This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The leaves and other green parts of a plant constitute a workshop of many compartments, in which the raw food materials are organized into more or less simple or complex substances for the building up of the various parts of the plant body. The green chlorophyll granules are the agents, under the influence of sunlight, whereby these remarkable chemical changes are brought about. Starch is the first visible product, and first makes its appearance in the form of small grains in the chlorophyll granules. The leaf breathes by absorbing oxygen from the air, and that gives it power or energy by which the other chemical changes are brought about. The oxygen attacks and destroys some of the material in the cells, unites with carbon to form carbon dioxide, which is given off into the air as in the breathing or respiration of animals. The leaf also absorbs carbon dioxide from the air, breaks it up, gives off the oxygen, and the carbon unites with the elements of water brought into the plant by the root. The more complex composition of the protoplasm is effected in the leaf by the addition of the other necessary elements of plant food absorbed by the roots (see p. 33). This process of building up organized matter from the raw materials is termed assimilation, and can only be carried on during daylight, though it may possibly be done artificially by electric light. Plants which have no leaves, like the Cacti, assimilate by means of the chlorophyll in their stems and branches.
The effect of light and shade on the leaves of plants is not fully appreciated by all gardeners. Too often we see plants crowded so much together that only a very small percentage of the leaves have any chance of being bathed in sunshine during the day. Fruit growers will have too many trees to the acre, probably thinking that the crops will be weighty in accordance with numbers. The exact reverse is really the case, for the simple reason stated above - that only under the influence of daylight can the starch and other building-up materials be formed in the cells of the leaves. The great bulk of the dry weight of any plant is obtained from the atmosphere, not from the soil; hence the necessity of allowing a fair amount of space between one plant or one tree and another. (See pp. 108, 141).
Another kind of work carried on by leaves is transpiration, or the passing off of watery vapour into the air. The cells bordering the intercellular spaces when gorged with water give off some of it in the form of watery vapour into the cavities communicating with the stomata. During daylight, and when the air is comparatively dry, the stomata open and the watery vapour passes out. The cells bordering the air cavities would soon get dry and flabby, but prevent this by absorbing water from cells behind them, and these in turn from cells more deeply seated. This is continued through the leaf, its stalk, the branches, stem, and roots until a current of water, known as the "transpiration current", is set up from the roots to the leaves. In the open air and on a windy day this current is often so great that the roots cannot supply it, more especially if the soil is dry, and the leaves flag as a consequence. This phenomenon may often be observed in the case of pot plants if allowed to get dry, whether under glass or outside. It can be remedied by watering the soil in the pots and by syringing the foliage. The undue loss of water from plants under glass can be more effectually prevented by closing the ventilators before syringing, and shading may be resorted to in extreme cases. The atmosphere then becomes saturated, thereby largely checking transpiration for the time being, and the leaves resume their wonted stiffness. This rapid ascent of water serves to keep the plants cool in hot weather, the cells turgid, and also brings in plant food. Transpiration is a vital process regulated by the protoplasm in the leaves and is somewhat different from evaporation pure and simple. For instance, a Stonecrop may be placed between sheets of paper, covered by a board and held down by a weight. It will continue to elongate and even open its flowers under such conditions; but if placed in a basin and some boiling water poured over it to kill it the stems and leaves will part with their moisture in a few days. At night the stomata close, and transpiration ceases. Another phenomenon may often be observed in the morning. Drops of water may be seen on the tips of the leaves of Aspidistras, Arum Lilies, Fuchsias, Chinese Primulas, and many others as a result of root pressure. Over the ends of the vascular bundles of the leaves of those plants water pores are situated, and unlike stomata they never close. The roots continue to absorb water night and day, and when transpiration ceases the cell walls become saturated. Water then filters into the cavities of the wood fibres and vessels under pressure from the roots until it reaches the water pores, where it escapes from the overgorged tissues.