ADIAXTL\M PEDATL'M. 127 



The original parenchyma outside the bundles of the 

 stem early thickens its walls. These thick walls con- 

 sist of several layers, the most prominent of which, the 

 median, is called the middle lamella. This layer, 

 according to Strasburger 30 and others, is the primary 

 cell wall, upon which thickening layers are deposited. 

 By other histologists it is held that the layers are 

 formed, as the thickening progresses, by the differenti- 

 ation of the wall. Growth in thickness, according to 

 the first view, is due to apposition ; according to the 

 second, to intussusception. 



The thickening layers of the wall are perforated by 

 numerous pits, through which probably pass threads of 

 protoplasm, not occupying the breadth of the pit, but 

 passing through much more minute openings in the 

 closing membrane of the middle lamella." 



In addition to serving to increase the strength of 

 the stem, the cortical part is a convenient storehouse 

 for reserves of food, as indicated by the quantity of 

 starch in its cells. 



The several cell layers of the leaf necessitate some 

 arrangement for allowing the entrance of gaseous food 

 and exit of the by-products of the cells' activity ; 

 hence the loose arrangement of the cells of the leaf, 

 forming' large intercellular spaces, which communicate 

 with the exterior by numerous stomata. The stomata 

 have here the form usual among the higher pteri- 

 dophytes and flowering plants, an elliptical slit, bounded 

 by two crescentic cells, which by their change of posi- 



90 Bauund Wachsthum der Zellhaute, p. 175. 



Sl Cf. Schaarschmidt, Protoplasm, Nature, xxxi, p. 290 ; Gardiner, 

 ibid, p. 390. 



