Ch. IV. 



STRUCTURE OF ^TE:\IS 



121 



Fig. , , . — A t\-picai lenticel. of Sambucus nigra, 

 infection: magnified. ( From Haberlaiidt.) 



follows the waning ^^tality, 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 ^-■J^''.Z^^^^- 

 layer becoming the '^Z''^'!c:^t^^''^^^'r'^^ri^^^^.z- 

 corky and waterproof '^"-^^'y^^^-yj^-r^^'-ZiX^'S^- 

 leaf-scar. The lenti- ->2'-%"" ~' ?5~5r ,^ 

 eels are physiologi- i^'::^ ^ :-::->^.?fe5£3S 

 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 i~ no longer needed 

 for photosj-nthesis, which ceases as cork develops, but is nec- 

 essary- for the respiration of the lining tissues within, as ■will 

 ^____^__ later be sho"«TL. The lenti- 



/ " eels are places where a loose 



tissue with inter-cellular 

 spaces is formed instead of 

 the imper\ious cork; and 

 this tissue by its growth 

 partially closes them in 

 winter and forces them open 

 the next spring (Fig. 77 ji 



The tissues of these tran- 

 sitional stems show verj' 

 clearly in cross section (Fig. 

 78 L Their most striking 

 feature is the sharp di^-ision 

 between bark and wood at 

 the cambium. The parts of 

 the bundles inside the cambium have grown greatly, and 

 show clearlv the characteristic forms and texture, while the 



3^ 



A cr05= section through 

 a winter twig of Tulip Tree ; X 10. 

 The lighter continuou; line is the cam- 

 bium, and the meduUarj- rays are dis- 

 tinct. 



Fig. 7s 



