It’s not easy being green: Immobility

non-motile

It’s harder than you might think to be a plant. They face the same obstacles as other organisms; acquiring nutrients, reproducing, responding to the environment, and avoiding being eaten. However, plants must respond to these trials somewhat differently than animals because plants are immobile.

The basic nutrients plants need (carbon dioxide, water, and sunlight) are so ubiquitous, they can acquire what they need to thrive in most environments. Plants don’t have to move in the same way that animals do. However, plants are capable of motion, just not on a scale noticeable by impatient humans. Plants grow and respond to their environment with much more subtle motions. They position their leaves for optimal harvesting of sunlight then close them at night when the sun is gone and there is no need to waste the energy holding them outward. Vines can wrap themselves around a support to give them strength as they grow. Check out this great site with time-lapse videos that bring plants’ motions up to a speed humans can appreciate.

Immobility also has consequences for the basic biological tenet of reproduction, but plants have quite a few reproductive options that many animals don’t have. They have a developmental flexibility that allows for the propagation of plants by regeneration from smaller plant parts like leaf cuttings and root cuttings. Plants also have no problems with asexual reproduction. No need to worry about finding a mate when you can do it all yourself. What about the issues of genetic robustness and diversity that can only come from sexual reproduction mixing up the genes of a population? Plants have that covered too, but they’ve developed strategies other than mating dances. Some plants rely on the wind for pollination, allowing for the shuffling of genetic material between plants miles apart. Other plants rely on insect and animal pollinators to do their moving for them. These options may not be the most romantic, but they are effective. Plants also have a wide array of seed dispersal strategies to make sure that their offspring do not become competitors for nutrients.

When the going gets tough, plants can’t get going. When environmental conditions change, plants must tolerate these changes somehow. Plants cannot migrate with the seasons like many animals.  They either have elaborate systems for enduring extreme weather conditions or complete their lifecycles on a timescale compatible with permissive conditions. When micronutrients are limiting, plants can’t pack up and move to another location to forage elsewhere. They must judiciously use the resources available to them or rely on another trick. Plants have the capability of indeterminate growth, that is, they continue to grow larger over the course of their lifetimes, allowing their root systems to explore new territory. Plants must deal with pathogens, as well as insects and animals that want to eat them. When it comes to flight or fight, for plants flight isn’t an option. They choose to fight, and their weaponry includes sharp thorns and toxic poisons.

Obviously plants are doing quite well for themselves despite the challenges of being immobile. The secret is that they only appear to be stoic to our human perception. On the outside, they seem to passively endure the afflictions nature throws at them, but on the inside they are a fury of activity. Plants are consummate biochemists. They create chemicals to ward off their predators and others to recruit beneficial symbionts. They compose an assortment of flavor compounds and colors to entice the pollinators they need and persuade humans to be their caretakers*. Their biochemistry is also essential to the creation of the various forms plants employ to physically address their difficulties, as in thorns to protect against herbivory or helicopter-shaped seeds for dispersal by the wind.

So, the next time you see a plant, stop to appreciate all they are actually doing. The thistle’s sharp prickles should make you think twice before partaking in its edible stalk. The scent of honeysuckle on a summer’s evening is the perfectly-timed biochemical lure for its preferred pollinator, a nocturnal moth. The pecan tree, so bare and forlorn for all of winter and early spring, is cautiously consuming its reserves and patiently counting the days until warm weather is here to stay. The feathery dandelion achenes are flying away for optimal dispersal across your waiting lawn. From this perspective, it seems like independent movement is the easy way out.

Johnna

*The idea of plants manipulating humans was most eloquently stated and elaborated by Michael Pollan in his book The Botany of Desire: A Plant’s Eye-View of the World. It’s worth reading for a different perspective on the relationship between plants and humans; who is really manipulating whom?

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