DEVELOPMENT OF PLANTS 295 



come in contact with the soil to become an independent plant 

 since it is provided with all the tissues necessary for photosyn- 

 thesis and absorption. Note also that the growth of the sporo- 

 phyte is becoming more prolonged, that the spore mother cells 

 appear later in its development and comprise but a small portion 

 of it, and finally, that the sporogenous and sterile tissues alter- 

 nate, forming a dome-like zone of cells. Structures very sugges- 

 tive of these features will appear among the mosses and ferns. 



Class B. Musci or Mosses 



107. General Characteristics. — ^The mosses are by far the 

 largest group of the Bryophyta and show marked advances over 

 the hepatics in the development of the gametophyte and sporo- 

 phyte. Particularly noticeable is the size and differentiation of 

 the sporophyte which now becomes among the majority of the 

 genera of nearly equal importance with the gametophyte (Fig. 

 200). Variations appear in the mosses that have been successful 

 in adapting them to a great variety of conditions, ranging from 

 submerged aquatics to pronounced xerophytes that live upon 

 exposed rocks. As a result, they are of common occurrence 

 everywhere. The mosses, however, do not show any variations 

 that have led to higher types of plants. In fact, they are re- 

 garded as a highly specialized group, like the Jungermaniales, 

 that have branched off from the hepatics in recent geological 

 times and become the dominant representatives of the Bryophyta 

 owing to their better adaptation to present conditions upon the 

 earth. This recent derivation of the mosses is indicated by the 

 remarkable uniformity 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 sections 



