300 VEGETABLE ORGANOGRAPHY. 



petiole, when particular causes, such as the sting of an 

 insect, affect them. 3d. Lastly, there are other organs 

 which have no buds at their base, and which fall off in 

 a manner so analogous to leaves, that it is impossible to 

 believe that facts so similar should be caused by causes 

 entirely different. 



Duhamel came near the cause of the phenomenon 

 when he compared the fall of leaves to the disease of the 

 Vine, in which the upper joints of the young shoots are 

 disarticulated when they are affected by early frosts, or 

 only, perhaps, by cold and damp seasons. The fall of 

 leaves resembles this phenomenon in its being a true dis- 

 articulation, but it differs in this, being a constant and 

 regular fact, which takes place at the same period, 

 whatever the external circumstances may be. 



Vaucher has, in fact, established, that leaves which are 

 continuous with the stem die without falling off, but 

 that all those which are articulated with it necessarily 

 drop at a certain period of their existence. The leaflets 

 of compound leaves, being articulated in the same 

 manner with a common petiole, may, therefore, fall off 

 independently of the leaf. Thus the fall of leaves, like 

 that of fruits, is determined beforehand, and is a neces- 

 sary consequence of the assistance of an articulation. 



It is true to say only that it is facilitated by different 

 causes ; such are the growth of the bud in the axil of 

 the petiole ; the cessation or diminution of vegetation, 

 which tends to dry up and twist the petiole ; the growth 

 of the trunk, which tends to disunite the fibres of the base 

 of the leaf; the action of atmospheric inclemencies, such 

 as frost, cold moisture, and especially hoar-frost, which 

 tend to diminish vegetation ; the actions of mechanical 

 agents, such as the wind, hail, or rain, which tend to 

 shake the base of leaves. All these different causes 

 explain the slight anomalies which the leaves of trees 



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