ORGANIZATION: PLANT TISSUES. 



357 



the epidermis of stems and leaves of many plants, as in the underground 

 stems of the bracken fern, the leaves of pines (fig. 415), etc. 



Fig. 414. Fig. 415. 



Transverse section of portion of Margin of leaf of Pinus pinaster, transverse 

 tomato stem. ep, epidermis; ch section, c, cuticularized layer of outer wall 

 chlorophyll-bearing cells; co, collen- of epidermis; *', inner non-cuticularized 

 chyma; cp, parenchyma. layer; c', thickened outer wall of marginal 



cell; g, ', hypoderma of elongated scle- 

 renchyma; p, chlorophyll-bearing paren- 

 chyma; pr, contracted protoplasmic con- 

 tentr. X8oo. (After Sachs.) 



699. Cork. In many cases there is a development of "cork" tissue 

 underneath the epidermis. Cork tissue is developed by repeated division 

 of parenchyma cells in such a way that rows of parallel cells are formed 

 toward the outside. These are in distinct layers, soon lose their proto- 

 plasm and die; there are no intercellular spaces and the cells are usually 

 of regular shape and fit close to each other. In some plants the cell walls 

 are thin (cork oak), while in 

 others they are thickened 

 (beech). The tissue giving 

 rise to cork is called "cork 

 cambium," or phellogen, and 

 may occur in other parts of 

 the plant. For example, 

 where plants are wounded the 

 living exposed parenchyma 

 cells often change to cork 

 cambium and develop a pro- 

 tective laver of cork. The ^- * l6 - 



., , . . Section through a lenticel of Betula alba show, 



walls ot COIK cells contain a i ng stoma at top, phellogen below producing rows 

 substance termed suberin, of flattened cells, the cork. (After De Bary.) 



which renders them nearly waterproof. 



