Wild Things: A multiplicity of green

From top, northern red oak, black ash, silver maple.

By ED KANZE

Most of us see green leaves whenever and wherever we go, at least when there’s daylight. Yet we easily take them for granted. Leaves are nearly as ubiquitous as the air we breathe and just about as important. Let’s spend a few minutes pondering their everyday miracles.

The leaves of flowering plants, of conifers, and of ferns and clubmosses are, for starters, vascular. This means they hold specialized tissues that spirit substances from one place to another. Early land plants lacked vascular tissue. Modern plants with ancient lineages such as mosses and liverworts still get by without it.

The evolution of vascular tissue allowed plants to move moisture and minerals from the soil to places often high above the ground where those items were needed. It also allowed sugars assembled in leaves from carbon, hydrogen, and oxygen to be exported from leaves and sent down into stems, trunks and roots. Without vascular tissue, plants hug the ground. With it, they can rise toward the sky and sun. A vascular plant’s plumbing serves a vital duty in connecting leaves to the rest of the plant.

Next time you pick up a leaf, look at the veins. And while you’re at it, next time you lift up an arm, look at the veins. You’re seeing vascular tissue.

In some plants, retired vascular tissue provides a sort of scaffold that holds a plant up. We call this retired plumbing “wood.” Wouldn’t it be great if we could retire our own vascular tissue and grow new rings of it every year? If we could, there’d be no need for coronary bypass surgery, statins and stents. New pipes would replace the old.

The great majority of leaves are green because they harbor tiny organelles called chloroplasts. These are solar panels, capturing the energy of sunshine and using it to bind together carbon dioxide and oxygen atoms as carbohydrates. The chloroplasts of a plant contain their own DNA. It is distinct from that of the plant itself. Chloroplasts descend from the original inventors of photosynthesis, bacteria that lived in ancient oceans and filled them with a byproduct we call oxygen. Chloroplasts are in essence symbiotic cyanobacteria (once known as blue-green algae) that flourish inside green plant cells.

Leaves come in an astonishing variety of sizes and shapes. Some are simple, consisting, generally, of a single blade and stem. Simple leaves attach to stems one at a time. You’ll find examples of this sort of leaf on birch, beech and maple trees.

Compound leaves, by contrast, consist of multiple parts linked together by vascular tissue, all connected to the Mother Ship via a single shared stem, or petiole. Trees with compound leaves include ashes, black locusts, black walnuts and butternuts. 

Vein patterns vary in leaves, too. The leaf veins of our woody plants generally fall into one of two types. Either they’re pinnately veined, or palmately. Most of our trees and shrubs have pinnately veined leaves. There’s a main bundle of vascular tissue, a vein, running up the middle, with lesser veins running toward each side and margin. Birches, beeches, and oaks have this sort of veining. Maples and sycamores do not. 

What maples and sycamore leaves do exhibit are lobes and major veins radiating out from each leaf’s base like fingers from a human palm. This is called a “palmate pattern.” Most of us recognize it and know maples at a glance. 

As for the edges of leaves, they vary in obvious and subtle ways. Some leaves have simple teeth along the edge like the sharp points of a handsaw. Others are double-toothed, with a small number of large teeth projecting beyond a larger number of small teeth. And there are plenty of leaves with no toothing at all. Teeth, though, are the rule. Why? Teeth may help leaves shed water and disperse heat. They may also stir air currents that aid a leaf’s ability to take in carbon dioxide, a critical resource required for sugar-making.

Without green leaves teeming with chloroplasts, chlorophyll and vascular tissue, the world would be a vastly diminished place.