492 



MOSSES AND FERNS 



CHAP. 



one of the cells a second, somewhat curved one, which strikes 

 the primary wall about half-way up. The cell thus cut off, 

 seen in longitudinal section, is triangular, and is the apical cell 

 of the stem (Fig. 257, A). The two other cells (leaf- 

 segments) now undergo division by a vertical wall, which 

 divides each into equal parts, and each of these pairs of cells 

 develops into a cotyledon. The apex of the young cotyledon 

 is occupied by a row of marginal cells in which divisions are 

 formed, like those in the apical cell of the stem, and in longi- 



A C. 



Cot. 



G. / F. 



Fig. 257. — Selaginella. Martensii (Spr.). Development of the embryo (after Pfeffer). A, B, D, E, 

 Successive stages in longitudinal section, X 340 ; C, apical view of a young embryo with four-sided 

 apical cell (x), x 340 ; F, longitudinal section of the primary root, X 205 ; G, apex of the young 

 sporophyte, showing the first dichotomy, X 340. 



tudinal section the apex of the cotyledon seems to have a 

 single apical cell, much like the stem (Fig. 257, E). From the 

 larger of the leaf- segments, by a more active growth of the 

 cells next the suspensor, the foot is formed, and by its growth 

 the stem apex is pushed to one side, and its axis becomes 

 almost at right angles to that of the suspensor. Each 

 cotyledon develops upon its inner side, near the base, an 

 appendage, the ligula (Fig. 258, /), which is a constant 

 character of all the later leaves. 



