120 



A TEXTBOOK OF BOTANY 



[On. IV, 2 



trees during the first winter (Fig. 75). 

 The leaves are now gone, not to reappear 

 on this part of the stem; but the LEAF- 

 ar A ffis remain, marked by a lighter Colored 

 c^rky layer, in which can be seen the 

 severed ends of the veins. Each scar of 

 course stands at a node, sometimes plain, 

 but often not, just above which is the now 

 prominent axillary bud, while a larger 

 terminal bucl ends the twig. The thin 

 epidermis has been replaced by a layer of 

 gray-brown waterproof cork, scattered over 

 which are the lighter colored warty ex- 

 crescences CallecLLENTICELS. 



The leaf-scars and lenticels need special 

 comment. Leaves fall from trees because 

 of the formation of a special ABSCISS-LAYER 

 -of tissue which develops across the base of 

 the leaf in late summer (Fig. 76). Gradu- 

 ally this layer closes the free communica- 

 tion between stem and leaf, though mean- 

 time the valuable materials of the leaf are 

 mostly transferred to the stem. Then 



FIG. 75. Winter 

 twig of Horse Chest- 

 nut; X {. 



a.l: 



FIG. 76. Vertical section through a 

 twig and petiole of Poplar, showing the 

 absciss-layer, a.l (From F. Darwin, 

 Elements of Botany.) 



