356 PART III. THE CLASSIFICATION OF PLANTS. 



sided, the leaves are arranged, in consequence of subsequent dis- 

 placement, in two rows, and the symmetry of the shoot is isobi- 

 lateral. Again, in Sphagnum, Polytrichum, Andresea, etc., the 

 leaves are not arranged in three rows with a divergence of -|> but 

 spirally with divergences of f, -f, etc., because the walls of the seg- 

 ments cut off successively from any one side of the three-sided 

 apical cell are not parallel to each other, but are inclined at an 

 angle. The symmetry of the shoot is, however, 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 (acrocarpous), one 

 (or more) of the lateral branches (termed innovations) close behind 

 the apex assumes the characters of the main shoot and carries on 

 the growth : the resulting branch-system is cymose, either 

 sympodial or dichasial according to the number (one or more) of 

 the innovations at each branching. When the growth of the 

 main stem is not thus arrested, the sexual organs being borne 

 on lateral branches (pleurocarpous), the branch- system is mono- 

 podial 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 

 size and shape from those of the primary shoot : in other forms 

 (e.g. Thuidium) the lateral branches have limited growth. 



The development of the branches, though never axillary, is 

 intimately connected with the arrangement of the leaves, since 

 the apical cell of a branch is developed from the same segment as 

 the corresponding leaf. Each branch is developed beneath the 

 corresponding leaf, either in the median line (e.g. Fontinalis), or 

 on one side of it (e.g. Sphagnum). However, a branch is not 

 developed in connexion with each leaf. 



In most cases the adult shoot does not present any differentia- 

 tion into a vegetative and a reproductive portion (gametophore), 

 but such a differentation is to be found in certain forms. Thus, 

 in Splachnum, the male organs are borne upon a leafless prolonga- 

 tion of the shoot : again in Schistostega the fertile and the sterile 

 shoots differ from each other in form ; the sterile shoot is leafless at 

 the base, but from the middle upwards it bears two rows of leaves 

 inserted longitudinally, so that the shoot resembles a small fern- 

 leaf ; the fertile shoot bears at its upper part a tuft of leaves in 

 five rows, inserted obliquely or transversely, whilst the lower part 



