DEVELOPMENT OF PLANTS 267 



phyte splits into two valves (Fig. 198, s f ), thus permitting the 

 scattering of the spores. The lower portion, however, may con- 

 tinue to elongate for several months in some species, forming 

 additional spores. 



It is evident that this sporophyte would become a self-support- 

 ing plant if its foot should reach through the gametophyte and 

 absorb substances from the soil. In comparing the sporophyte 

 in forms like Ricciocarpus, Marchantia and the leafy hepatics 

 with Anthoceros, we see that it has undergone a gradual evo- 

 lution and has finally reached a point where it only needs to 

 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 alternate, 

 forming a dome-like zone of cells. Structures very suggestive 

 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 equal importance with the gametophyte (Fig. 200). 

 Variations appear in the mosses that have been successful in adapt- 

 ing 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 regarded 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 



