226 



BOTANY 



segments, each of which gives rise to a leaf and a portion of the stem. 

 From the base of the young shoot rhizoids grow out, which fasten it 

 to the ground. These rhizoids may, under proper conditions, give 

 rise to new protonemal filaments. 



Apical Growth. With very few exceptions (e.g. Fissidens), the 

 growth of the shoot in the Musci is from the activity of a tetrahedral 

 apical cell (Fig. 189), and the shoot is radially symmetrical. Each 

 segment of the apical cell gives rise to a leaf and a portion of the 



A. 



FIG. 188. Tetraphis pellucida. A, leafy shoot with gemmae (X 6). B, upper part 

 of the same (X 50). C, young gemma (X 600). I), older gemma (X 300). 



stem. The branching is always lateral, the apical cell of the branch 

 being cut out from an outer cell of the stem, below one of the young 

 leaves. 



The Leaf. The growth of the young leaf is from a two-sided apical 

 cell, whose growth is limited. The later growth is basal. In Sphag- 

 num, Fontinalis, and a small number of other Mosses, the leaf de- 

 velops no midrib; but the typical moss-leaf shows a median thickened 

 strand, whose central cells are for purposes of conduction. The rest 

 of the leaf is usually composed of a single layer of uniform green 

 cells ; but in Sphagnum there are two kinds of cells, large empty 



