The greenery’s just for the eyes

Originally published 7 May 2002

There comes a moment in New Eng­land wood­lands in the spring when up through last sea­son’s decay­ing leaves and pine nee­dles comes the first green. Like a car­pet rolled out overnight, sud­den­ly the greedy leaves of the Cana­da mayflower are everywhere.

The Cana­da mayflower—also called wild-lily-of-the-val­ley — insin­u­ates a web of run­ners beneath the leaf lit­ter, col­o­niz­ing dark con­ti­nents of the for­est floor. From this hid­den infra­struc­ture of com­mu­ni­ca­tion and trans­port, the plant throws up paired green leaves, one to the right and one to the left, like sup­pli­cant hands. In a few weeks time, there will be tiny white flow­ers and the plant will be busy with arrange­ments for the next gen­er­a­tion. But, for the moment, the busi­ness is pure energy.

Cap­tur­ing sun­light. Soak­ing up rays.

There is lit­tle enough sun­light on the wood­land floor and the Cana­da mayflower must aggres­sive­ly court its share. I am remind­ed of a line from a poem of Andrew Mar­vell: “…a green Thought in a green Shade.”

Each small plant has one thought in mind, so to speak: Catch what­ev­er light it can before the decid­u­ous trees com­plete their leafy canopy.

When you think about it, plants have no real need to rise high on stems or trunks. Only so much sun­light falls upon the Earth. There would be just as much ener­gy to go around if plants just spread them­selves out on the ground and shared as share alike. But, of course, in nature, com­pe­ti­tion, not shar­ing, is the rule.

Plants evolved tall stems and trunks so they could put their neigh­bors in the shade. The oaks, maples and hick­o­ries stretch sky­ward, putting out buds, doing their best to leave the Cana­da mayflow­ers in the lurch. But the spring plants of the wood­land floor man­age to sur­vive on what­ev­er dol­lops of light slip through the canopy. They get a jump on the trees by bloom­ing early.

No law of physics is more basic than the law of entropy, the ten­den­cy of the uni­verse to move toward dis­or­der and death. But life bucks that tide, using avail­able free ener­gy wher­ev­er it can get it, and around here the most abun­dant source of ener­gy is sun­light. The suc­cess of life on Earth was ensured by pho­to­syn­the­sis, the chem­i­cal process by which green plants store up sun­light in ener­gy-packed carbohydrates.

Biol­o­gists are not sure how pho­to­syn­the­sis evolved, but that it hap­pened ear­ly in the his­to­ry of life is cer­tain. Every high school stu­dent learns the basic equa­tion: Car­bon diox­ide plus water plus sun­light yields sug­ar and oxy­gen (with all the Cs, Hs and Os appro­pri­ate­ly bal­anced). The equa­tion does­n’t near­ly con­vey an appre­ci­a­tion for the com­plex reac­tions that con­nect one side of the reac­tion arrow to the other.

Cru­cial to the process is a boxy mol­e­cule with a mag­ne­sium heart and a long tail. Chloro­phyll does the actu­al cap­tur­ing of sun­light. Atom­ic elec­trons in the mol­e­cule are bumped up in ener­gy. As they return their boun­ty, they ener­gize reac­tions that cre­ate inter­me­di­ate prod­ucts called ATP and NADPH, which then move along the assem­bly line. When all is said and done, it is sug­ar that appears at the fac­to­ry door.

All of this mol­e­c­u­lar activ­i­ty is going on in the splayed leaves of the Cana­da mayflower. Who would have guessed? The only hint of this hum­ming man­u­fac­to­ry is the col­or of the leaves. Chloro­phyll absorbs ener­gy from the blue and red parts of the solar spec­trum — the ends of the rain­bow. The green mid­dle of the spec­trum is spurned, and that’s what reflects off the leaves and enters our eyes.

Green is not the col­or of pho­to­syn­the­sis, as every­one assumes.

Rather, green is the unused left­overs of the solar feast. If absorp­tion were more effi­cient and the entire solar spec­trum were used by plants, their leaves would be black. It’s as if nature gra­cious­ly lim­it­ed her con­sump­tion so that we might have a green plan­et, gob­bling up the bulk of sug­ar-pro­duc­ing light but gen­er­ous­ly leav­ing us scraps of pigment.

Like all ani­mals, we burn sug­ar when we respire; that’s where we get our pep. We suck in oxy­gen with every breath and set lit­tle fires alight in our lungs. Burn­ing sug­ar keeps us warm and active. But we can’t make sug­ar our­selves; for that we need plants. We need to eat green plants, or the flesh of ani­mals that eat green plants. We need the prod­ucts of pho­to­syn­the­sis as vital­ly as we need air and water.

Bee­tles, spi­ders, ele­phants, great blue whales — every ani­mal on Earth needs its share of the plants’ Midas wealth. It is esti­mat­ed that humans cur­rent­ly com­mand between one-third and one-half of the prod­ucts of all ter­res­tri­al pho­to­syn­the­sis, as food for our­selves and our domes­ti­cat­ed ani­mals, but also for fuel, build­ing mate­r­i­al, and fiber — the lion’s share, we might say, except the lion gets slim pickings.

The Cana­da mayflower, at least, is not on our list for consumption.

It’s two green leaves sat­is­fy anoth­er human hunger — our need for beauty.