132 M us CINE ^ 



SECOND SUBDIVISION. 

 MUSCINE^. 



The line of demarcation between tbe "S^ascular Cryptogams and the 

 plants immediately below them in the scale of organisation, the 

 Muscineae, 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 j 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. AVithin 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 thalhcs. 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 of only a 

 single layer of cells. AVe 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 overlapping 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- 



