14 



In common with most other cryptogams mosses present the 

 phenomenon known as alternation of generation. In their case the 

 spore produces, by the intervention of a branching filamentous 

 stage, known as the protonema, the sexual moss plant,which even- 

 tually produces the oosphere or egg-cell, from which on fertiliza- 

 tion the sporogonium or capsule, the second and asexual 

 generation of the moss is developed. The capsule contains the 

 spores from which the cycle of life is again commenced. It is 

 generally small in proportion to the vegetative and sexual plant 

 which forms the moss as we recognize it. The arrangement forms 

 a striking contrast with the alternation of generation presented 

 by the ferns, in which the asexual or spore-bearing generation is 

 by far the more important. 



The stem in the higher mosses consists of a firm central por- 

 tion called the central string, and which, although its functions 

 are very imperfect is regarded by Rabenhorst as the rudiment of 

 of the fibro-vascular bundles, which play such an important part 

 in the higher plants. It is surrounded by the parenchymatous 

 cells of the ground tissue which are generally rich in chlorophyll, 

 and outside this there are often one or more layers of colourless 

 cortical cells, which are well developed in the genus Sphagnum. 

 The lower part of the stem of all mosses, except Sphagnum, is 

 clothed with rootlets or rhizoids whose principal function is to fix 

 the plant to the substratum on which it grows, though it has 

 recen tly been proved that they to some extent possess the functions 

 of true roots and can derive nutriment from the subtratum. In the 

 leaves of mosses a wonderful variety of form is found based on a 

 very simple plan. The leaf is never compound and never possesses 

 a leaf-stalk as in the higher plants. It usually consists of a single 

 layer of cells which are very variable in form and it never has 

 stomata. In shape every intermediate is found between the narrow 

 linear leaves as exhibited by campylopus and the almost spherical 

 leaves of Mnium subglobosum. Leaves sometimes occur which 

 merely consist of a thin layer of cells of similar shape throughout 

 as in Fterygophyllum liicens, but more often they possess a central 

 midrib or nerve which offers every gradation of development from 

 the short ill-defined nerve of many species of Hypnum to that of 

 the genus Lencobryum where it occupies practically the whole 

 leaf. Owing to the comparatively large size of the cells and of 

 the chlorophyll granules the formation of starch under the 

 influence of light can be more readily observed in some mosses than 

 in the higher plants. As the blade of a leaf offers a comparatively 

 small surface for assimilation, the interchange of gases only taking 

 place through the outer cell wall, since there are no stomata. 



