Unit 12
Plants.
How a tree lifts water, how a seedling finds the light and tells up from down, and how a flower becomes a fruit.
Before this unit
The unit, lesson by lesson
Lessons
5 lessons · 27 practice questions · about 2 hours
Water through a tree
A tree lifts tens of litres of water an hour from the soil to its leaves, and almost all of it leaves as vapour: through the stomata, which the plant opens in the light and closes in the dark, pulled from above rather than pushed from below.
What speeds up transpiration
Dry air, wind and sunshine speed up transpiration, and moist air slows it, even when the leaf warms up. What drives the flow is the difference between the saturated air inside the leaf and the air outside, and when the tree closes its stomata to keep from drying out.
Growing towards the light
A plant grows only at its tips, and turns to the light when the shaded side of its shoot grows faster: auxin flows to it from the tip. The shoot stops between the lamp and upright, where the pull of the light and gravity balance.
Which way is down
A seedling laid on its side lifts its shoot and turns its root down within hours: heavy grains of starch in the root cap show it where down is, and the same auxin speeds up the shoot and slows the root. The bend stays in the stem, even once the tip is straight.
From flower to fruit
The parts of a flower, pollination, the race of the pollen tubes down the style, fertilisation, and how the number of seeds decides whether there is a fruit and how big it grows. And why many flowers reject their own pollen.