334 PART IV. CLASSIFICATION. 



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 in- 

 timately 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 corres- 

 ponding 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 (gametophorej, 

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

 Splachnum, the male organs are borne upon a leafless prolongation 

 of the shoot. 



In Sphagnum the apex of the female shoot grows out, after fer- 

 tilisation of the -archegonium, into a long leafless shoot termed a 

 pseudopodium, which bears the sporogonium (here destitute of a 

 seta) at its apex. In Aulacomnium and Tetraphis there is a some- 

 what similar terminal shoot, likewise termed a pseudopodium, 

 which bears at its apex a cluster of gemmae. 



The Leaves present considerable variety in size and form : they 

 may be divided, in the first instance, into foliage-leaves and in- 

 volucral leaves. 



The foliage-leaves are simple and sessile ; they are usually 

 inserted transversely on the stem, and are closely packed. They 

 are generally larger towards the upper than towards the lower 

 part of the shoot. In most pleurocarpous Mosses the leaves of the 

 lateral branches differ more or less from those of the main stem. 

 In some forms (e.g. Bryum roseum, Climacium, etc.), where the 

 branches take the form of creeping runners or stolons, the leaves 

 of these branches are reduced to scales (cataphyllary leaves). 



The involucral leaves are arranged in one or more whorls, form- 

 ing an involucre round the sexual organs. Those surrounding a 

 group of male organs are commonly larger than the foliage-leaves, 

 and in some cases (e.g. Polytrichacese) are coloured red or yellow. 

 Those surrounding a group of female organs differ but little from 

 the foliage-leaves: the more internal leaves are smaller than the 

 external : the innermost leaves, distinguished as perichcetial leaves, 

 are quite rudimentary when the archegonia are mature, but after 

 fertilisation has taken place they grow up round the base of the 

 seta of the sporogonium. 



