is associated with overcoming dormancy, while far-red partly restores it.

In the short-day Japanese morning glory, flowering follows the length of the night, and five minutes of red light can impair it. That is why such a plant needs a long enough dark period, not merely a short day. In one model of long-day detection, light falls into a dark-expecting phase of the clock, and scientists still test hourglass against circadian timers.

Vernalization is flowering sped up by prolonged cold. In Arabidopsis, cold represses the flowering repressor FLC and silences it, so the effect outlasts winter. Expression of FLC and its regulator VIN3 tracks temperature extremes more closely than the average. Day length is a separate control: in wheat, PPD1 governs the photoperiod response and VRN1 the cold requirement, and spring wheats carry VRN1 alleles active without cold.

An internal clock times stomatal opening, photosynthesis, leaf movement, and flowering. Root elongation cycles too, and a clock mutant loses that rhythm under constant conditions, which points to an internal timer rather than a simple response to light. The value lies in anticipation: the cabbage looper usually feeds at noon, and jasmonate defenses rise early in the day and peak at noon.

Land plants have a haploid-diploid life: a multicellular diploid sporophyte releases spores, and a spore germinates into a gametophyte with its own sexual organs. Most related streptophyte algae differ, because their only diploid cell is the zygote. The name stays the same across lineages, but which generation you see as the plant does not.

Follow what each cell does next. A spore grows into a whole gametophyte with no partner, while a gamete must fuse to make a zygote. The names hide the trap: in a fern, the sporophyte's reproductive job is to release haploid spores, and the gametophyte grown from one makes the gametes. Pollen shows the order too, with meiosis first and then two mitotic divisions.

In bryophytes the sporophyte stays attached to the gametophyte. In vascular plants the sporophyte is free-living, larger, and more complex, and the gametophyte keeps shrinking, until in most flowering plants the female and male forms are each just a handful of cells. Ferns sit between, with a tiny gametophyte that lives independently.

Most ferns make one spore type, and the gametophyte's sex can depend on its neighbors. Heterospory splits spores into small microspores and larger megaspores. In a simplified account of the seed's origin, one megaspore is then retained in its sporangium and wrapped in an integument to form the ovule. Heterospory alone is no seed: it arose several times, including in aquatic ferns.

An 1887 botany text sets out the pattern. Annuals grow from seed, fruit, and die root and all. Biennials such as carrots store food in a thickened root the first season, then flower from that store and die, so both groups flower only once. Perennials flower year after year, and a perennial herb may die to the ground while buds below survive. The same text says all gradations exist, so a single label can mislead.

Monoecious species such as birches and oaks carry separate male and female flowers on one plant. Dioecious species such as hollies carry them on different plants, so seed on a female depends on pollen from another individual. Sex can be flexible: silver maple can be monoecious, dioecious, or polygamo-monoecious. Most conifers are monoecious, but junipers are dioecious.

Self-compatible plants can reproduce alone, but a fully selfed fern gametophyte yields a completely homozygous sporophyte. Timing is one guard: in dichogamy male and female organs mature at different times, as in southern magnolia, whose stigmas are receptive before its pollen is released. Separate sexes and genetic self-incompatibility push the same way, toward crosses and diversity.

Mosses, liverworts, and hornworts lack the hardened conducting tissue of vascular plants, and mosses take in water directly through their surface. Many mosses have simple hydroid and leptoid cells, so no conducting cells at all goes too far. Nor are they all tied to wet places: some survive deserts by drying out and reviving when moisture returns.

Rhizoids attach a moss to its surface. Water and nutrients come in directly through surface tissues, so a moss can live on rock or bark with no soil at all. Without hardened conducting tissue, water and food cannot move efficiently through the body, the difference that calling rhizoids roots hides.

The green body you recognize is the gametophyte, and gametophytes make the sperm and eggs. A fertilized egg grows into the sporophyte, a slender stem rising straight out of the gametophyte, with a capsule at the tip that releases spores. A spore then forms a flat filamentous protonema, which matures into new gametophytes. A fern reverses which generation is the big one.

Flagella let sperm swim to the archegonia, a trip that needs liquid water and in several species is set off by rain. That confines sexual reproduction in most non-seed plants to wet seasons with a continuous layer of moisture. A fern sperm can swim for up to an hour. The limit applies to sex, not to survival: some mosses endure deserts, and a fern's rhizome can clone it without any sperm.

A frond unfurls from a tightly curled fiddlehead, though some non-ferns unfurl this way too. Spores form in sori, typically on the underside, and their shape and placement help you tell species apart. The rhizome is an underground horizontal stem that can break off and root as a clone. Ferns are vascular, unlike mosses, but they produce neither flowers nor seeds.

The large plant is the diploid sporophyte, the reverse of a moss. A spore grows, given enough moisture, into a prothallus: the small, short-lived gametophyte that lives independently and makes gametes. Sperm then swim to an egg, which starts a new sporophyte. Textbooks draw a heart-shaped hermaphrodite, but depending on the species a gametophyte can be male, female, hermaphroditic, or asexual.

In the Pennsylvanian, lycophytes reached up to 100 feet and left biomass that later became coal, and horsetails reached 30 feet. Modern club mosses are generally only a few inches tall. Do not lump the two together as fern allies: genetic studies group horsetails with the ferns, while club mosses and quillworts are not closely related to ferns.

A seed holds an embryo, a nutritive tissue, and a protective coat formed from the integuments. Compare a fern, whose free spore must first grow into a gametophyte. A dispersed seed can also wait, because dormancy may block germination even when conditions look good, and some seeds lie in soil for years before they sprout.

Pollen protects sperm from drying out and carries it by air or on pollinators, a key adaptation that let seed plants thrive even in very dry environments. In conifers, gnetophytes, and flowering plants, the sperm no longer swim at all. Cycads and ginkgo are the exception worth remembering: they have pollen, yet their sperm remain flagellated and motile.

The basic split from flowering plants is whether an ovary encloses the ovules. In a conifer cone, two inverted ovules form on the upper surface of each ovulate scale. Naked does not always mean dry or fully bare: juniper scales fuse into a berrylike structure, yew seeds develop in fleshy arils, and gnetophyte seeds are partly enclosed.

Short-lived pollen cones shed pollen mainly by wind, and pollination drops on receptive seed cones capture grains and carry them into the ovule. The timing varies by group. Most pines take three years from bud to seed, because pollen tube growth pauses and fertilization waits until the next spring. The gap between pollination and fertilization runs from 3 weeks in Engelmann spruce to 15 months in Monterey pine.

Small, wax-coated needles use less water, and keeping them for years saves the cost of regrowing foliage annually. Winter makes this a drought problem: cold soil limits uptake while sun and warm air raise demand, so the cuticle matters once stomata shut. At one Pyrenees treeline, thinner cuticles went with more winter water loss, which supports an idea that remains debated.

Conifers are the core group, with about 58.5% of living gymnosperm species. Ginkgo is a single species, and it shares flagellated sperm with the cycads, whose trunks are unbranched or forked. Gnetophytes are the outliers, with vessels in their secondary xylem and double fertilization, two traits you might otherwise reserve for flowering plants. How these groups relate is still argued over.

Softwood means wood from gymnosperms, not wood that is soft; some softwoods are not. Water moves through tracheids, long imperforate cells that make up most of the wood, while hardwoods have vessel elements with perforation plates, visible as pores. Resin canals appear in softwoods such as Douglas-fir and spruce but are no universal marker, because other softwoods lack them in normal wood.

From outside in, sepals enclose the bud, petals attract pollinators, stamens make pollen, and pistils hold the ovules, all set on the swollen receptacle. Only stamens and pistils are required for normal seed production, which is why the outer two are called accessory parts. Expect variation: tulip and lily sepals are colored like petals.

The filament is just the stalk that holds the anther up; pollen forms in sacs inside the anther. A microspore mother cell goes through meiosis and then two mitotic divisions, which is why a pollen grain is a tiny male gametophyte rather than a gamete. Sperm are made inside it, sometimes only after the grain has been shed and germinated.

The sticky stigma catches pollen, the style connects it to the ovary, and the ovary holds the ovules. A flower can have many pistils. After fertilization the ovule ripens into a seed, while the carpel develops into the fruit, sometimes with help from other floral parts. Keep the pairing straight: ovule to seed, ovary to fruit.

Complete asks whether calyx, corolla, stamens, and pistil are all present and working. Perfect asks only whether stamens and pistil both work. So a flower lacking a corolla can be incomplete yet perfect, while a unisexual flower must be incomplete. Cherries are complete; willows and hazels lack both calyx and corolla.

These definitions come from an 1887 botany text. In a raceme each flower has its own stalk along a common axis. Remove those stalks and you have a spike. An umbel's stalks all spring from one point, like the ribs of an umbrella. A head packs stalkless flowers on a very short axis, so a sunflower is many flowers: the disk flowers are small and tubular, and the showy rays are sterile.

A radially symmetric flower can be reached from any direction by many pollinators, while a bilateral one tends to promote visits from specialists. The orchid genus Ophrys goes further, drawing male wasps with a lip resembling a female insect. Bilateral symmetry evolved many times with many reversals, so treat it as a strong clue about pollination, not a rule.

An 1890 botany text gives monocots one cotyledon, mostly parallel-veined leaves, and floral parts in threes in the majority of species. Older texts give many other flowering plants two cotyledons, netted veins, and parts on a plan of four or five. Treat these as tendencies rather than a test, because a single trait can fool you: wild yam is a monocot with broad, netted-veined leaves.

The flower itself belongs to the sporophyte. Inside it, the embryo sac is the female gametophyte, just a handful of cells including the egg and central cell, and the pollen grain with its tube is the male gametophyte. A conifer differs: its large female gametophyte doubles as the seed's food store.

Pollination moves pollen to a receptive surface; fertilization comes later, when a tube delivers sperm to the egg. Between them, many grains never germinate and many tubes abort. The gap can be 24 hours or less in most flowering plants and 12 to 14 months in some oaks. Late freezes and rain during pollination can also interfere, so a pollinated flower is no promise of seed.

A germinating grain sends a tube between the cells of the stigma and style toward an ovule, carrying two nonmotile sperm as passive cargo. Synergid cells of the embryo sac secrete attractants, and the tube enters through the micropyle before bursting to release the sperm. Usually only one tube enters an ovule, and the others abort.

One sperm joins the egg to form the diploid zygote. The other joins the two polar cells, making triploid endosperm that feeds the embryo, with genes from both parents. Many Gnetales also fertilize twice, but mostly get extra embryos rather than a food tissue. That is why the nourishing endosperm is the mark to remember, and why a pine's food tissue should not be called endosperm.

Wind pollination trades attraction for quantity: small flowers, most without petals, no special color, scent, or nectar, stamens and stigmas exposed to the air, and feathery stigmas to catch grains. The pollen is small, smooth, and light and comes in enormous numbers, which is why wind-pollinated plants may be allergens while animal-pollinated ones seldom are. Most conifers use this route, along with about 12% of flowering plants.

Treat a syndrome as a forecast, not a guarantee. Bee flowers tend to be bright with nectar guides and a landing platform. Bird flowers are often scarlet, with ample nectar deep in a funnel and no odor. Bat flowers are dull and musty, and moth flowers smell strong and sweet, both at night. Butterfly flowers pair a narrow spurred tube with a wide landing pad.

In many bee flowers a region of low ultraviolet reflectance near each petal's center marks the way in; you cannot see it, but a bee can. Guides may do more than point. In one field study, wild bees favored Rudbeckia with oversized UV-absorbing guides, which suggests a role in recruiting visitors. That is one genus, so read it as evidence and not a general law.

Self-incompatibility is self-rejection between the pistil and the pollen, so it can block seed even in a flower with both sexes. The gametophytic form, controlled at the S-locus, is the most widespread known system. A crop review counted 134 animal-pollinated crops that are self-sterile, some only partly, so their yield can hinge on a compatible pollen donor. Self-sterility has other causes too, such as early-acting inbreeding depression.

S. E. McGregor's 1976 USDA handbook sorted crops three ways: grains that are wind-pollinated or self-pollinated, crops such as soybeans, peas, and cotton that are largely self-pollinating with some benefit from insects, and most fruits, vegetables, and nuts, which depend on insects. Those are its categories for its time, not a current list. A recent review adds that even self-fertile crops can gain seed and fruit quality from crossing.

The embryo is a miniature plant: radicle for root, hypocotyl for stem, plumule for bud, and cotyledons. Food may sit in endosperm, in the cotyledons, or in both, depending on the genus. Gymnosperms store it in female gametophyte tissue instead. The coat comes from maternal tissue, and it often carries the dormancy that decides when germination can begin.

In monocots the endosperm still supplies the seedling during germination, and palms keep it as their main store. In many dicots, such as hickory and catalpa, the endosperm is used up as the embryo develops and the cotyledons store the food. This is no fixed rule: persimmon and magnolia are dicots that keep endosperm, and ash stores food in both places.

A fruit develops from the carpel after fertilization, sometimes with other floral parts joining in. An 1887 botany text classes the tomato as a berry, the cucumber as a pepo, or hard-rinded berry, and a grain of wheat or corn as a fruit fused to its seed, whatever a grocery calls them. Fertilization is not always required: parthenocarpy forms fruit without it.

Count flowers and carpels; these examples come from botany texts of 1887 and 1890. A cherry is simple, a one-seeded stone fruit from a single carpel. An aggregate comes from many carpels of one flower, as a strawberry does, and a blackberry is one too, despite the name. A multiple fruit, such as a mulberry or pineapple, forms from several flowers consolidated into one mass.

Some fruit flesh is not ovary wall. Gray's 1887 botany text says the great mass of a strawberry is receptacle, and modern research still speaks of receptacle fruit development. The same text says the apple's principal thickness is calyx, with papery pods at the core, which is an older account. So the eaten part of a fruit is not a safe guide to which part of the flower made it.

Dry fruits divide by whether they open. Dehiscent ones split at maturity, such as capsules, legumes along two sutures, and follicles along one. Indehiscent achenes, nuts, and samaras stay closed. Among fleshy fruits, a berry is pulpy with a skin, a drupe has a stony endocarp around one seed, and a pome has a papery core. The stone of a drupe is fruit wall, not seed coat.

Wind-carried seeds are mostly small, with hairs or wings large relative to the embryo. Food and color recruit animals, with rodents moving seeds locally and birds spreading them widely. Water dispersal runs on flotation, and among oaks, overcup acorns are noted for floating. Ceanothus and witch-hazel fruits split explosively as they dry. Some seeds travel in phases, such as through an animal and then on water.

In climacteric fruits, ethylene output and respiration surge together, and ethylene signals then shape color, softening, sugar, and aroma. Do not generalize this to all fruit. Grapes and strawberries show no such rise, and their ripening is governed by hormones such as abscisic acid. So an ethylene blocker such as 1-MCP addresses only part of the ripening picture.

A dormant seed could germinate but does not, even in favorable conditions, which separates it from one that simply lacks water. The block may lie in the coat, which often keeps out water or gases, in the embryo, often through inhibitors such as ABA, or in an embryo not yet fully grown. Delayed germination and true dormancy shade into each other, so the boundary is not sharp.

Match the treatment to the block. Hard coats need scarification by abrasion, hot water, or acid, and all such methods carry risk to viability. Internal dormancy usually needs stratification, moist chilling at 1 to 5 C for 1 to 6 months that mimics winter. Basswood needs both, in that order. In nature, gut passage can scarify hard coats but may also damage embryos, and fire melts resin seals so serotinous cones can open.

Moisture, temperature, light, and aeration all act, but unevenly. Oxygen is rarely limiting unless a seed is buried too deep or submerged. Temperature mostly changes the rate of germination rather than the total. Light stimulates most woody species but is absolutely necessary for only a few, and in some species chilling or high temperatures can overcome a dark block.

Water uptake runs in three phases: rapid imbibition, a slow lag, then rapid uptake as metabolism climbs and the coat splits. The first phase also happens in dead seeds, so swelling alone does not prove a seed is alive. Stored foods then become soluble and move to the embryonic axis, with lipids passing through starch in oily seeds and proteins breaking down to amino acids.

In epigeal germination the hypocotyl arches and straightens, pushing the cotyledons above the soil, as in pine and magnolia. In hypogeal germination the epicotyl elongates and the cotyledons stay below ground, feeding the seedling for weeks, as in oak and walnut. The form is normally the same within a genus, but the cherry and plum genus has both.

These methods rely on a plant part regrowing what it lacks. An 1887 botany text gives the old core: a cutting forms roots from its cut end, and a layered branch roots while still attached before being cut free. Auxin rooting powders now help cuttings. A rhizome that breaks off and roots makes a clone, a graft joins a scion to a rootstock, and in tissue culture the auxin-cytokinin balance steers callus toward roots or shoots.

In a grafted apple tree the scion decides the variety, while the rootstock decides tree size, when bearing starts, and resistance to soil diseases, and carries nutrients up. Dwarfing rootstocks hold a tree near 10 feet instead of 20 to 30. Graft junctions are not perfect pipes: they often show disorganized vascular tissue, and certain scions do better on certain rootstocks.

Constitutive defenses are in place before attack; induced ones follow it. Conifers show both, keeping terpenes and phenolics on hand and adding more when attacked. Plants must also keep balancing growth against defense, a constraint that sharpens under stress. For bark beetle protection, the relative roles of the two are still being worked out.

Bark gives conifers a lignified physical barrier. Silica, which builds up mainly in grasses, hardens leaf tissue and may wear down chewing mouthparts, though some studies question that wear and piercing-sucking insects can avoid it. In tomato, trichomes can repel or trap pests and attract their natural enemies. Prickles arise from epidermal cells. The line with chemistry blurs because glandular trichomes also secrete compounds.

Pyrrolizidine alkaloids protect plants against insect herbivores. Terpene-based resin in conifer bark and related phenolics act against beetles and their fungi, and phenolics such as tannins are produced abundantly after infection. The levels are not fixed: fungal inoculation raised bark terpenes 3- to 9-fold in one Norway spruce study.

Plants first respond to broad pathogen signals, then to specific ones: resistance proteins recognize a pathogen's effectors, and a rapid, strong response kills cells at the site. Systemic acquired resistance follows, an induced defense in healthy tissues of a locally infected plant. It depends on salicylic acid and related signals, and the search for its long-distance signals is still going on.

Abscisic acid closes stomata to save water, and osmotic adjustment helps cells cope, while heat shock proteins protect cell components. Each has a limit. Stomatal closure cannot fully stop water loss, because transpiration through leaky stomata and cuticle can still dehydrate a tree. One review's ladder of drought responses ends with abscisic acid imposing full reproductive arrest under extreme drought.

When a plant freezes, ice forms outside the cells, lowering the water potential there and pulling water out, so cells dehydrate and shrink. Hardening is a separate step: low but non-freezing temperatures trigger cold acclimation, a multigenic process that builds up soluble sugars and raises freezing tolerance. Timing matters, so late spring frosts can kill flowers on trees that came through winter.

Salt imposes two stresses at once, osmotic stress and ion toxicity; high sodium limits water uptake, so a plant can run short of water in wet salty soil. Waterlogging displaces soil oxygen, and roots can lose nearly all of it within hours because diffusion through saturated soil collapses. Air channels called aerenchyma help but generally fail in full floods. The two stresses often occur together.

A species name has two parts, the genus and a specific epithet, in Latin. One plant can carry several common names, but its scientific name points to one accepted combination. Stable does not mean frozen: an author in parentheses marks a transfer, as when Rhus radicans became Toxicodendron radicans (L.) Kuntze.

Moving down the taxonomic ranks, from order to family to genus, shared traits grow more specific and each group gets smaller. A shared family can flag shared structure, but it may not separate the members: an 1890 text says mustards have such uniform flowers that their classification leans mostly on fruit and seeds.

Each step offers two opposite choices, one that fits and one that does not, and your answer sends you to the next pair until one name is left. A key is only for identification, unlike a taxonomy, so sitting next to another species in a key does not make it a relative. Its steps use characters you can observe, such as how a conifer's needles are grouped.

An 1890 text, using older family names, still shows the flags. Grass leaves have a split sheath around the stem. Legumes such as peas have a large standard petal, two wings, and a keel. Rose flowers are usually five-parted with five to thirty stamens. Mustards vary mainly in fruit and seed. Composites gather separate flowers on a receptacle. The nightshade family spans potato, tomato, and tobacco.

Producers make their own food by photosynthesis, and herbivores and everything above them live on that captured energy. A chlorophyll-free ghost plant shows the idea is about green plants, since it must take its carbon from elsewhere. In a fjord the same role falls to phytoplankton, so plants anchor land food webs rather than every food web.

Primary succession starts where no soil exists, such as land exposed by a glacier, and secondary succession follows a disturbance that leaves soil intact. Each stage changes the site for the next: at Kenai Fjords, alders enrich the soil, and later shade-tolerant hemlock seedlings thrive under spruce. Even a so-called climax community is not static, because succession never truly stops.

Non-native is not the same as invasive. Tomatoes and lettuce are non-native in the US, and the legal definition adds harm, or likely harm, to the economy, environment, or human health. Common traits include heavy seed production, far-traveling seed, aggressive roots, and growth-inhibiting chemicals. One ARS study of 243 European species found fast-growing weedy plants tend to have the most enemies to escape.

Breeders cross plants with promising traits, then screen the seedlings hard; one USDA apple rootstock program loses 70 to 80 percent to test pathogens alone. Heterosis is an F1 hybrid outperforming its parents, though its genetics remain incompletely understood. The advantage does not carry on: hybrid corn keeps its vigor for only a single generation, so farmers buy new hybrid seed each year.

A hardiness zone reports only the average annual extreme minimum winter temperature over 30 years, not the coldest night ever or heat tolerance. Zone is a first filter. Wind, soil moisture, soil type, and winter sun can greatly affect survival, and a plant hardy in your zone can be injured if soil is too dry going into dormancy. Species also differ in shade tolerance and soil pH range.

Watering too much and too often results in shallow roots, along with weeds, disease, and fungus. EPA's simple lawn test: if stepped-on grass springs back, it does not need water. Plants also differ, so trees and shrubs with deep roots take longer, less frequent watering. Schedule by soil and sun, and expect to lose up to 30 percent to evaporation at midday.

Cut just outside the branch bark ridge and collar. A flush cut injures stem tissue and can start decay, while a stub delays wound closure. Prune for health by removing diseased or insect-infested wood and crossing branches. Timing depends on bloom: spring bloomers set flower buds the year before, so prune right after flowering, and summer or fall bloomers during dormancy.

A rotation is a planned sequence of crops on the same ground, and it works against pests by interrupting their life cycles, with several years of perennial cover as the stronger break. Legumes in the sequence supply nitrogen to the crops that follow, provided the right rhizobia are present. Cover crops, planted for seasonal vegetative cover, can also suppress weeds and raise organic matter.

IPM is a sequence of decisions, not one method. Set an action threshold first, because sighting a single pest does not always mean control is needed. Monitor and identify, since many organisms are harmless. Prevent through rotation and resistant varieties. Control starts with less risky options, and broadcast spraying of non-specific pesticides is a last resort. IPM is not the same as organic, and it can include pesticides.

Set a plant at the depth it grew in the nursery and straighten or cut encircling roots, which can later cause girdling. Topping cuts large upright branches between nodes, which invariably brings epicormic sprouts or dieback to the next lateral. The sprouts are weakly attached and may end up held by a decaying branch, so the regrowth is the weak point.

Stand structure shapes what comes next. Thinning from below removes ladder fuels to reduce crown fire, though even spacing may lessen habitat variety. Fire can also be a regeneration event: lodgepole's serotinous cones release seed onto a clear, sunlit floor. If fire is kept out, shade-tolerant spruce and fir usually take over. After a disturbance, seed reach and light tolerance help decide which trees return.