TISSUES AND SIMPLE ORGANS. 



57 



ous cells of the character of primary parenchyma called " phel- 

 loderm," or " cork-parenchyma cells." The various layers formed 

 outwardly are not all equal : there may be alternate layers with 

 thick and thin cell-walls (Betula alba). 



The well-known " peeling" or "scaling" of bark will occur 

 very readily along the thin-walled layers, because they are only 

 slightly extensible as compared with the thick-walled layers, in 

 which cellulose predominates. The thin-walled layers consist 

 essentially of suberin, a fatty substance, which, besides other con- 

 stituents, contains stearin (v. HOHNEL, KUGLER). The microscopist 

 recognizes cuticularized membranes by their insolubility in con- 

 centrated sulphuric acid. According to AMBEONN, fat-crystals may 

 readily be detected in the cuticle (cuticula) by means of polarized 

 light. 



FIG. 35. Transverse section of Ribes nigrum from a twig one year old. 



e, Epidermis; h, hair-cell; r, bark -parenchyma; K, product of the cork-cambium c; fc, cork- 

 cells; pd, chlorophyll-bearing cells; 6, bast-cells. (After Sachs.) 



When and where is cork-formation necessary? Harmonizing 

 with the properties of cuticularized cells-walls, a corky protective 

 tissue is required on the following plant-structures : at points 

 where the cuticle and epidermis are ruptured because of the growth 

 in thickness of the stem or root ; on delicate plant-structures which 



