Ch. IV, 2] 



STRUCTURE OF STEMS 



12i 



Fig. 77. — A typical lenticel, of Sambucus nigra, 

 in section; magnified. (From Haberlandt.) 



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 

 structures, for they 

 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 

 _ r ,,,, Y _ later be shown. The lenti- 



fe^ ce * s are places where a loose 



tissue with inter-cellular 

 spaces is formed instead of 

 the impervious cork; and 

 this tissue by its growth 

 partially closes them in 

 winter and forces them open 

 the next spring (Fig. 77). 



The tissues of these tran- 

 sitional stems show very 

 clearly in cross section (Fig. 

 section through 78). Their most striking 

 feature is the sharp division 

 between bark and wood at 

 the cambium. The parts of 

 the bundles inside the cambium have grown greatly, and 

 show clearly the characteristic forms and texture, while the 



Fio. 78. 



a winter twig of Tulip Tree; X 10. 

 The lighter continuous line is t he cam- 

 bium, and the medullary rays are dis- 

 tinct. 



