THE LEAVES OF VASCtiLAR PLANTS. 299 



seen them produced ; whilst those of succulent plants, or 

 of trees of a hard and coriaceous texture, both of which, 

 although from diiFerent causes, evaporate but little, often 

 remain for several years. It may be said in general 

 then, that the duration of the life of leaves is in inverse 

 ratio to the activity of their evaporation. When this 

 period arrives, the leaf dries up by degrees, and finally 

 perishes ; but the death of the leaf must not be con- 

 founded with its fall : these are two phenomena 

 which, although frequently connected, are entirely 

 different. All leaves die at a certain period, but some 

 are gradually destroyed by external causes, without 

 falling off; and others fall off, being detached from the 

 stem at their base. 



At the time when the death and fall of leaves were 

 confounded, Mustel thought that this fall was caused by 

 the state of fulness which leaves acquired at the end of 

 their life ; but this state, which may be considered as a 

 cause of death, is not of itself a cause of their fall. 



M. Vrolyck has sought to establish, that when the leaf 

 is dead, the living part of the tree tends to cast it off, as 

 the living parts of animals throw off dead parts, as is 

 seen in the phenomena of Gangrene and Necrosis ; but 

 this explanation, ingenious as it is, goes too far, since 

 there is a number of leaves which die without being 

 detached from the stem which bears them. 



Senebier began to distinguish the death from the fall, 

 and attributed the latter to the growth of the bud of 

 the following year, which in the summer is developed in 

 the axils of leaves. I do not deny that the growth of this 

 bud may facilitate the fall, but it cannot be the essential 

 cause of it; for — 1st. There are leaves, and especially 

 stipules, which have no buds in their axil, and which 

 fall as others. 2d. There are leaflets which have no 

 buds, and which become detached from a common 



