Ill 



THE JUNGERMANNIALES 



103 



what unequal size. The next wall formed divides the larger 

 of the two primary cells into an inner and an outer cell (Fig. 

 50, A), so that the young segment now consists of three cells, 

 an inner one and two outer ; the latter in the dorsal segments 

 correspond to the two lobes usually found in the dorsal leaves. 

 The two outer cells now divide by walls in two planes, and 

 rapidly grow out above the level of the apical cell and form 



Fig. so. — Porella Bolanderi. A, Median longitudinal section of a vegetative axis; 

 B, a cross-section of the apex of a similar one, X500; x, the apical cell; h, hair; 

 d, dorsal surface; v, ventral surface; C, male; D, female branch. 



lamellae which remain single-layered, and undergo but little 

 further modification beyond an increase in size. From the 

 base of the young leaves simple hairs develop, but remain small 

 and inconspicuous. The inner of the three first formed cells 

 of the segment, by further division and grow^th in all direc- 

 tions, produces the axis of the plant. This in cross or longi- 

 tudinal section shows almost perfectly uniform tissue. No 

 distinct epidermis, or central strand, like that found in most 

 Mosses, can be seen. 



