94 



This dropping is due, not to breaking, but to growing off by the forma- 

 tion of a cleavage plane lietween both the twig, petiole and the parent 

 stem. Thus the reason for the scars left by the sliedding. 



lu the shedding of stems, the cleavage plane is gradually developed 

 across the fibro-vascular system separating the stem from the parent 

 stem with the exception of the ))ark and a few layers of wood cells which 

 are easily broken. The scar is virtually formed l)efore the falling of the 

 stem. Marked illustrations of this habit from the deciduous trees are 



found in the family Salicacea^ L. The l)ranches and twigs begin to fall 

 before the shedding of the leaves and continue tiu-oughout the period of 

 leaf fall. The twigs shed are green, many bearing large winter buds 

 upon their tips. Of the conifers, the Tsiif/u- Ciiuddenxis Can:, illustrates 

 this hal)it very markedly. However, their twigs, when shed, are dead.* 



In the shedding of leaves, Ave find the formation of the cleavage 

 plane the same as in the stem. The most common method is that of 

 a separation between the petiole and stem, as shown l)y the scars on the 

 stems. 



The Self-pruning of Woody Plants.— John II. Schaffner, Ohio Nat. I., 1902, pp. 171-147 



