FUNCTION.] FALL OF THE LEAF. 207 



the leaf exercises its functions for a while, and enjoys the 

 fullness 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 

 frequently 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 drop at once, 

 either already dead, or dying, or simply unhealthy. 



The main cause is, however, to be sought in the different 

 rates at which leaves and bark grow. So long as they con- 

 tinue to grow at the same rate they adhere to each other : 

 but if, from whatever cause, the leaf grows more slowly than 

 the bark, it drops ; or if the tissue of a leaf contracts sud- 

 denly, while the bark remains distended, we obtain the same 

 result. The latter seems to be one of the reasons why leaves 

 are instantly cast in the autumn after a sudden frost ; the 

 former is the obvious explanation of the fall of evergreen 

 leaves upon the renewal of growth in the spring. 



