Plant reproduction and responses
Plants cannot move to find mates or escape danger, so they have evolved clever ways to reproduce and respond to their surroundings.
Flowers
Flowers are reproductive organs. Stamens are the male parts: anthers produce pollen on filaments. Carpels are the female parts: a sticky stigma catches pollen, a style leads down, and the ovary holds ovules. Petals often attract pollinators with colour and scent.
Pollination
Pollination transfers pollen from anther to stigma. Insects, birds, and bats carry pollen while seeking nectar; wind carries pollen of grasses and many trees, which is why their flowers are small and plain. About three-quarters of major food crops depend at least partly on animal pollinators.
Fertilisation, seeds, and fruit
A pollen tube grows down to the ovule, delivering sperm. After fertilisation, the ovule becomes a seed containing an embryo and stored food, and the ovary becomes the fruit. Fruits help disperse seeds: eaten by animals, carried by wind like dandelions, or hooked onto fur like burrs.
Germination
Seeds stay dormant until conditions are right: water, oxygen, and a suitable temperature. During germination, the seed absorbs water, enzymes break down stored food, and the root emerges first, then the shoot. Some seeds need fire, cold, or passing through an animal's gut before they will sprout.
Asexual reproduction
Many plants also reproduce asexually: strawberries send out runners, potatoes grow from tubers, and many plants grow from cuttings. Offspring are clones, identical to the parent. This is fast and keeps successful traits but produces no genetic variety to cope with change.
Plant hormones
Auxin promotes cell elongation and causes bending toward light. Gibberellins stimulate stem growth and seed germination. Ethylene, a gas, triggers fruit ripening, which is why one ripe banana speeds up ripening of others nearby. Abscisic acid closes stomata during drought and keeps seeds dormant.
Tropisms
Tropisms are growth responses toward or away from stimuli. Phototropism bends shoots toward light, as auxin collects on the shaded side and makes those cells grow longer. Gravitropism makes roots grow down and shoots grow up. Thigmotropism makes vines coil around supports they touch.
Life cycles and alternation of generations
Plant life cycles alternate between a multicellular diploid stage (sporophyte) and a haploid stage (gametophyte). In mosses the gametophyte is the main green plant; in ferns and flowering plants the sporophyte dominates, and in flowering plants the gametophytes are reduced to pollen grains and tiny structures in the ovule.
Key takeaways
Flowers hold stamens and carpels. Pollination by animals or wind is followed by fertilisation; ovules become seeds and ovaries become fruits that disperse them. Seeds germinate when conditions suit. Asexual reproduction makes clones. Hormones such as auxin and ethylene control growth and ripening, and tropisms orient growth toward or away from stimuli.
Flashcard
Male part: anther and filament
Check your understanding
A houseplant on a windowsill bends toward the window. This happens because: