Unit 13: Plants
How a plant standing still solves every problem an animal solves by moving: feeding, breathing, and reproducing.
6 topics · 18 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Plant Structure: Roots, Stems, and Leaves
- How it worksHow do roots anchor a plant and absorb water while stems transport it upward to the leaves?
- Common mistakeWhy doesn't a plant's root system exist mainly to anchor it rather than to absorb nutrients?
- Ties togetherHow does a leaf's flat, thin shape maximize the surface area it exposes to sunlight for photosynthesis?
Your turn1 of 2
A plant is rooted in one fixed spot for its entire life, unable to move toward better resources the way an animal can. Why does a plant need three completely different organs, roots, stems, and leaves, just to survive in one fixed spot?How Water Moves Up a Plant
- How it worksHow do cohesion and adhesion let water form an unbroken column being pulled up through narrow xylem tubes?
- How it worksHow does water evaporating from leaf pores pull the entire column of water upward behind it?
- Common mistakeWhy isn't root pressure alone strong enough to push water all the way to the top of a tall tree?
Your turn1 of 2
Transpiration pulls water upward through a tree's xylem at a steady rate of 5.72 meters per hour. About how many hours would it take for water to travel from the roots to a leaf 60 meters up the trunk?Gas Exchange in Leaves
- How it worksHow do guard cells open and close a stoma to control gas exchange and water loss?
- Going furtherWhat happens to a stoma's opening on a hot, dry day when the plant needs to conserve water?
- Common mistakeWhy doesn't a plant only exchange gases during the day and stop completely at night?
Your turn1 of 2
A leaf's surface is covered with thousands of microscopic pores called stomata rather than one large opening for gas exchange. Why does a leaf need thousands of tiny pores instead of one big opening?Plant Reproduction: Flowers, Pollination, and Seeds
- How it worksHow does pollen traveling from a flower's anther to another flower's stigma lead to fertilization?
- How it worksHow does a fertilized ovule develop into a seed while the surrounding ovary develops into a fruit?
- Common mistakeWhy isn't a fruit simply a food the plant makes for animals with no purpose of its own?
Your turn1 of 2
A wind-pollinated grass releases enormous quantities of pollen into the air with no need to attract any animal at all, while many flowering plants invest heavily in bright petals and sweet nectar instead. Why does a flower need to attract insects instead of just releasing pollen into the wind?Seed Germination and Plant Growth
- How it worksHow does a seed use its stored food reserves to grow before it can photosynthesize on its own?
- Worked exampleFind which of three seeds germinates first at temperatures of 5, 25, and 40 degrees Celsius.
- Common mistakeWhy doesn't every seed need direct sunlight just to begin germinating?
Your turn1 of 2
A seed buried in dry desert soil can remain dormant for years, seemingly lifeless, then sprout within days of a single heavy rainfall. Why can a seed sit dormant for years and then suddenly sprout after one good rain?Plant Responses: Tropisms and Hormones
- How it worksHow does the hormone auxin build up on a stem's shaded side to bend it toward light?
- Going furtherWhat happens to a plant's growth direction if you rotate the pot so the light source switches sides?
- Common mistakeWhy doesn't a plant's roots bending downward mean roots are somehow choosing to avoid light?
Your turn1 of 2
A houseplant sitting on a windowsill gradually bends its stem toward the window rather than continuing to grow straight upward. Why does a houseplant bend its stem toward the light instead of growing straight up?