1034 



A TEXTBOOK OF THEORETICAL BOTANY 



1 



causing the detachment of the leaf. In the simplest cases this is all that happens 

 up to the time of separation, but subsequently in the simplest cases and in 

 all others also before separation, there is produced a protective layer on the 

 inner side of the separation layer, which remains as a cover to the scar after 

 leaf fall. This layer is characterized by ligno-suberization of its walls, and 

 its formation may sometimes, though not always, involve cambial activity. 

 Later, a phellogen appears on the inner side of the protective layer and a 

 periderm is formed which is continuous with the stem periderm. There is 

 also, in some plants, an additional lignified layer, w^hich is formed on the 

 outer side of the separation layer. This is shed with the petiole. 



Cork cambium 



Abscission 

 layer 



^ Cork layer 



Vascular 

 strand 



Fig. 1025. — Aesculus hippocastamwi. Longitudinal median 

 section through a node with two leaf bases, showing the 

 development of the abscission layer prior to leaf fall. 



The vascular elements are not interrupted by these layers across the 

 petiole and they are independentlv sealed off, firstly by the formation of 

 tyloses or of wound gum or of both, and ultimately by crushing, when the 

 growth of the new phellogen begins. The sealed-off ends of the bundles 

 usually remain visible and are characteristic features of the leaf scar, which 

 is left on the stem when the leaf has dropped (Fig. 1026). 



Modifications of Leaf Form. 



(Modifications associated with special conditions of the environment are 

 dealt with separately in Volume IV). 



I. Leaf Spines. — Leaves of any type, including stipules, may be spinose, 

 and all stages of modification may occur, from foliage leaves like that of the 

 Holly {Ilex) in which the spines are merely hardened teeth at the apex or on 



