CHAP. IV. LEAVES. . 273 



ence; but, by degrees, in conseqvience 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 failing ; 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 iii others. 



T 



