Prof. Vauchcr on the Fall of Leaves. 331 



writers upon this branch of Natural History. But by far 

 the best and most philosophical account of this periodical de- 

 foliation that has fallen under our notice, is contained in a 

 memoir upon this subject by Professor Vaucher of Geneva. 

 As the memoir of this excellent observer does not seem to be 

 generally known, we make no apology for presenting an ab- 

 stract of its contents. 



There are few phenomena more remarkable than the fall of 

 leaves. Trees, which during summer preserve their foliage in 

 spite of the changes of the atmosphere and the effects of winds, 

 despoil themselves naturally on the approach of autumn, what- 

 ever be the temperature of the season, and the cireumstances 

 in which they are placed. The only exceptions to this law of 

 nature, says Professor Vaucher, are what are called evergreens, 

 of which the defoliation does not take effect till the lapse of 

 years, and trees of which the leaves lose their vitality at the 

 same time with the others, though they do not separate from 

 the stem till the era of spring. 



Many theories have been formed by ingenious men to ac- 

 count for the periodical fall of leaves. Some have believed 

 that leaves fall from trees in autumn, because the bud which 

 they shelter, increasing in size, separates the leaf-stalk insensi- 

 bly from the stem. Others have imagined, that this fall is 

 caused by a disease in the leaf itself, occasioned by the super- 

 abundance of the juices it receives in autumn, and the diminu- 

 tion of insensible transpiration ; while others have attributed 

 this phenomenon to the difference of growth between the cir- 

 cumference of the twig and the leaf-stalk, the effect of which 

 is to break the fibres which attach it to the stem. None of 

 these explanations, however, seem sufficient, in M. Vaucher's 

 opinion, to account for the fact. As to the first, it is evident 

 from simple inspection that it cannot be admitted. This pres- 

 sure of the bud, which, like a wedge, ought to detach the leaf- 

 stalk from the stem, almost never operates in this way; and, 

 besides, if it did, its action ought to be as general as the fall 

 of the leaves. Hut leaves which have no buds at their axil, 

 or which have them very small, fall as quickly as the others ; 

 and in compound leaves, the leaflets, which have no buds, are 

 separated from the principal leaf-stalk before it is detached 



