Page:A primer of forestry, with illustrations of the principal forest trees of Western Australia.djvu/19

13 How a Tree Breathes.—Plants breathe, although not quite in the same way as animals. Both inhale the air, keeping the oxygen and expelling the carbon dioxide gas. The difference is that a tree has no lungs, but breaths over its whole surface, leaves, branches, trunk and roots. Even though the roots are covered with soil, they breathe the air which is present in the tiny air-spaces between the soil particles. A plant will be just as surely drowned if its roots are kept under water for long periods as a man will be if his head is kept under water for a certain time. A few plants have special adaptations to enable them to grow in swamps and wet places, but they are exceptional. This process, which is called "respiration," is the exact opposite of photosynthesis. The difference between the two processes may be set out as follows:—

These two processes—photosynthesis and respiration—go on together, but, whereas respiration goes on always in all the livings parts of the tree, photosynthesis only takes place in the green parts and in the day time. During the day the photosynthesis is the more active process and more oxygen is returned to the air than is retained. At night respiration alone is going on. Trees therefore have the effect, through the giving off of oxygen, of purifying the air in the day time and of vitiating it at night, but, upon the whole, the purifying action is much greater than the other.

Transpiration.—It has already been explained that the leaves of trees breathe or respire. They have another function: they transpire; that is, certain matter passes out through them into the atmosphere. We know already that trees suck up water and certain other substances through their roots, but, under normal conditions, the quantity taken up by the roots and sent up the trunk and through every part of the tree is in excess of its requirements. A tree requires certain mineral substances for its nutrition, and, as these are present only in very small quantity in the water, the roots have to suck up a large quantity of water in order to get sufficient of these mineral substances for the use of the tree. The tree does not need all the water its roots supply, and the excess is evaporated through all parts of the tree above ground, but principally by the leaves. This process is called "transpiration." It can be readily perceived that a good deal of power must be exerted to drive the water from the roots to all parts of great trees. What that power is, is still a question for final decision. It is known, however, that the roots exercise some kind of pumping function and that there is above ground a sucking action due to transpiration from the leaves and other parts. These two forces, the latter in particular, may be the main factors in producing a steady flow of water from lowest root to topmost leaf.

The Growth of a Tree.—The source of growth of a tree lies in the additions to it made by the food which it has drawn from the ground and absorbed from the atmosphere. But the food does not increase the bulk of the whole tree in the same way. Except in the case of leaves, buds, fruit and twigs of less than a year's age, the digested food material is deposited in the form of a thin layer over the whole tree between the wood and the bark. This layer is made up of wood cells, or wood fibre. Year by year these successive layers of wood cells are deposited and the tree increases in size. The new twigs grow in length by a kind of stretching, but only