OF THE LEAVES.
The leaf, folium, fig.16 and 17, is a very general organ of vegetables, yet not absolutely necessary to all plants, for the stems and stalks occasionally perform its functions. What those functions are we shall in a compendious manner explain Leaves are generally so formed as to pre sent a large surface to the atmosphere : when they are of any other hue than green, they are said in botanical language to be coloured. Their duration is for the most part annual, but in some trees and shrubs, they survive two or more seasons, and such plants being always in leaf are denominated evergreens. The internal surface of a leaf is highly vascular and pulpy, and is clothed with a cuticle very various in different plants, but its pores are always so constructed as to admit of the requisite evaporation or absorption of moisture, as well as to admit and give out air. Light also acts through this cuti cle in a definite manner. That air and moisture and light have considerable, and even the most important effects upon the leaves of plants, has long been known to those who have studied the subject; that heat and cold affect them is familiar to every one. The experiments of Hales, Bonnet, and others, have thrown much light upon the absorption and perspira tion of leaves, while those of Priestley and Ingenhouz have explained their ef fects upon the atmosphere, and the man ner in which air and light particularly act upon them. Leaves have a natural ten dency to present their upper surface to the light, and turn that surface towards it, in whatever direction it is presented to them. When trees in leaf are nailed to a wall, and the position of their leaves is consequently disturbed, they soon reco ver their natural direction. Light evi dently acts as a wholesome stimulus to their upper surfaces, and as a hurtful one to the under. When the latter is forcibly presented for a long period to its rays, destruction is the consequence. Leaves seem to require occasional repose from the action of light on their upper surface ; for, when it is withdrawn from them, ma ny leaves close or fold themselves toge ther,as Win a state of relaxation, and spread themselves forth again at the returning beams of the morning. This is more es
pecially the case with winged leaves, as those of the pea kind. Those of the white acacia, robinia, pseudo-acacia, have been remarked by Bonnet to be over excited by the sun of a very hot day, and to fold their upper sides together, in a man ner directly contrary to their nocturnal posture. The effect of moisture upon leaves every one must have observed. By absorption from the atmosphere, they are refreshed, and by evaporation, espe. cially when separated from their stalks, they soon fade and wither. Aquatic ve getables, whose leaves are immersed in the water, both absorb and perspire with peculiar facility. Anatomical investiga tions have shown that the nutritious juices, imbibed from the earth, and be come sap, are carried by appropriate ves sels into the substance of the leaves. Mr. Knight, in his papers in the Philosophi cal Transactions, has demonstrated that these juices are returned from each leaf, not into the wood again, but into the bark. Hence a new and curious theory of vegetation has been established. It appears that the sap is carried into the leaves for the purpose of being acted up on by air and light, with the assistance of heat and moisture. By all these agents a most material change is wrought in its component parts and qualities, differing widely according to the diversity of the species. Thus the resinous, oily, mucila ginous, saccharine, bitter, acid, or alka line secretions are elaborated. The heed less observer of a leaf is little aware of the wonderful operations constantly going on in its delicate substance, nor can the most enlightened philosopher explain more than a very small part of the chemi cal processes of which it is the immediate agent. It is scarcely necessary to ob serve how materially plants differ in the flavour and qualities of their leaves, all which must depend in a great measure on the oper.ttion of the leaf itself; for the common sap of plants, from which all their secretions are made, differs very little in plants whose qualities are very unlike to each other ; those qualities de pending upon the secreted fluids elabo rated principally by the leaves.