268 NATURE OF MOSSES 



derivation of the mosses is indicated by the remarkable uni- 

 formity of structure that characterizes the entire group. 



While the plants themselves are relatively small, they often 

 form large swards or mats, owing to the extensive branching 

 and prolonged growth of the stems and their rapid multiplication 

 through the dying off of the older parts and the independent 

 growth of the branches that are thus set free. Owing to this 

 habit of growth and multiplication the mosses often furnish the 

 conspicuous features of the vegetation. Especially is this true 

 in northern regions and in bogs and barrens where great sec- 

 tions of the country are carpeted with them. The mosses are 

 leafy stemmed plants like the Jungermaniales, but they usually 

 possess the decided advantage of having erect stems and radi- 

 ally arranged leaves (Fig. 200). Even in the prostrate forms 

 where but two rows of leaves can be developed, the resemblance 

 to the leafy hepatics is only superficial. The moss leaves are 

 not lobed and the central portion is usually traversed by a strand 

 of cells which serve for conduction like the vascular bundles of 

 higher plants (Fig. 206, B). The differentiation of the tissues 

 of the stem also show a marked advance over preceding forms. 

 A central conducting region of elongated cells that may be 

 compared to a rudimentary vascular system and a cortical zone, 

 often with thickened cells and rudimentary epidermis, are fre- 

 quently to be seen (Fig. 206, A). Asexual reproduction is al- 

 most entirely confined to the detachment of branches as stated 

 above. In only a few genera have the formation of gemmae been 

 noticed. The sexual reproduction and the development of the 

 sporophyte present some interesting features that will be noticed 

 in the following orders. 



108. Order a. Sphagnales. Bog or Peat Mosses. — A single 

 genus. Sphagnum, of about 20 species is the sole representative 

 of this order which forms in several respects a transition from 

 the hepatics to the mosses proper. These pale-green mosses 

 ( Fig. 201 ) grow on bogs and moors and other places where 

 they are subject to drainage containing organic matter, as humic 

 acid, derived from the decay of plant and animal life. The 

 majority of plants are unable to endure these conditions, which 



