- The respiration of plants is a very complicated function, having its seat in all the green parts, but more especially in the leaves. It results in the elaboration or assimilation of the sap, which through this agency becomes available for building up and nourishing the vegetable fabric. Looked upon from this point of view, then, the leaves are analogous to the lungs of animals, as those organs form the principal laboratory for the production and purification of the blood. We shall see that light exercises considerable influence over this important function.

When the ascending sap reaches the leaves and fills their cells, it is rapidly concentrated by the evaporation or transpiration of its surplus water. This water, which is no longer separated from the outward air except by the thin cell-walls, penetrates them and is dispersed in the atmosphere in the form of invisible vapour, to which the stomates give issue. The drier the air, the greater the transpiration from the leaves, and the greater the consequent tax upon the roots to keep up an adequate supply by absorption. This is one of the most important points connected with economy of plant-life for the consideration of the practical gardener. The principal cause of loss by death of newly transplanted trees and shrubs, especially if removed, as in the case of evergreens, in full foliage, or shortly before deciduous species put forth their leaves, is due to the inability of the roots to maintain the necessary supply of water to compensate for that transpired, causing the tissues of the foliage to shrivel and ultimately become unable to perform the operation. This will frequently happen when the greatest care has been exercised in the removal to secure all the roots without injury, and when abundance of water is within their reach. This may arise from a paucity of small roots; and when a tree has but a few large coarse roots, it is almost impossible to lift it with a ball of earth; and a very short exposure of the fibrils to a drying wind will destroy their delicate spongioles or absorbing parts. Some plants will readily replace their fibrils, whilst others, and especially those with coarse roots, do so with difficulty, or not at all, if the circumstances be unfavourable. In instances of this kind it is useless to confine one's attentions to the roots. The top must be cared for, and this is best done by covering it over with bast mats in very hot sunny or drying weather. From time to time the mats should be moistened to moderate the transpiration of the plant. The trouble involved in this operation would be amply repaid by the preservation of choice shrubs.

After a prolonged drought the leaves of many herbaceous plants, whose roots do not penetrate far into the soil, droop during the day when exposed to the fierce rays of the sun, because the evaporation is in excess of the absorption. But the dews of night are sufficient, as a rule, in our climate, to afford them moisture enough to rise again and recover their former firmness. In hotter, drier climates, however, as in some parts of tropical and sub-tropical Africa, a great number of plants succumb, that would otherwise have enjoyed a much longer existence. In dull, rainy weather the amount of evaporation is almost nil.

From the differences in their structure, plants possess greater or less power of resisting the effects of continued dry weather. These differences are in texture, number of stomates, thickness of epidermis, and also depth reached by the roots. Relatively, annual and perennial herbs suffer more from drought than woody and arborescent plants; and aquatic plants wither almost direetly they are taken from the water. Trees with coriaceous leaves, like the common Box, with hard wood or deep roots, suffer little from long droughts; and the same may be said of most plants with succulent, fleshy leaves or stems, owing to their thick epidermis and few stomates. The plants belonging to the latter category abound in hot, dry countries.

The condensation of the sap in the tissues of the leaves is insufficient of itself to give it the necessary nutritive properties which should distinguish it. It has yet to undergo important modifications in its chemical composition. A certain temperature is indispensable to accomplish this change; but it is sunlight that plays the principal role.

Delicate and complicated experiments have demonstrated the fact, that (in many instances, at least) during the day, and especially when exposed to the direct rays of the sun, all green parts of plants exhale oxygen; and that, on the contrary, during the night, or when in darkness, they give off carbonic acid. Oxygen is obtained by the decomposition of the carbonic acid contained in the sap, which the plants have drawn from the soil or absorbed directly from the atmosphere through their leaves. Under the influence of light, the oxygen of the carbonic acid is liberated, and the carbon fixed in the tissues of the plant, entering itself into fresh combinations, termed proximate principles. Although plants exhale a certain proportion of carbonic acid under the conditions here indicated, and perhaps also under other conditions, the quantity of oxygen disengaged is greatly in excess, as the immense annual growth of wood in all parts of the world goes to prove. In fact, it is beyond dispute that plants are mainly instrumental in preserving the equilibrium of the constituent parts of the atmosphere by their decomposition of the carbonic acid respired by animals, and restitution of the greater portion of the oxygen to the air.

All parts of plants naturally deprived of light, as roots, tubers, rhizomes, etc., do not themselves fix the carbon contained in their tissues, nor become green; but it is assimilated for them in the leaves, and returned through the circulating channels. Nevertheless, these parts when partially exposed to light become green, and are capable of elaborating their sap. There is a considerable variation in the degree of light required by different plants to enable them to form the chlorophyll, or green colouring matter common to most vegetable organisms, of which carbon is one of the chief ingredients. For the greater part, direct exposure to the sun in a climate suitable to their nature, other things being equal, is necessary for the perfect development of plants. But Ferns, and some other plants, will flourish and reproduce themselves in semi-darkness. This well-known action of light and darkness on plants is taken advantage of by gardeners in blanching Endive, Seakale, etc., and thereby rendering them more fit for food, as this process not only changes the colour of a plant, but also greatly modifies its other secretions.

The sources of nitrogen and many other constituents of the ternary and quaternary proximate principles of plants, and by what agencies these combinations are effected, are subjects still engaging the attention of scientific men.