STEMS 



661 



neath the outer bark there is green tissue that is of great interest because 

 of the relative darkness in which chlorophyll is developed and food is 

 manufactured (figs. 972, 1033). As the bark increases in thickness, 

 the chlorophyll gradually decreases, finally disappearing from old stems. 

 It has been suggested 

 that the food-making 

 activity of deciduous 

 trees may not cease 

 upon leaf fall, but 

 that the stem chloro- 

 phyll, like the leaf 

 chlorophyll of coni- 

 fers, may manufac- 972 

 ture food at tempera- 

 tures favorable for 

 synthesis during the 

 late autumn, winter, 

 and early spring. 



The structural fea- 

 tures of lenticels. 

 During the first vege- 

 tative period of a 

 woody stem, a cork 

 cambium or phello- 

 gen layer (p. 705) 

 makes its appearance 



FIGS. 972, 973. Lenticels: 972, a young lenticel as seen 

 in a cross section through the outer part of a growing stem 

 of the privet (Ligustrum vulgar e) ; s, a stoma beneath which 

 a lenticel is developing; p, the phellogea layer from which 

 closing layer of cork later develops; x, chlorenchyma, 

 in the cortex, giving composed of thin-walled cells beneath the lenticel and of 

 to a protective thick-walled cells elsewhere; h, epidermal hairs; 973, a 

 cross section through the outer part of a stem of an elder 

 (Sambucus nigra), showing a lenticel during the summer oi 

 its second year; e, epidermis; c, cork layer of the preceding 

 year, which has been ruptured at a by the development 

 of complementary tissue ('); c', developing cork layer 

 of the current season ; x, chlorenchyma ; both figures highly 

 magnified. Fig. 973 from HABERLANDT. 



rise 



cylinder of cork, 

 which cuts off com- 

 munication between 

 the cortex and the 

 stomata. Under- 



neath some of the 



stomata, however, cork does not develop for a time, the phellogen 

 giving rise instead to a loose tissue composed of rounded cells, the 

 complementary cells, between which are conspicuous air spaces (figs. 

 972, 973). This tissue, by reason of the cell turgidity and the air 

 spaces, takes up much more space than does the cork, rarely 



