26 



MOSSES AND FERNS 



CHAP. 



apical cell which lies much nearer the ventral than the dorsal 

 surface (Fig. 2, x). From this are cut off successively dorsal 

 and ventral segments. Each segment next divides into an 

 inner and an outer cell. From the outer cells of the dorsal 

 segments the sexual organs arise, and from those of the ventral 

 segments the overlapping lamellae upon the lower surface of the 

 thallus, and also the root-hairs. The rapid division of the inner 

 cells of the segments, especially those of the dorsal ones, causes 

 the thallus to become rapidly thicker back of the apex. 



Fig. 3. — Riccia glajica (X-)- Horizontal sections of the growing point. A, B, X525; C, X about 260. 

 C shows the dichotomy of the growing point ; x, x , the two new growing points ; L, the lobe 

 between them ; ar, a young archegonium. 



Sections made parallel to the surface of the thallus, and pass- 

 ing through the growing point (Fig. 3), show that the margin 

 is occupied by a group of cells that look very much alike. 

 Sometimes one of these cells is somewhat larger than the others, 

 but more commonly it is impossible to decide with certainty that 

 a single initial is present. From a comparison of the two sec- 

 tions it is at once evident that the initial cells have nearly the 

 form of the segment of a disc, and that in addition to the 

 dorsal and ventral segments lateral ones are cut off as well. 

 In the region just back of the apex the tissue of the thallus is 



