10.2 Plant Structure, Function, and Transport
Key Takeaways
- Xylem carries water and dissolved minerals upward only, using the transpiration pull created by evaporation from leaves, while phloem carries dissolved sugars in either direction from source to sink.
- Stomata are pores flanked by guard cells that open to admit carbon dioxide and close to conserve water, which is why most plants close their stomata during the hottest part of a Texas afternoon.
- Leaf structure matches photosynthetic function: a transparent epidermis admits light, a waxy cuticle limits water loss, palisade mesophyll packs chloroplasts near the upper surface, and spongy mesophyll provides air spaces for gas diffusion.
- Vascular plants have true roots, stems, and leaves with xylem and phloem, allowing them to grow tall; nonvascular plants such as mosses lack this tissue and stay small and close to moisture.
- Tropisms are directional growth responses controlled by hormones: auxin accumulates on the shaded side of a stem, elongating those cells so the stem bends toward light.
Plants Are Half of the Structure-Function Statement
Competency 011 states that the teacher "analyzes how structure complements function in tissues, organs, organ systems and organisms," explicitly naming plants and animals. Candidates prepare the animal half thoroughly and the plant half thinly, which is a scoring risk: plants also anchor photosynthesis, ecosystem energy flow, the water cycle, and the carbon cycle, so plant items appear in Domains III and IV alike.
Levels of Plant Organization
Cell → tissue → organ → organism, the same hierarchy as in animals.
- Tissue types: dermal (protective outer covering), ground (photosynthesis, storage, support), and vascular (xylem and phloem transport).
- Organs: roots, stems, and leaves are the vegetative organs; flowers, fruits, and seeds are the reproductive organs of flowering plants.
Roots
Roots anchor the plant, absorb water and dissolved minerals, and often store food.
| Structure | Function | Structural feature that enables it |
|---|---|---|
| Root hairs | Absorb water and minerals | Thin extensions of epidermal cells that multiply surface area enormously |
| Root cap | Protects the growing tip as it pushes through soil | Continuously replaced layer of tough cells |
| Taproot | Deep water access; storage | Single thick primary root (carrot, mesquite, dandelion) |
| Fibrous root system | Erosion control; shallow water capture | Dense network of similar-sized roots (grasses, corn) |
The taproot-versus-fibrous distinction is a good Texas example: mesquite taproots reach water many meters down, which is why they persist through drought, while native prairie grasses use fibrous roots that hold topsoil in place.
Stems
Stems support leaves, transport materials, and in some species store food or perform photosynthesis. A stem contains xylem and phloem arranged in vascular bundles. Herbaceous stems are soft and green; woody stems add layers of secondary xylem each year, which is what produces growth rings. Modified stems include potato tubers (storage), strawberry runners (asexual reproduction), and cactus pads (water storage and photosynthesis).
Leaves and Photosynthetic Structure
The leaf is the clearest structure-function example in botany.
| Layer | Structure | Function |
|---|---|---|
| Cuticle | Waxy, transparent coating | Reduces water loss while admitting light |
| Upper epidermis | Single transparent cell layer, few chloroplasts | Protection without blocking light |
| Palisade mesophyll | Tightly packed column cells crowded with chloroplasts | Primary site of photosynthesis, positioned where light is strongest |
| Spongy mesophyll | Loosely packed cells with large air spaces | Allows CO₂ and O₂ to diffuse to and from photosynthetic cells |
| Vascular bundle (vein) | Xylem above, phloem below | Delivers water, removes sugar |
| Lower epidermis with stomata | Pores flanked by guard cells, mostly on the underside | Gas exchange; positioned away from direct sun to limit evaporation |
Xylem, Phloem, and Transpiration
| Feature | Xylem | Phloem |
|---|---|---|
| Transports | Water and dissolved minerals | Dissolved sugars (mainly sucrose) and some amino acids |
| Direction | Upward only, roots to leaves | Both directions, from source to sink |
| Cell condition at maturity | Dead, hollow, reinforced tubes | Living cells (sieve tubes with companion cells) |
| Driving force | Transpiration pull plus cohesion and adhesion | Active loading of sugar; pressure flow |
Transpiration is the evaporation of water from leaf surfaces, mainly through stomata. As water molecules leave, hydrogen bonding pulls the next molecules up behind them — cohesion holds the water column together and adhesion to the xylem walls keeps it from slipping. The result is a continuous column of water lifted from root to canopy without any pump. Transpiration also cools the leaf and returns enormous quantities of water to the atmosphere, which is why it appears as an arrow in every water-cycle diagram.
Guard cells control the trade-off. When water is plentiful, guard cells take up water, swell, and bow apart, opening the stoma so CO₂ can enter. Under water stress or high heat they lose turgor and close, conserving water at the cost of halting photosynthesis. A wilted plant has lost the turgor pressure that normally keeps its cells firm against their cellulose walls.
Reproductive Structures
In flowering plants, the stamen (anther plus filament) produces pollen and is the male structure; the pistil or carpel (stigma, style, ovary) is the female structure. Pollination transfers pollen to a stigma; fertilization occurs when a sperm nucleus fuses with an egg in an ovule. The ovule becomes the seed and the ovary becomes the fruit, which is why a tomato, a bean pod, and an acorn are all botanically fruits.
Seed dispersal mechanisms are structural adaptations worth naming: wind (maple samaras, dandelion parachutes), animal transport (burrs, fleshy fruits), water (coconut), and mechanical ejection (touch-me-not). Each is a structure-function pair a student can observe directly.
Tropisms: Structure Responding to Environment
A tropism is directional growth in response to a stimulus, and it links plant anatomy to the regulatory material of Competency 014.
- Phototropism — growth toward light. The hormone auxin accumulates on the shaded side of a stem, causing those cells to elongate more, so the stem bends toward the light.
- Gravitropism (geotropism) — roots grow downward with gravity (positive), shoots grow upward against it (negative).
- Thigmotropism — growth in response to touch, as when a vine tendril coils around a support.
- Hydrotropism — root growth toward moisture.
The auxin explanation matters because students commonly assume the lit side grows faster. It is the shaded side that elongates, and the resulting asymmetry tips the shoot toward the light — a mechanism students can test by masking one side of a seedling box.
A student girdles a young tree by removing a complete ring of bark and the tissue just beneath it, leaving the inner wood intact. Over the following weeks the leaves stay green and hydrated, but the roots weaken and eventually the tree dies. What does this demonstrate?
On a hot, dry August afternoon in Central Texas, a plant closes its stomata. What is the immediate trade-off?
A seedling on a windowsill bends toward the window. Which explanation is scientifically accurate?