170 MOSSES AND FERNS CHAP. 



At the base of the young plant very delicate colourless 

 rhizoids are developed, and these show the oblique septa so 

 general in the rhizoids of other Mosses. As the plant grows 

 older these almost completely disappear. 



The apex of the stem and branches is occupied by a pyram- 

 idal apical cell with a very strongly convex outer free base. 

 From the lateral faces of the apical cell, as in the acrogynous 

 Liverworts, three sets of segments are formed. The whole 

 vegetative cone is slender, especially in the smaller branches. 

 The first division in the young segment is parallel to its outer 

 face, and separates it into an inner cell, from which the central 

 part of the axis is formed, and an outer cell which produces the 

 leaves and cortex. 



The second wall, which is nearly horizontal, divides the 

 outer cell of the segment into an upper and a lower cell, the 

 former being much broader than the latter, which is mainly 

 formed from the kathodic half of the segment, which is higher 

 than the anodic half (Leitgeb (i)). The next wall divides 

 the upper cell into an upper and a lower one, the former being 

 the mother cell of the leaf, the lower, with the other basal cell, 

 giving rise to the cortex. Growth proceeds actively in the 

 young leaf, which soon projects beyond the surface of the stem, 

 and by the formation of cell walls perpendicular to its surface 

 forms a laminar projection. The position of the cell walls in 

 the young leaf is such that at a very early period a two-sided 

 apical cell is established, which continues to function for a long 

 time, and to whose regular growth the symmetrical rhomboidal 

 form of the cells of the young leaf is largely due (Fig. 90). 

 The leaves do not retain their original three-ranked arrange- 

 ment, but from the first extend more than one-third of the cir- 

 cumference of the stem, so that their bases overlap, and the 

 leaves become very crowded, and the two-fifth arrangement is 

 established. The degree to which the central tissue of the stem 

 is developed varies with the thickness of the branch. In the 

 main stem it is large, but in the small terminal branches it is 

 much less developed, as well as the cortex, which in these small 

 branches is but one cell thick. Later the cortex of the large 

 branches becomes two-layered (Fig. 89, B), and is clearly sep- 

 arated from the central tissue, whose cells in longitudinal sec- 

 tion are very much larger. In such sections through the base 



