THE EPIDERMAL SYSTEM. 91 



portion of the underlying mass is also removed. In many 

 of the Thallophytes there is absolutely no differentiation of 

 an epidermal portion. 



118. In the Bryophytes there is in general a poor epider- 

 mal development ; it is composed for the most part of one 

 or more weakly defined layers of smaller cells, which, how- 

 ever, pass by insensible gradations into the inner tissue 

 mass. Here, however, the first true epidermal hairs make 

 their appearance. 



119. In one group of the Liverworts the Marchantiacem 



Fig. 78. Longitudinal section of erect portion of thallus of Marchantia pnlymor- 

 1>ha. o, epidermis ; 8, walls beiween air-spaces, the latter filled with rows of chloro- 

 phyll-bearing cells, chl ; sp, a stoina ; y, a large parenchyma-cell. X 550. After Sachs. 



there is an epidermal system of a high degree of perfection, 

 and composed of epidermis proper and stomata (Fig. 78). 

 The epidermis consists of a single layer of somewhat tabu- 

 lar cells arching over the air-cavities which occupy the upper 

 surface of the plants ; it is perforated here and there by sto- 

 mata or breathing pores, composed of four to eight circular 

 rows of cells placed one above the other (sp in the figure). 

 These chimney-like structures originate by the division of a 

 single cell into four or six radiating daughter-cells ; in the 

 centre of this group an intercellular pore is formed by the 

 lateral growth of the cells (Fig. 79) ; and by a subsequent 



