Plants

Plants

Why do some desert plants grow tall and thin like organ pipes? Why do most trees in the tropics keep their leaves year round? Why in the Arctic tundra are there no trees at all? After many years without convincing general answers, we now know much about what sets the fashion in plant design.

Using terminology more characteristic of a thermal engineer than of a botanist, we can think of plants as mechanisms that must balance their heat budgets. A plant by day is staked out under the Sun with no way of sheltering itself. All day long it absorbs heat. If it did not lose as much heat as it gained, then eventually it would die. Plants get rid of their heat by warming the air around them, by evaporating water, and by radiating heat to the atmosphere and the cold, black reaches of space. Each plant must balance its heat budget so that its temperature is tolerable for the processes of life.

Plants in the Arctic tundra lie close to the ground in the thinlayer of still air that clings there. A foot or two above the ground are the winds of Arctic cold. Tundra plants absorb eat from the Sun and tend to warm up; they probably balance most of their heat budgets by radiating heat to space, but also b warming the still air that is trapped among them. As long as Arctic plants are close to te ground, they can balance their heat budgets. But if they should stretch up as a tree does, they would lift their working parts, their leaves, into the streaming Arctic winds.Then it is likely that the plants could not absorbed enough heat from the Sun to avoid being cooled below a critical temperature. Your heat budget does not critical temperature. You heat budget does not balance if you stand tall in the Arctic.

Such thinking also helps explain other characteristics of plant design. A desert plant faces the opposite problem from that of an Arctic plant -----the danger of overheating. It is short of water and so cannot cool itself by evaporation without dehydrating. The familiar sticklike shape of desert plants represents one of the solutions to this problem: the shape exposes the smallest possible surgace to incoming solar radiation and provide the largest possible surface from which the plant can radiate heat. In tropical rain forests, by way of contrast, the scorching Sun is not a problem for plants because there is sufficient water.

This working model allows us to connect the general characteristics of the forms of plants in different habitats with factors such as temperature, availability of water, and presence or absence of seasonal differences. Our Earth is covered with a patchwork quilt of meteorological conditions, and the patterns of this patchwork are faithfully reflected by the plants.

rb