CHAP. IV. 
LEAVES. 
273 
ence ,* but, by degrees, in consequence of exhaling perfectly 
pure water, and preserving in its 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 an- 
other, evaporate but little, often last several years. We may, 
therefore, 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 
confounded with its fall ; for these two phenomena, although 
frequently confounded, are in reality very different. All 
leaves die some time or other; but some are gradually de- 
stroyed 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 the other kind of tissue, separately, which produce it, but 
the two combined ; the one acting principally in some cases, 
and the other in others. 
T 
