XIII] 



ABSCISS-LAYER 



105 



layers beneath than the now dying epidermis could do; 

 and we notice that this layer of cork-tissue also stretches 

 across the base of the leaf-stalk (p f ) as a plate parallel to 

 the plate of tissue Ab. 



Fig. 58. Diagrammatic vertical section through the insertion of a leaf 

 to show the method of abscission preceding the leaf-fall, e epidermis ; 

 pet base of petiole; st vascular bundles and other elements of the 

 central column passing into the petiole st'; b axillary bud; p periderm 

 or cork passing across the petiole at p'; Ab absciss-layer crossing the 

 narrowed vascular bundles ; sep line of separation. 



Soon after the completion of this plate of cork, changes 

 occur in the median plane of the layer Ab, along the line 

 sep in the figure, which result in a loosening and separa- 

 tion of the parts along this plane, and the now top-heavy 

 leaf, aided, it may be, by wind or dew, &c., falls away 

 (Fig. 57), leaving a bare patch, or scar, the shape and size 

 of which depend on the sectional area of the leaf-stalk in 

 the region pet (Fig. 58). The thin layers of tissue between 

 sep and p' now dry up, and would of course expose to the 

 exigencies of wind and weather the tissues of the shoot- 

 axis on the inside, were it not that the layer of cork (p') 



