Dr. Lindley thus explains this phenomenon.

"In the course of time a leaf becomes incapable of performing its functions; its passages are choked up by the deposit of sedimentary matter; there is no longer a free communication between its veins and the wood and liber. It changes colour, ceases to decompose carbonic acid, absorbs oxygen instead, gets into a morbid condition, and dies; it is then thrown off. This phenomenon, which we call the fall of the leaf, is going on the whole year. Those trees which lose the whole of their leaves at the approach of winter, and are called deciduous, begin, in fact, to cast their leaves within a few weeks after the commencement of their vernal growth; but the mass of their foliage is not rejected till late in the season. Those, on the other hand, which are named evergreens, part with their leaves much more slowly; retain them in health at the time when the leaves of other plants are perishing; and do not cast them till a new spring has commenced, when other trees are leafing, or even later. In the latter class, the functions of the leaves are going on during all the winter, although languidly; they are constantly attracting sap from the earth through the spongelets, and are therefore in a state of slow but continual winter growth.

"It usually happens that the perspiratory organs of these plants are less active than in deciduous species." - Theory of Horticulture.

With all due deference to Dr. Lindley, whose scientific acquirements are of a high order, we cannot admit that his theory in this particular is entirely satisfactory. Nature has ordained that vegetables shall perform certain functions, and that which may appear to be the cause of change, is in fact only a result of the action of established laws.