THE EPIDERMAL SYSTEM. 91 



portion of the underlying mass is also removed. In many 

 of the Thallophytes there is absolutely no diiferentiation of 

 an epidermal i^ortion. 



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 ti"ue epidermal hairs make 

 their appearance. 



119. — In one group of the Liverworts — the Marchantiacem 



Fig. 78. — Longitudiaal section of erect portion of tiiallus of Marckantia polymor- 

 pha. 0, epidermis ; S, walls between air-spaces, the latter filled with rows of chloro- 

 phyll-bearing cells, ckl ; sp,& stoma ; gr, 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' 



