Most human thinking about hierarchy of nature goes in order from humans to mammals to vertebrates to other animals, and then to plants. We express this in our laws, which protect humans and a few animals, but certainly not plants, and in our eating habits, wherein some of us refuse to eat the meat of mammals or even fish, but no one feels guilt about eating plants. If asked why we place plants so low in this hierarchy, most of us would respond that plants don't move, don't act, and don't feel, and thus are inferior to animals and certainly to humans. The only animals with similar characteristics are corals and sponges, which we consider the "lowest" animals.
This is an essay trying to persuade you otherwise - that if we must think of "higher" and "lower" organisms, then plants are instead more advanced lifeforms than animals. At the behavioral level, at the cellular level, and at the biochemical level, I'll argue, plants have the superiority.
Let's start with behavior. Why are plants immobile? They're immobile because they are so splendidly adapted to their environments that they don't need to move. They can feed themselves without moving - without having to graze over or chase down, and thereby steal, their food. Furthermore, they feed themselves so well during their growing season that they can maintain themselves for the rest of the year, and so they can stay in one place all year. They also deal so well with the threats of their environments - rain, snow, storms, and even predation and fire - that they can stay in one place all year. That's a very advanced behavior compared to all the mobile and even migratory food-stealing organisms whose behavior we find so remarkable because they must move to keep themselves fed and secure.
The ability of plants to feed themselves comes from the advanced nature of plants at the cellular level, and from one of the great events in cellular evolution. Biologists now recognize that eukaryotic cells have evolved by progressive inclusion of other kinds of cells as organelles. For example, the mitochondria of plants, animals, and fungi are descended from bacterial cells that were trapped in early eukaryotic cells and made part of those cells' machinery. Only later did an even more useful symbiotic event occur: an ancestral plant cell took in a cyanobacterium (a blue-green photosynthesizing bacterium) that became the chloroplast (the photosynthesizing organelle) of plants. That was very good news for plants. Animals, fungi, and the like, however, had already split off the evolutionary tree and so missed out - they were standing behind the door when the cellular machinery for self-feeding was passed out. That's why animals have been doomed to food-stealing, and by necessity to mobility as a means to allow food stealing, whereas plants have become marvelously self-sufficient.
Another reason that plants can be immobile, whereas food stealers must move, lies in the external and biochemical adaptations that allow plants to fend off food-stealers. Thorns and bark are external traits that keep attackers at bay - consider thistles and cacti standing impervious amidst hungry grazers who dare not attack the thorny plants. Biochemistry, on the other hand, is an internal defense, like that found in the "milk" of milkweed, fending off predators, or in the tannin of redwoods, holding fungi at bay. Why flee when you can stand and defend yourself? Grasses have taken a different tack but gained the same result: they grow back from their bases, rather than their tips, and thus grow back quickly if attacked by a food-stealer. They thus stand impervious to attack, whereas a food stealer like you or me flees predation.
At this point, a food-stealer like yourself will probably object that active behavior - the song of birds, the cunning hunting of carnivorous mammals - is what puts your kind on a higher plane. You should remember that plants turn to the sun, respond to the length of the day, and even summon insects to do their bidding in carrying pollen to and fro. In short, they have "behavior" too. What about a herd of buffalo circled against wolves is more noble than a grove of oaks standing together against the wind? How is a fox chasing field mice more cunning than a venus-fly-trap luring its victim in? How is a sonnet more beautiful than the color and scent of a newly-opened rose?
At this point, a human food stealer might object that the measure of an organism is how much it modifies its environment. Humans, after all, construct small-scale environments (buildings) in which to live, and they have begun to modify the global environment, if inadvertently, by such actions as adding CO2 to the atmosphere. Plants, however, did that on a massive scale eons ago, converting an anoxic atmosphere in which animals could not have lived into the oxygen-rich atmosphere we take for granted. Animals, including humans and all their artifices, have hardly done anything in comparison to the change of the global environment enacted by plants. So which is the superior kind of organism? Presumably it's the self-sustaining, world-changing kind, rather than the ineffectual parasitic kind.
My point here is not that you should never eat broccoli again. My point is that, if you're inclined to be hierarchical about life forms in a way that puts yourself at the top of the list, you might think again. Plants really are more highly evolved in the cellular sense, and they are not so lowly in other senses as we would infer from their immobility. If we infer nobility from indifference, we should consider how plants stand so indifferent and resolute while we flee the raging wind, the burning sun, the cold of winter, or the threatening wolf. Our mobility and clever actions are driven by weaknesses that our green cousins will never know.
Acknowledgments: This essay was inspired by the comments of Haruko Kazama, as quoted by Roald Hoffman in American Scientist (v. 91, p. 308, July-August 2003). Hoffman's paper discusses some other biochemical defenses emplyed by plants.
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