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On the Trails: Plant behavior

Posted: July 19, 2013 - 12:02am
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A long-leaf sundew uses several sticky traps to hold a damselfly.   Photo courtesy of Bob Armstrong
Photo courtesy of Bob Armstrong
A long-leaf sundew uses several sticky traps to hold a damselfly.

Long ago and far away, when I was a kid, I took plants for granted. They stood there, being green, and that was that. Animals were way more interesting; they ran or climbed or flew, they barked or snorted or sang, they did things.

Considerably (embarrassingly) later, I began to understand that plants and animals lead parallel lives in many ways. I’m not referring here to the obvious facts that both plants and animals grow, reproduce and die. I’m thinking in terms of behavior — whole books and thousands of scholarly papers have been written about animal behavior, but plant behavior has been largely neglected, except by Darwin and a few others.

Plant behavior? Well, why not!

Consider the following parallel: Each species of animal has particular behaviors involved in competition for mates, courtship and mate choice. Depending on the animal species, maybe the males dance up and down, or chirp or sing, or wave their antennae or legs in a certain way. Our male humming birds whir in great swinging loops, male goldeneye ducks [filtered word] their heads far back and throw out their chests, male sooty grouse hoot from conifer trees, and so on. The visible and audible displays of animals are all regulated by hormones. Plants cannot dance or sing, or fly, or show off fancy plumage, but their behavior is regulated by hormones, just as it is in animals.

Plants certainly can and do compete for mates. All those varied and lovely flowers are a plant’s way of being attractive to visiting animals that will pick up pollen and carry it to another flower of the same kind. The equivalent of courtship takes place subtly in plants; it’s conducted largely by chemistry between a pollen grain and the female parts of the flower that it landed on. The plant receiving the pollen can even exert some choice of which pollen grains are acceptable, depending on the chemistry of the interaction. The details of many of the male and female interactions are still being worked out, but there is no doubt that they occur.

Plants are generally stationary; once they get established in a spot, they typically stay there until they die. Most animals, in contrast, can leave a place if it becomes unsatisfactory. However, even though the plants themselves can’t move (except in Hobbitland and similar countries), parts of plants can move in self-generated, hormonally regulated ways. Thus, we can observe in some orchids, for example, that once a pollinator has removed pollen, certain portions of the flower move to expose the receptive female surfaces, ready to receive pollen from the next visitor. In a number of flowering plants, if no pollen has been received from another flower, the male parts of the flower will bend until the flower’s own pollen is deposited on the female surfaces, effecting self-pollination. The little woodland plant called “shy maiden” has one flower, which demurely faces down, until it is pollinated and the fruit starts to ripen; then the flower turns its face upward and becomes a “brazen hussy.”

Some plants can signal to their potential pollinators by color changes in the flower. A good example from the local flora is lupine, which is pollinated by bees. The simple version of the story goes like this: The upper (“banner”) petal of each flower is usually white. But when the flower has been pollinated, that petal turns pink or purple. This tells other bees that the flower has been visited and its nectar removed, and therefore it would be more profitable for those bees to visit other flowers, ones that still have white banner petals.

The leaves of some plants “go to sleep” at night, folding up and reducing water loss; others fold up when touched, perhaps moving away from an inquisitive herbivore’s mouth. Tendrils of climbing plants twist and curl until they touch a support, and then they twine around that structure. When an insect contacts a sticky hair on sundew plant leaves, the sticky hair bends; this draws the insect into contact with more sticky hairs, so the insect is more securely trapped, and the sundew is ensured a good meal. Darwin, a very close observer, wrote a book on plant movements.

Plants can even communicate with each other in some cases. Some do it through root grafts, a physical connection among individuals, even those of different species, passing nutrients and hormones back and forth. More surprising, perhaps, is that when certain kinds of plants are attacked by herbivores, they send out aerial chemical signals to which nearby plants can respond by ramping up their own anti-herbivore defenses. And, of course, each plant communicates regularly with itself (just as animals do), using hormones to control growth, reproduction, wound-healing and other activities.

So plant behavior is a real thing. It’s much less obvious than much (but not all!) of animal behavior and therefore commonly requires very close observation. But don’t give it short shrift, just because it may be hard to see!

• Mary F. Willson is a retired professor of ecology.

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