GROUP II. BRTOPHYTA : MUSCI. 



333 



The growth of the adult shoot and its branches is effected by 

 means of an apical growing-point with a single apical cell which is 

 generally a three-sided pyramid. 



Each segment cut off from the apical cell gives rise to a leaf : 

 hence the arrangement of the leaves, and the symmetry of the 

 shoot, is generally determined by the form of the apical cell. 

 Thus in Fissidens, the leaves are arranged in two rows and the 

 symmetry of the shoot is isobilateral : in other cases (e.g. Fontin- 

 alis) the leaves are in three rows, and the symmetry of the shoot 

 is radial. 



Branching is confined to perennial shoots, and is lateral, never 

 dichotomous. When the growth of the main shoot is arrested by 

 the formation of 

 sexual organs at 

 the apex (acrocar- 

 pous), one (or 

 more) of the lateral 

 branches (termed 

 innovations) close 

 behind the apex 

 assumes the cha- 

 racters of the main 

 shoot and carries 

 on the growth : the 

 resulting branch- 

 sj'stem is cymose, 

 either sympodial or 

 dichasial according 

 to the number (one 

 or more) of the in- 

 novations at each 

 branching. When 

 the growth of the 

 main stem is not 

 thus arrested, the 

 sexual organs be- 

 ing borne on lateral branches (pleurocarpous), the branch-system 

 is monopodial and racemose. 



The branches (except the innovations) frequently differ in 

 various ways from the primary shoot. Thus, in Sphagnum and 

 other pleurocarpous Mosses, the leaves of the branches differ in 



FIG. 204. Longitudinal section through the apical region 

 of a stem of Fnntinalis antijtyretica, a Moss growing in water 

 (after Leitgeb) : v the apical cell of the shoot, producing three 

 rows of segments which are at first oblique and afterwards 

 placed transversely (distinguished by a stronger outline). 

 Each segment is first of all divided by the wall a into an 

 inner and an outer cell; the former produces a part of the 

 inner tissue of the stem, the latter the cortex of the stem and 

 a leaf. Leaf-forming shoots ari^e beneath certain leaves, a 

 triangular apical cell (z) being formed from an outer cell of 

 Hie segment, which then, like v, produces three rows of seg- 

 ments ; and each segment here also forms a leaf. 



