CH. IV, 2] STRUCTURE OF STEMS 121 



follows the waning vitality, cessation of chlorophyll forma- 

 tion, appearance of autumn coloration, and finally, by a 

 weakening of the 

 walls of the absciss 

 layer, the fall of the 

 leaf itself, the absciss 

 layer becoming the 

 corky and waterproof, 

 leaf-scar. The lenti- 

 cels are physiologi- 

 cally important 



9triiH-iirp! for thpv FlG> 77 -~~ A typical lenticel, of Sambucus nigra, 

 6S, 1 tiey in sect ion ; magnified. (From Haberlandt.) 



replace the stomata 



(which disappear of course with the epidermis), as avenues 

 of gas exchange between the interior of the stems and the 

 external atmosphere. This exchange is no longer needed 

 for photosynthesis, which ceases as cork develops, but is nec- 

 essary for the respiration of the living tissues within, as will 



later be shown. The lenti- 

 cels are places where a loose 

 tissue with inter-cellular 

 ||k spaces is formed instead of 



J|H 1% the impervious cork; and 



->^~^:^M^^i i& A | this tissue by its growth 



partially closes them in 

 winter and forces them open 



^ Wr *ke next s P r i n S (Fig- 77). 



TJi$ The tissues of these tran- 



sitional sterns show very 



^^^^M^^^ clearly in cross section (Fig. 



FIG. 78. A cross section through 78). Their most striking 



a winter twig of Tulip Tree ; x io. f ea t u re is the -sharp division 



The lighter continuous line is the cam- . 



bium, and the medullary rays are dis- between bark and WOOd at 



tinct - the cambium. The parts of 



the bundles inside the cambium have grown greatly, and 

 show clearly the characteristic forms and texture, while the 



