Structure, transport, reproduction, and responses — how plants capture light, move water, reproduce, and sustain life on Earth
Walk through almost any outdoor space and plants are everywhere — covering the ground, climbing walls, arching over paths, floating on water. They are so familiar that it is easy to overlook how remarkable they are. Plants make their own food from sunlight, water, and air. They anchor soil, filter water, regulate temperature, and produce the oxygen that nearly every organism on Earth depends on. They are the foundation of most food webs on land.
But plants are more than a backdrop for other life. They are sophisticated organisms with internal systems for transporting water and nutrients, detecting light and gravity, timing their reproduction to the season, and responding to damage, drought, and competition. A redwood tree moving hundreds of liters of water from roots to canopy every day, a Venus flytrap snapping shut on an insect, a sunflower tracking the sun across the sky — these are not passive events. They are the work of biological systems.
This chapter explores what makes plants distinctive as a group of organisms, how their cells and tissues are organized, how their roots, stems, and leaves serve interconnected functions, how water and nutrients move through vascular tissue, how plants reproduce, and how plants respond to their environments. It also examines the deep connections between plants and the rest of life on Earth.
A large oak tree may move several hundred liters of water from its roots to its leaves on a hot summer day — with no pump, no heart, and no muscle. How is this possible? By the end of this chapter, you will understand the physical and biological mechanisms that make this feat routine for plants, and why it depends on a simple property of water molecules clinging to one another.
Plants are multicellular, eukaryotic organisms that carry out photosynthesis using chlorophyll. They are the primary producers in most land ecosystems — organisms that capture energy from sunlight and convert it into organic molecules that other organisms consume. Several features set plants apart from all other groups of living things.
Cell walls made of cellulose. Every plant cell is surrounded by a rigid cell wall composed largely of cellulose, a carbohydrate polymer. Cell walls give plant cells structural support, help plants maintain shape without a skeleton, and allow plants to build upright stems and tall trees.
Chloroplasts. Plant cells that carry out photosynthesis contain chloroplasts — organelles that capture light energy and use it to produce sugar from carbon dioxide and water. Chlorophyll, the green pigment inside chloroplasts, absorbs primarily red and blue light while reflecting green light, which is why most plants appear green.
Alternation of generations. Plants have a life cycle that alternates between two distinct stages: a diploid stage called the sporophyte and a haploid stage called the gametophyte. In familiar flowering plants and trees, the sporophyte is the large, visible plant. The gametophyte stage is reduced and, in many plants, microscopic — pollen grains and the structures inside ovules are gametophytes.
Stationary lifestyle. Unlike animals, plants cannot move from place to place to find resources, escape stress, or locate mates. Their biology reflects this: they evolved extensive root systems to find water and nutrients, chemical signaling systems to respond to threats, and reproduction strategies that allow their offspring to disperse.
Adaptation to land. Plants evolved from aquatic algal ancestors. The transition to land required solving major challenges: preventing water loss, supporting upright structures against gravity, transporting water from soil to tissues, and reproducing without surrounding water to carry reproductive cells.
| Group | Key Features | Examples |
|---|---|---|
| Nonvascular plants | Lack vascular tissue; small; require moist environments and liquid water for reproduction | Mosses, liverworts, hornworts |
| Seedless vascular plants | Have vascular tissue; reproduce by spores, not seeds | Ferns, horsetails, club mosses |
| Gymnosperms | Vascular; seeds on surface of cones or similar structures; no fruit enclosing seeds | Pines, spruces, cycads, ginkgo |
| Angiosperms | Vascular; seeds enclosed within fruits; flowers; largest and most diverse plant group | Grasses, oaks, roses, wheat, corn |
Angiosperms enclose their seeds within a fruit — the mature ovary wall. Gymnosperms produce "naked seeds" on the surface of cone scales. This distinction is one of the most important in plant classification. Angiosperm means "enclosed seed"; gymnosperm means "naked seed."
Plant bodies are built from several distinct cell types, each specialized for particular functions, organized into three tissue systems that run throughout the plant body.
| Cell Type | Key Feature | Primary Function |
|---|---|---|
| Parenchyma | Thin, flexible cell walls; living and metabolically active at maturity | Photosynthesis, storage, repair, general metabolism |
| Collenchyma | Unevenly thickened cell walls; living at maturity | Flexible structural support in growing regions |
| Sclerenchyma | Thick, lignified walls; dead at functional maturity | Rigid structural support (fibers, stone cells) |
| Xylem (tracheids, vessel elements) | Thick secondary walls; dead at maturity; hollow interiors | Water and mineral conduction upward |
| Phloem (sieve tube elements) | Living but lack most organelles; associated with companion cells | Transport of dissolved sugars from sources to sinks |
| Guard cells | Bean-shaped cells surrounding stomata; change shape with turgor | Regulate stomatal opening and closing |
Dermal tissue — the outer covering; epidermis, cuticle, root hairs. Protects against water loss and physical damage; absorbs water in roots.
Ground tissue — the bulk of most plant organs; parenchyma, collenchyma, sclerenchyma. Carries out photosynthesis, storage, and structural support.
Vascular tissue — xylem and phloem running through roots, stems, and leaves as continuous bundles. Transports water, minerals, and sugars throughout the plant body.
Roots serve three primary functions: anchoring the plant in soil, absorbing water and dissolved minerals, and storing nutrients. In many plants, roots also house beneficial microorganisms and play a role in vegetative reproduction.
At the tip of each root is the root cap — a thimble-shaped mass of cells that covers and protects the root apical meristem, a zone of actively dividing cells responsible for root growth. As the root pushes through the soil, the root cap is worn away and continuously replaced.
Just behind the root tip is the zone of cell division, where the meristem produces new cells. These cells then enter the zone of elongation, where they lengthen dramatically, pushing the root tip forward. Further back is the zone of maturation, where cells differentiate into the specialized cell types of functional root tissue.
The epidermis of the zone of maturation is studded with root hairs — thin extensions of individual epidermal cells that vastly increase surface area available for water and mineral absorption. A single plant may have billions of root hairs. Water and dissolved minerals enter through root hairs largely by osmosis and active transport, then move through the cortex to the vascular tissue at the root's core.
Surface area with hairs = 2 cm² × 10 = 20 cm²
Absorption capacity increases by the same factor — approximately 10×
| Root Type | Structure | Function / Examples |
|---|---|---|
| Taproot system | One large primary root with smaller lateral branches | Deep anchoring; nutrient storage (carrots, beets, dandelions) |
| Fibrous root system | Many roots of similar size spreading widely near soil surface | Soil holding; widespread water absorption after rain (grasses) |
| Prop roots | Roots growing from the stem downward to the ground | Additional structural support (corn, mangroves) |
| Storage roots | Enlarged roots accumulating starch, sugars, or water | Energy reserves (sweet potatoes, turnips) |
| Nodule-bearing roots | Roots hosting nitrogen-fixing bacterial symbiosis | Obtain usable nitrogen from atmosphere (legumes: soybeans, clover, beans) |
Legume roots form nodules housing bacteria that convert atmospheric nitrogen gas (N₂) into forms the plant can use. This is why farmers rotate legume crops with others — legumes enrich soil nitrogen, reducing the need for nitrogen fertilizer. This natural symbiosis is one of the most agriculturally important ecological relationships on Earth.
Stems serve multiple functions: they support leaves and flowers, position photosynthetic surfaces to receive light, and provide the main transport pathway connecting roots to leaves. Some stems also carry out photosynthesis and store water or nutrients.
In monocots (grasses, lilies, corn, and their relatives), vascular bundles are scattered throughout the ground tissue of the stem. In eudicots (most broadleaf plants, including beans, tomatoes, and oaks), vascular bundles form a ring near the stem's outer edge, arranged around a central pith.
Primary growth lengthens the plant. It originates from apical meristems at the tips of roots and stems — zones of cell division that add new cells that elongate and differentiate, extending the plant body upward and downward.
Secondary growth increases girth. It occurs in most gymnosperms and in eudicot trees and shrubs, but not in monocots. The vascular cambium (a lateral meristem) produces new xylem inward and new phloem outward. The xylem produced by secondary growth is wood. Annual tree rings form because xylem produced in spring (larger, lighter cells) differs from xylem produced in late summer (smaller, darker cells).
Primary growth: Lengthens the plant. Originates at apical meristems (tips of roots and shoots). Occurs in all vascular plants.
Secondary growth: Increases girth; produces wood and bark. Originates at lateral meristems (vascular cambium, cork cambium). Occurs in most gymnosperms and eudicot trees and shrubs. Does NOT occur in monocots — explains why palm trees, grasses, and corn do not produce wood rings.
| Modified Stem | Description | Example |
|---|---|---|
| Rhizome | Horizontal underground stem; allows vegetative spread | Ginger, many ferns, irises |
| Stolon (runner) | Horizontal above-ground stem; produces new plants at nodes | Strawberries |
| Tuber | Swollen underground stem storing nutrients; "eyes" are buds | Potatoes |
| Succulent stem | Thick, water-storing, photosynthetic; leaves reduced to spines | Cactus |
Leaves are the primary photosynthetic organs of most plants. Their structure reflects an evolutionary solution to a design challenge: how to maximize light capture and gas exchange while minimizing water loss.
A leaf is typically a flat, thin blade — a shape that maximizes surface area for capturing light while keeping diffusion distances for gases small. The outer surfaces of a leaf are covered by the epidermis, coated with a waxy cuticle that reduces evaporative water loss. The underside of most leaves (and sometimes both sides) is punctuated by stomata — microscopic pores that allow the exchange of carbon dioxide and oxygen with the atmosphere. Each stoma is flanked by two guard cells whose shape changes to open or close the pore.
The interior of the leaf is called the mesophyll. In many eudicot leaves, the palisade mesophyll near the upper surface consists of elongated, tightly packed cells densely loaded with chloroplasts — this is where most photosynthesis occurs. Below it, the spongy mesophyll contains cells with large air spaces between them, facilitating the movement of gases to and from photosynthetic cells. Vascular bundles — the veins visible in leaves — branch throughout the mesophyll, delivering water and minerals and collecting the sugars produced by photosynthesis.
Desert plants have thick cuticles, sunken stomata, and reduced leaf surfaces to minimize transpiration. CAM plants (cacti, agaves) open stomata only at night, fixing CO₂ when temperatures are lower and water loss is reduced. Conifer needles have a small surface area and thick cuticle — adaptations for cold, dry conditions where liquid water may be unavailable for months.
The vascular system of a plant is its internal plumbing — a continuous network of tubes running from root tips to leaf edges, connecting every part of the plant body. It moves two separate streams: water and minerals moving upward through xylem, and sugars moving in both directions through phloem.
Xylem conducts water and dissolved minerals from roots to the rest of the plant. Water movement through xylem is driven by transpiration-cohesion-tension:
Step 1 — Transpiration: Water evaporates from leaf cells and exits through stomata. This creates negative pressure (tension) in the xylem of the leaf.
Step 2 — Cohesion: Water molecules cling to one another (cohesion) and to xylem walls (adhesion), forming a continuous water column through the xylem tubes.
Step 3 — Tension: The tension created by transpiration at the leaf pulls the water column upward throughout the xylem — all the way from roots to leaf tips. No pump is needed.
Step 4 — Replacement: As water is pulled up, new water is drawn in at the roots by osmosis from surrounding soil.
This passive mechanism can move water more than 100 meters to the tops of the world's tallest trees — coast redwoods and mountain ash. On a warm summer day, a single large tree may transpire hundreds of liters. No energy input beyond the sun evaporating water from leaves is required.
At the root, water crossing the cortex must pass through the endodermis — a layer of cells with a waxy band called the Casparian strip in their cell walls. This band blocks water from moving between endodermal cells; water must pass through the interior of endodermal cells instead. This selective passage allows the plant to control which solutes enter the vascular system — functioning like a gateway.
Phloem transports dissolved sugars — primarily sucrose — produced by photosynthesis from wherever they are produced or stored (sources) to wherever they are consumed or stored (sinks). Growing shoot tips, roots, and developing fruits are sinks; mature leaves carrying out active photosynthesis are sources. Unlike xylem, phloem flow is bidirectional.
Phloem transport operates by pressure flow. At a source, sugar is actively loaded into phloem sieve tube cells, raising solute concentration and drawing in water by osmosis — creating high pressure. At a sink, sugar is unloaded, water follows by osmosis, and pressure drops. The pressure difference drives flow from source to sink through the interconnected phloem.
| Feature | Xylem | Phloem |
|---|---|---|
| What is transported | Water and dissolved minerals | Dissolved sugars (sucrose) and other organic molecules |
| Direction of flow | Upward only (roots to leaves) | Both directions (source to sink) |
| Conducting cells alive? | Dead at functional maturity | Alive (sieve tube elements supported by companion cells) |
| Driving force | Transpiration-cohesion-tension (passive) | Pressure flow from high sugar concentration to low (active loading at source) |
Plants can reproduce both sexually and asexually. Sexual reproduction involves the union of gametes and produces genetically variable offspring — important for adaptation and evolution. Asexual reproduction produces genetically identical offspring (clones) from a single parent — a rapid, reliable way to multiply when conditions are favorable.
Many plants reproduce vegetatively — producing new individuals without gametes. This occurs naturally through runners (strawberries), rhizomes (many grasses and ferns), bulbs (tulips and onions), and fragmentation. Humans exploit plant asexual reproduction extensively in agriculture through cuttings, grafting, and layering to produce genetically identical plants.
Sporophyte (diploid, 2n): The large, visible plant you see. Produces spores by meiosis.
Gametophyte (haploid, n): Produces gametes (eggs and sperm). In flowering plants, the gametophyte is microscopic and dependent on the sporophyte. Male gametophyte = pollen grain. Female gametophyte = small structure within the ovule.
Fertilization: Sperm from pollen fertilizes egg in ovule → diploid embryo (new sporophyte generation).
In flowering plants, reproduction centers on the flower. A typical flower includes sepals (protective outer structures), petals (often colorful, attracting pollinators), stamens (male reproductive structures bearing pollen-producing anthers), and carpels (female reproductive structures with stigma, style, and ovary containing ovules).
Pollination is the transfer of pollen from anther to stigma. A pollen grain that lands on a compatible stigma germinates, producing a pollen tube that grows down the style to the ovule. Two sperm travel through the pollen tube. One fertilizes the egg to produce a diploid embryo; the other fuses with other cells to produce the endosperm — tissue that will nourish the embryo. This process of double fertilization is unique to flowering plants.
Flower color, shape, fragrance, and timing often reflect the sensory abilities and behaviors of specific pollinators. Red tubular flowers attract hummingbirds (which see red well but lack a strong sense of smell). Pale, heavily scented flowers attract night-flying moths. Landing platforms and guide markings direct bees to nectar and pollen. These are the results of millions of years of coevolution — mutual adaptation between flowers and their pollinators.
A seed contains three key components: the embryo (the developing sporophyte), endosperm (nutrient tissue), and a seed coat (protective outer layer derived from the ovule wall). Seeds protect the embryo, provide it with stored nutrients for germination, and allow it to survive unfavorable conditions — sometimes for years.
Germination — the resumption of embryo growth — is typically triggered by water absorption, appropriate temperature, and sometimes light or a period of cold (stratification). As the embryo emerges, it draws on the endosperm for nutrients until its leaves begin photosynthesis.
A fruit is a mature ovary, often with associated structures. Fruits develop from the ovary wall after fertilization and typically contain seeds. Their structure reflects how their seeds travel.
| Fruit Type | Dispersal Mechanism | Examples |
|---|---|---|
| Fleshy fruit | Animals eat fruit; seeds deposited elsewhere in droppings | Berries (tomato, grape), drupes (peach, cherry), pomes (apple) |
| Winged/feathery dry fruit | Wind carries lightweight structures | Maple samaras, dandelion achenes with pappus |
| Hooked/barbed dry fruit | Catches on fur or clothing | Burdock, cocklebur |
| Explosive dehiscent fruit | Pod dries and snaps open, flinging seeds | Witch hazel, squirting cucumber |
| Buoyant fruit | Floats on water to new locations | Coconut, water lily |
The distinction between fruits and vegetables is culinary, not botanical. Botanically, a fruit is any mature ovary containing seeds. By this definition, tomatoes, cucumbers, peppers, squash, beans, and corn kernels are all fruits — they develop from fertilized ovaries. Roots (carrots, beets), stems (celery), and leaves (spinach, lettuce) are the structures a biologist would classify as non-fruit plant organs.
Plants cannot move from place to place, but they are far from passive. They respond to light, gravity, touch, water availability, temperature, day length, and damage. Without neurons or a nervous system, plants coordinate these responses through chemical signals — hormones — and through electrical signals.
Plant hormones are organic molecules produced in one part of the plant that travel to target tissues and alter their growth or activity. The same hormone can have different effects in different tissues or at different concentrations.
| Hormone | Produced In | Key Effects |
|---|---|---|
| Auxin (IAA) | Shoot tips | Promotes cell elongation; drives phototropism; controls apical dominance |
| Cytokinins | Roots | Promote cell division; delay leaf aging; interact with auxin to regulate root/shoot balance |
| Gibberellins | Young leaves, seeds, roots | Promote stem elongation, seed germination, and flowering |
| Abscisic acid (ABA) | Various tissues | Stress signal; closes stomata during drought; promotes seed dormancy |
| Ethylene | Ripening fruit, aging tissues | Promotes fruit ripening, leaf senescence, and response to mechanical stress; gaseous |
| Tropism | Stimulus | Typical Response | Example |
|---|---|---|---|
| Phototropism | Light | Shoots grow toward light; roots grow away | Sunflower, houseplants bending toward windows |
| Gravitropism | Gravity | Roots grow downward; shoots grow upward | Seedling emerging sideways corrects orientation |
| Thigmotropism | Touch / mechanical contact | Stems or tendrils wrap around supports | Peas, grapes, morning glories climbing |
Many plants flower in response to the length of the dark period each night — a response called photoperiodism. Short-day plants (more accurately long-night plants) flower when nights exceed a critical length — they bloom in late summer or fall. Chrysanthemums, poinsettias, and soybeans are short-day plants. Long-day plants flower when nights fall below a critical length — they bloom in spring or early summer. Spinach, lettuce, and many grain crops are long-day plants. Day-neutral plants flower regardless of day length.
Florists and growers use controlled lighting to trigger or prevent flowering out of season. Keeping chrysanthemums under extended artificial darkness during summer induces them to flower in time for winter holidays. Briefly interrupting long dark periods with flashes of light prevents "short-day" plants from flowering prematurely — a technique used in commercial greenhouses worldwide.
In most terrestrial ecosystems, plants are the primary producers — the foundation of the food web. Through photosynthesis, they convert solar energy into chemical energy stored in organic molecules. Every heterotroph that eats plants, and every predator that eats herbivores, ultimately depends on this energy. Plants also fix carbon from atmospheric carbon dioxide into organic matter and release the oxygen that aerobic organisms require.
Plants are central participants in nutrient cycles. They absorb carbon dioxide from the atmosphere and incorporate carbon into their tissues. When they die and decompose, that carbon returns to the atmosphere or soil. Their roots stabilize soil, reducing erosion and keeping nutrients in place. Forests, grasslands, and wetlands — all dominated by plants — are major reservoirs of carbon. The clearing and burning of forests releases stored carbon, contributing to atmospheric carbon dioxide accumulation.
No other organism group has shaped human civilization as fundamentally as plants. Human settlements became possible when people began cultivating plants — grains (wheat, rice, corn), legumes (beans, lentils, soybeans), roots and tubers (potatoes, cassava, yams), and fruits and vegetables. Agriculture made food surpluses possible, which enabled cities, specialization of labor, and the development of complex societies.
| Plant Contribution | Examples | Significance |
|---|---|---|
| Food | Wheat, rice, corn, soybeans, potatoes, fruits, vegetables | Foundation of human nutrition and civilization |
| Fiber | Cotton, linen, hemp | Clothing, textiles, ropes, paper |
| Timber | Oak, pine, mahogany | Construction, furniture, fuel |
| Medicine | Aspirin (willow), morphine (poppy), taxol (Pacific yew), quinine (cinchona) | Treatment of pain, infection, cancer, malaria |
| Oxygen | All photosynthetic plants | Essentially all atmospheric O₂ is produced by photosynthesis |
| Habitat structure | Forests, grasslands, wetlands | Physical environment and food supply for most terrestrial species |
Plants do not merely participate in ecosystems — they largely create them. The physical structure of a forest, grassland, or wetland is defined by its plant community. These ecosystems provide what ecologists call ecosystem services: flood prevention, water filtration, climate regulation, soil building, pollination, and carbon storage. In Chapter 27 you will study how populations and communities are organized, and in Chapter 28 you will examine energy flow and nutrient cycles — both of which have plants at their foundation.
Correction: Plants perform cellular respiration continuously, consuming oxygen and releasing carbon dioxide just as animals do. During daylight, the rate of photosynthesis in green cells exceeds respiration, so the net exchange is CO₂ uptake and O₂ release. But at night — or in non-photosynthetic tissues such as roots — plants consume oxygen and release carbon dioxide.
Correction: Plants make their own food through photosynthesis. Roots absorb water and dissolved minerals — not organic food molecules. The sugars that fuel plant growth are produced in leaves and distributed through the plant via phloem. This is why plants are called producers: they produce their own organic matter.
Correction: Flowers are reproductive structures. Their colors, shapes, fragrances, and structures are adaptations that attract specific pollinators and facilitate pollen transfer. Their visual appeal to humans is incidental to their biological function — a happy coincidence of pollinator and human sensory preferences.
Correction: The rate of photosynthesis depends on light, water, carbon dioxide, temperature, and the amount of photosynthetic tissue — not simply on leaf size. Desert plants with small or reduced leaves minimize water loss at the cost of photosynthetic surface area, which is adaptive where water rather than light is the limiting resource.
Correction: "Short-day plants" actually respond to the length of the uninterrupted dark period each night, not to day length itself. A long-night plant would be a more accurate name. A single brief flash of light in the middle of an otherwise long night can prevent flowering in these plants — demonstrating that it is darkness, not day length, that matters.
Each problem shows a student's claim or reasoning. Identify the error and write a correct explanation.
What is wrong with this explanation? What mechanism actually drives water movement through xylem?
Is this botanically correct? What is the botanical definition of a fruit, and how does the tomato fit it?
Identify the error. In what types of plant cells does photosynthesis actually occur, and what structure is required?
What is wrong? Which of these plants can produce wood, and why can the other not?
Correct this claim. What determines the direction of phloem flow, and give an example of phloem moving upward.
Chapter 21 examined plants — stationary, photosynthetic organisms that form the energy foundation of most terrestrial ecosystems. Chapter 22 turns to animals — the most behaviorally complex and motile group of organisms on Earth. Animals face different challenges: they must find and consume food rather than produce it, navigate their environments, find mates, and respond rapidly to threats. Chapter 22 will explore how animal body plans, tissues, organs, and systems solve these challenges, survey the diversity of major animal groups from simple invertebrates to complex vertebrates, and examine how animals interact with ecosystems and with human life.