Unit 14 explains how a plant is built, how it moves water and sugar without a heart, how it reproduces, and how it senses its surroundings without a nervous system. By the end, you should be able to connect a plant structure to its job, trace water and sugar through the plant, follow a plant life cycle, and predict how a plant will respond to light, gravity and day length.
Read the overview first, then work through each topic note and answer its checkpoint before you check the answer.
Plants solve the same problems animals do. They have to get materials in, move them around, reproduce and react to changes. Most stay in one place, though some float or drift. Many grow toward resources instead of moving their whole bodies.
Honors Biology Unit 14 covers plant structure, function and reproduction. You learn the tissue types (meristematic, dermal, ground, vascular) and the organs they build (roots, stems, leaves, flowers). You learn how xylem moves water by cohesion-tension and how phloem moves sugar from sources to sinks. You follow alternation of generations, pollination, double fertilization and seed and fruit formation. You also study plant responses to stimuli, including tropisms, hormones like auxin, and photoperiodism involving the light receptor phytochrome. Exact content and order vary by school.
Root hairs add huge surface area for absorbing water. Thin, flat leaves catch light, and guard cells open and close stomata to balance gas exchange against water loss. When you see a structure, ask what problem it solves.
Xylem pulls water up from the roots, powered by evaporation from the leaves, so the movement itself costs the plant no ATP. Phloem moves sugar from where it's made or stored to where it's used. Some plants use ATP to load sugar into phloem, while others load it passively. Phloem flow can go up or down depending on where the sinks are.
The sporophyte (2n) makes spores, and the gametophyte (n) makes gametes. Over plant evolution the gametophyte shrank until, in flowering plants, it's just a pollen grain or a few cells inside an ovule. Seeds and fruits then protect and spread the next generation.
A plant can't move toward water or away from shade, so it grows toward resources and controls what crosses its surfaces. Its tissues handle support, transport and exchange, and its hormones and light receptors decide when and where to grow, flower or go dormant. Every topic in this unit is one piece of that strategy.
A plant body is built from three tissue systems.
Xylem and phloem are separate pipelines with different power sources.
Plants alternate between two multicellular generations.
Plants respond to their surroundings by growing unevenly or by switching processes on and off.
A plant body is built from three tissue systems. Dermal tissue covers the outside, vascular tissue moves materials, and ground tissue fills the rest and does most of the photosynthesis and storage. Meristems are regions of dividing, unspecialized cells that produce cells for plant growth. Some differentiated cells can also resume division, such as during wound repair.
Apical meristems at root and shoot tips cause primary growth, which makes the plant longer. Lateral meristems (the vascular cambium and cork cambium) cause secondary growth, which makes woody stems and roots wider. In many trees, the vascular cambium makes a visible layer of secondary xylem each year. Some conditions can produce an extra ring or no visible ring in a year.
Ground tissue has three cell types. Parenchyma has thin walls and handles photosynthesis and storage. Collenchyma has unevenly thickened walls that give flexible support, like the strings in celery. Sclerenchyma has thick walls hardened with lignin and is usually dead at maturity, like the gritty bits in a pear.
The organs follow the same logic. Roots anchor the plant and absorb water through root hairs. Stems hold leaves up to the light and carry vascular bundles, which sit in a ring in eudicots and are scattered in monocots. Leaves are thin and flat to catch light, and a waxy cuticle limits water loss. Flowers are the reproductive organs of angiosperms.
Check yourself: A nail is hammered into a tree trunk 1 meter above the ground. Ten years later, how high is it? (Answer: Still about 1 meter. A trunk only gets longer at its apical meristems at the tips. Lower down, the vascular cambium makes the trunk wider but not taller.)
Xylem and phloem are separate pipelines with different power sources. Xylem carries water and minerals up from the roots. Phloem carries sugar from sources to sinks.
The cohesion-tension model explains xylem flow. Water evaporates from leaf cells and exits through stomata (transpiration), which creates tension that pulls on the water in the xylem. Hydrogen bonds hold water molecules to each other (cohesion) and to the xylem walls (adhesion), so the whole column rises as one chain. The plant spends no ATP moving the water. Root pressure adds a small push at night, but it can't lift water to the top of a tall tree.
The pressure-flow model explains phloem. At a source like a mature leaf, sucrose enters sieve tubes. In some plants, companion cells use ATP to help load it; in others, loading is passive. Water follows by osmosis, pressure builds, and sap flows toward a sink like a root or fruit, where sugar is unloaded. A potato tuber is a sink in summer and a source in spring.
Worked example. A leafy shoot in a potometer takes up 1.8 mL of water in 30 minutes in still air. The rate is 1.8 mL ÷ 30 min = 0.06 mL/min. With a fan on, it takes up 3.0 mL in 30 minutes, which is 0.10 mL/min. The percent increase is (0.10 - 0.06) ÷ 0.06 × 100 = 67%. Moving air sweeps away humid air near the leaf, so water evaporates faster.
Check yourself: On a hot, dry, windy afternoon a plant closes its stomata. What happens to water movement in its xylem, and why? (Answer: It slows down sharply. Closed stomata cut transpiration, so there is much less tension pulling water up the xylem.)
| Feature | Xylem | Phloem |
|---|---|---|
| What it carries | Water and dissolved minerals | Sugars (mainly sucrose) and other organic compounds |
| Direction | One way, roots to leaves | Source to sink, up or down |
| Conducting cells | Tracheids and vessel elements | Sieve tube elements with companion cells in flowering plants; sieve cells in ferns and gymnosperms |
| Alive at maturity? | No, the cells are hollow and dead | Yes, though mature conducting cells lack a nucleus |
| Driving force | Transpiration pull (cohesion-tension) | Pressure differences produced by sugar loading and unloading (pressure-flow) |
| Uses ATP? | Not for the flow itself | Sometimes, for active sugar loading or unloading |
Plants alternate between two multicellular generations. The diploid (2n) sporophyte makes haploid spores by meiosis. Each spore grows into a haploid (n) gametophyte, which makes eggs and sperm by mitosis. Sperm plus egg makes a 2n zygote that grows into a new sporophyte. In mosses the gametophyte is the green plant you see. In ferns, conifers and flowering plants the sporophyte dominates and the gametophyte is tiny. In angiosperms the pollen grain is the male gametophyte, and the embryo sac inside the ovule is the female gametophyte.
In a flower, the stamen (anther and filament) makes pollen, and the carpel (stigma, style and ovary) holds the ovules. Pollination is pollen landing on a stigma. Fertilization comes later, after a pollen tube grows down the style and delivers two sperm. One sperm fuses with the egg to make the zygote. The other fuses with two polar nuclei to make triploid endosperm, a food supply for the embryo. This is double fertilization. The ovule becomes the seed, and the ovary becomes the fruit.
Worked example. Corn has 2n = 20. A sperm and an egg each carry n = 10 chromosomes, so the zygote has 10 + 10 = 20. The endosperm gets one sperm (10) plus two polar nuclei (10 + 10), for 30 chromosomes, which is 3n.
Many plants also clone themselves through runners, tubers or cuttings. The offspring are usually genetically identical to the parent, though mutations can introduce variation.
Check yourself: A pea pod holds 7 peas. What is the smallest number of pollen grains that had to land on the flower's stigma, and why? (Answer: 7. Each pea is a fertilized ovule, and each ovule needs its own pollen tube to deliver sperm, so at least 7 pollen grains had to germinate.)
| Feature | Sporophyte | Gametophyte |
|---|---|---|
| Chromosome number | Diploid (2n) | Haploid (n) |
| What it makes | Spores, by meiosis | Eggs and sperm, by mitosis |
| Dominant in | Ferns, gymnosperms, angiosperms | Mosses and other nonvascular plants |
| Example in a flowering plant | The whole visible plant | Pollen grain (male) and embryo sac (female) |
Plants respond to their surroundings by growing unevenly or by switching processes on and off. A tropism is growth toward (positive) or away from (negative) a directional stimulus. Shoots show positive phototropism and negative gravitropism. Roots show positive gravitropism. Vine tendrils coil around a support through thigmotropism, a response to touch.
The hormone auxin drives most tropisms. When light hits one side of a shoot, auxin moves to the shaded side, those cells elongate more, and the shoot bends toward the light. Roots respond the opposite way to the same hormone. In a root lying sideways, starch-filled statoliths settle and auxin builds up on the lower side, where it slows elongation, so the root curves down. Other hormones have other jobs. Gibberellins promote stem growth and germination, cytokinins promote cell division, ethylene ripens fruit, and abscisic acid closes stomata during drought.
Photoperiodism lets a plant time flowering to the season, and what it really measures is uninterrupted darkness. The pigment phytochrome switches between two forms, Pr and Pfr, as red and far-red light hit it, which lets the plant sense light.
Worked example. A short-day plant has a critical night length of 11 hours. On 14 h light and 10 h dark, the night is shorter than 11 h, so it doesn't flower. On 9 h light and 15 h dark, the night is longer than 11 h, so it flowers. If a flash of red light breaks that 15 h night at hour 7, the longest dark stretch is 8 h, so it doesn't flower.
Check yourself: A long-day plant with a critical night length of 10 hours is grown on 10 h light and 14 h dark, with a brief flash of red light in the middle of the night. Will it flower? (Answer: Yes. The flash splits the night into two 7-hour stretches, and 7 hours is shorter than the 10-hour critical night, so the plant responds as if it were getting long days.)
| Term | Definition |
|---|---|
| Meristem | A region of dividing, unspecialized plant cells, found at root and shoot tips (apical) or along stems and roots (lateral). |
| Xylem | Vascular tissue that carries water and minerals, mainly from roots to leaves. Its water-conducting cells are dead at maturity. |
| Phloem | Vascular tissue that carries sugars from sources to sinks. Its living conducting cells include sieve tube elements in flowering plants and sieve cells in ferns and gymnosperms. |
| Stomata | Pores in the leaf epidermis, controlled by guard cells, that let carbon dioxide in and water vapor and oxygen out. |
| Transpiration | The evaporation of water from plant leaves, which creates the pull that moves water up the xylem. |
| Alternation of generations | A life cycle that alternates between a diploid sporophyte that makes spores and a haploid gametophyte that makes gametes. |
| Double fertilization | The angiosperm process that forms both a diploid zygote and triploid endosperm from two sperm cells. |
| Endosperm | Triploid tissue in a seed that stores food for the developing embryo. |
| Auxin | A plant hormone that controls cell elongation and directs bending in phototropism and gravitropism. |
| Photoperiodism | A plant's response to day and night length. For many flowering responses, the length of uninterrupted darkness is especially important. |
Leaves are usually high on the plant, so it's easy to picture sugar sliding downward. Phloem actually moves sugar from any source to any sink, so it can flow up to a growing shoot tip or a developing fruit. Find the source and the sink first, then decide the direction.
Pollination is pollen landing on the stigma. Fertilization happens later, after the pollen tube grows down the style and the sperm reach the ovule. A flower can be pollinated and still never be fertilized if the pollen tube doesn't grow.
The names come from older research, but plants actually track how long the night lasts without interruption. A short-day plant flowers when nights are longer than its critical length. That's why a flash of red light in the middle of the night can stop it from flowering.
In shoots, extra auxin speeds up elongation, so the side with more auxin grows longer. In roots, the same buildup slows elongation, which is why a root curves toward the side where auxin collects. Always check whether the question is about a root or a shoot.