DECIDUUS, in botany, a term expres sive of the second stage of duration in plants, but, like caducus, susceptible of differ6nt senses, according to the par ticular part of the plant to which it is applied. A leaf is said to be deciduus which drops in autumn ; petals are de cidttus which fall off with the stamina and pistillum ; and this epithet is applied to such flower-cups as fall after the ex pansion, and before the dropping ofill.he flower. This last is exemplified in her berry, and the flowers of the class Tetra dynamia.
Most plants in cold and temperate cli mates shed their leaves every year. This happens in autumn, and is generally an nounced by the flowering of the common meadow saffron. The term is only appli ed to trees and shrubs; For herbs perish down to the root every year, losing stem, leaves, and all. All plants do not drop their leaves at the same time. Among large trees, the ash and walnut, although latest in unfolding, are soonest divested of them ; the latter seldom carries its leaves above five months. On the oak and horn-bean the leaves die and wither as soon as the colds commence ; but re main attached to the branches till they are pushed off by the new ones, which unfold themselves the following spring.
These trees are doubtless a kind of ever. greens; the leaves are probably destrsy ed only by cold ; and, *perhaps, would continue longer upon the plant, but for the force of the spring-sap, joined to the moisture.
With respect to decidnus trees, the fall ing off of the leaves seems principally to depend on the temperature of the atmo shere, which likewise serves to hasten or retard the appearance in question. An ardent sun contributes to hasten the dropping of the leaves. Hence, in hot and dry summers, the leaves of the lime tree and horse-chesnut turn yellow about the 1st of September ; whilst, in other years, the yellowness does not appear till the beginning of October. Nothing, how ever, contributes more to hasten the fall of the leaves than immoderate cold or moist weather in autumn ; moderate droughts, on the other hand, serve to retard it. It deserves to be remarked, that an evergreen tree, grafted upon a deciduns, determines the latter to retain its leaves. This observation is confirmed by repeated experiments, particularly by grafting the laurel, or cherry-bay, an ever green, on the common cherry; and the Hex, or ever-green oak, on the oak.