132 MUSCINE^ 



SECOND SUBDIVISION. 

 MUSCINEyE. 



The line of demarcation between the Vascular Cryptogams and the 



plants immediately below them in the scale of organisation, the 



Muscinese, is a very sharp one, and their genetic relationship to one 



another presents considerable difficulties. The lower type of structure 



is, however, chiefly manifested in the vegetative organs. The mode of 



sexual reproduction which occurs throughout this group corresponds in 



its most important features with that in Vascular Cryptogams ; and we 



have here also a division of the life-history of the plants into a sporo- 



phyte and an oophyte generation, a true alternation of generations, 



although the phenomenon differs in one important point from that which 



we have seen in Vascular Cryptogams, viz. in almost the whole of the 



vegetative system belonging to the oophyte instead of to the sporophyte 



generation. To this we have already seen an approach in Gymnogramme 



(p. 65). The vegetative system is invariably of small size, and almost 



entirely destitute of vascular bundles and of all other strengthening 



tissues. Within the group the boundary line is crossed between Cormo- 



phytes and Thallophytes ; and in the lower orders we entirely lose the 



differentiation of the vegetative organs into cauline and appendicular 



organs — in other words, into stem and leaves ; the entire vegetative 



system consisting of an undifferentiated thallus. The mature plant is 



almost invariably terrestrial in habit, and is attached to the substratum 



by rhizoids. The appendicular organs, when present, are minute leaves, 



which never contain true vascular bundles, and usually consist cf only a 



single layer of cells. We find, however, the first stage towards the 



epidermal and fibrovascular structures characteristic of the leaves of 



vascular plants, in a distinct midrib and edging of elongated cells with 



somewhat thicker cell-walls overlappmg one another at the extremities, 



and partially or altogether destitute of chlorophyll. In one group 



(Sphagnaceae) the leaves are composed of cells of two different kinds, 



small cells containing chlorophyll interspersed among much larger empty 



cells. The leaves, being usually unilamellar, cannot, of course, be pro- 



