The tree obtains nourishment from the soil through the roots, and from the air through the leaves. Both roots and leaves are adapted by nature for the role they play. The leaves of broad-leaved trees consist of the stalk, or petiole, and a thin lamina or blade, which provides the greatest possible surface of contact with the air. Atmospheric carbon dioxide is the tree’s main source of food. Together with water and by the means of chlorophyll and radiant energy from the sun it is processed by the leaf cells into organic compounds that go into building the major part of the tree’s organs. This process is known as photosynthesis.
Because the amount of carbon dioxide in the air is very low (0.03 per cent on average), the tree has to process great quantities of air. This must take place on the greatest possible leaf surface, which is why the leaves of woody plants are so thin, and why a mature tree has tens or hundreds of thousands of them. To make the most of the sun’s energy the leaves are spread out as advantageously as possible, this being aided both by the complex network of branches and by the varying lengths and positions of the stalks.
Another essential element for the life of the tree is water and the dissolved minerals it contains. The tree absorbs the water from the soil through its roots, chiefly through the young parts. From the roots the water passes to minute tubes or vessels, through which it is transported to the trunk. In the trunk these tubes form a continuous column that carries the water to the branches at the very top of the tree. From there it travels via the petioles to the leaf blades where it is distributed by the veins to all parts of the leaf surface. The main force which serves to transport the water to heights of thirty to fifty metres is the cohesion of the water column and the force of transpiration (evaporation). The individual cells and tissues along the way take the amount of water they need for the various chemical processes and the remainder is pulled up to the leaves, where part of the water is used during photosynthesis to manufacture sugar. But a great quantity still remains unused. Woody plants absorb more water than they can use to obtain the necessary amount of mineral substances contained in it; the excess is eliminated by the process known as transpiration. This takes place in the leaves and consists of the evaporation of water, regulated to a certain degree by a system of pores that can be opened or closed. It may also be limited by the curling or drooping of the leaves, and increased by the movement caused by wind. The evaporation of water during transpiration also serves to cool the leaf surface and prevents the leaf tissues from being damaged by high temperatures.
The amount of water transpired by trees into the atmosphere is very great and varies not only according to the size and species of the plants but also according to the conditions of the environment — soil moisture, relative humidity of the air, temperature, strength of the wind. Etc. The rate of transpiration of broad-leaved trees is several times greater than that of conifers. The poplar, aspen, alder, birch and ash have a particularly high rate.