332 PART IV. CLASSIFICATION. 



CLASS IV. MUSCI (Mosses). 



A. The GAMETOPHYTE. The protoncma is more conspicuous in 

 the Musci than in the Hepaticse : it sometimes persists until the 

 sporogonia are developed and the spores are ripe (e.g. Ephemerum), 

 and in many cases the subterranean portion persists from year to 

 year. It is generally filamentous and much branched ; but in 

 some forms it is a flattened expansion (e.g. Sphagnum), or cylindri- 

 cal branched and shrubby, or it bears lateral flattened expansions 

 which are assimilatory organs (e.g. Tetraphis, etc.). The filamen- 

 tous protouema consists of a subaerial and of a subterranean 

 portion, which differ in that the cells of the former contain chloro- 

 plastids, their walls are colourless, and the septa are transverse ; 

 whereas those of the latter do not contain chloroplastids, and their 

 walls are brown and their septa oblique. The protonema presents, 

 in fact, a certain differentiation into shoot and root, the term 

 rhizoids being applied to the root-like filaments. This differ- 

 entiation is, however, of little morphological value, since the 

 differences between the shoot- and root-filaments depend entirely 

 on external conditions : thus, if the rhizoids be exposed to light 

 they assume the characters of the subaerial filaments. The flat- 

 tened protonema of Sphagnum bears rhizoids on its margins and 

 under surface. 



The growth in length of the protonemal filaments is apical : the 

 terminal cell behaves as an apical cell from which segments are 

 successively cut off by transverse or oblique walls. 



The Adult Shoot arises as a lateral bud on the subaerial portion 

 of the protonema. In some cases (Bryinese) the subterranean 

 portion gives rise to lateral buds : these are small, spherical or 

 lenticular, multicellular bodies of a brown colour, filled with re- 

 serve materials, and are termed bulbils ; when they are brought to 

 the surface they give rise to adult shoots, either directly or with 

 the intervention of protonema. 



The adult shoot is in all cases differentiated into stem and 

 leaves, and may be branched or unbranched. Its symmetry is 

 commonly radial or isobilateral, less commonly dorsiventral. In 

 the former case it is attached to the soil by rhizoids springing 

 from its basal portion ; in the latter, by rhizoids developed on its 

 under surface. In Sphagnum, rhizoids occur only on young 

 shoots. 



