CHAP. VI. 
OF LEAVES. 
329 
the necessary communication between the branch and the leaf 
is destroyed, and the latter falls off. 
De Candolle explains the matter otherwise and better. 
The increase of leaves, he says, whether in length or in 
breadth, generally attains its term with sufficient rapidity ; 
the leaf exercises its functions for a while, and enjoys the 
plenitude of its existence ; but, by degrees, in consequence of 
exhaling pure water, and preserving in the tissue the earthy 
matters which the sap had carried there, the vessels harden 
and their pores are obstructed. This time in general arrives 
the more rapidly as evaporation is more active : thus we find 
the leaves of herbaceous plants, or of trees which evaporate 
a great deal, fall before the end of the year in which they 
were born ; while those of succulent plants, or of trees with 
a hard and leathery texture, which, for one cause or another, 
evaporate but little, often last several years. We may, there- 
fore, in general say that the duration of life in leaves is in 
inverse proportion to the force of their evaporation. When 
this time has arrived, the leaf gradually dries up, and finishes 
by dying : but the death of the leaf ought not to be con- 
founded with its fall ; for these two phenomena, although fre- 
quently confounded, are in reality very different. All leaves 
die some time or other ; but some are gradually destroyed by 
exterior accidents, without falling ; while others fall, separating 
from the stem at their base, and fall at once, either already 
dead, or dying, or simply unhealthy. 
It is probable that both these explanations are required to 
understand the phenomena of the fall of the leaf ; and that it 
is neither the rupture of the spiral vessels, nor the choking up 
of other kinds of tissue, separately, which produce it, but the 
two combined; the one acting principally in some cases, and 
the other in others. 
