Many plants produce sticky or hooked hairs (technically called trichomes) that are useful in a dazzling array of ways. Some ways are quite straightforward, and others present more complex stories.
Some plants use these hairs for feeding, capturing insects that are digested, leaving shriveled husks of their bodies to fall off, eventually. Think of our two species of sundews, or the butterwort, living in damp, nutrient-poor habitats, where they supplement their ‘income’ with the juices of hapless bugs. Research has shown that the supplemental income, garnered by sticky hairs on the leaves, is circulated through the plant. Well-fed plants are more likely to produce flowers and fruit and may also store nutrients in the roots for next year.
Other plants produce seeds or fruits with sticky hairs that glom onto fur or feathers or socks, getting transported to new sites for seed germination and the growth of young plants. The weed called tick trefoil or beggar’s lice (Desmodium) is a good example. The hairy fruit is rather like a long, thin pea pod, containing several seeds. When the fruit is mature, it easily breaks up into sections, each of which holds one seed. When an animal brushes past, the hairy segments of the fruit stick, and so the seed is dispersed.
Hops plants (Humulus) are often grown ornamentally. They grow as climbing, clinging vines. Every plant is covered thickly with hairs, including the tendrils that coil around supporting structures. The dense covering of hairs on the tendrils, as well as the rest of the plant, enables the plant to hang on to trellises–or other plants—as it grows, sometimes extending its length many feet upward.
A research project demonstrated that hairs act as sensors on tomato plants. They detect a caterpillar or other insect delicately walking over the plant and send a signal to the plant that danger from herbivorous insects is at hand. The plant thus is induced to produce more defensive chemicals to deter the possible herbivore. Some alders are also reported to have induced defenses, producing more hairs on later growth after an earlier attack by herbivores.
Many species use sticky or spiky hairs as a defense mechanism, deterring or poisoning herbivorous insects by chemicals in the sticky resin or gumming down small herbivores that would damage the plant (an example might be the hairs on Cannabis buds). There is a plant sometimes called blazing star (Mentzelia) that grows in the American Southwest on which the hair story has extra layers. The plant is covered with barbed hairs that trap and hold many insects. The trapping hairs work so well that researchers found it hard to imagine that any insects could penetrate the defense, but it turned out that there is a kind of aphid that seems to specialize on Mentzelia. The aphid walks gingerly among the spiky hairs as it feeds on the plant, and as it does so, the aphid is actually protected from predatory ladybird beetles that generally (but not always) get caught by the hairs.
Not long ago, a friend gave me a reference that provides yet another wrinkle on hairy methods of defense. There is a species of columbine (Aquilegia) in California that is very sticky (but our local species is not). Lots of insects get caught and die there, including both herbivores and some that just happen to land there without damaging intent. The dead bodies attract carrion-feeding and predatory insects that apparently do not get caught but attack other visiting herbivorous insects and so reduce damage to the plant. Thus the plants provision their helpful mutualists with carrion bait.
Some years ago, a friend and I wondered about the sticky stems of Tofieldia glutinosa (a.k.a. sticky false asphodel), which grows in some muskegs around here. Many tiny insects get caught on the stems. We did not detect that the plant is insectivorous, but our experiments were not completed. Could the sticky hairs defend the plant from crawling herbivores? Or provide carrion bait for helpful predators that would attack would-be herbivores? This is a project waiting for an interested student!
I am willing to bet that this is not a complete list of the possible uses of sticky hairs by plants!