116 EVOLUTION OF PLANTS 



They occur in almost all situations except in salt water 

 and actual deserts, some being submersed aquatics, 

 others growing upon the ground, or upon rocks and 

 trees, and indeed in any situation where they can occa- 

 sionally obtain moisture. Many of them may be com- 

 pletely dried up for an indefinite period without losing 

 their vitality. 



In the growth of the stem and leaves, as well as in the 

 structure of the reproductive organs, the gametophyte 

 is very uniform. Both leaves and stem show a definite 

 apical growth, and the leaves are, with few exceptions, 

 arranged spirally about the stem. The sexual organs 

 are in the main like those of the Hepaticse, but show a 

 definite apical growth in both archegonium and anther- 

 idium. 



The sporophyte is highly specialized and shows a 

 certain degree of independence in the development of a 

 well-marked assimilative system of tissues, as in Antho- 

 ceros and Sphagnum. It differs in its growth from that 

 of the liverworts in the presence of a definite single 

 apical cell, to whose regular divisions the early growth 

 of the embryo is due. Later this apical growth is re- 

 placed by a basal growth much as in Anthoceros. 



The young sporophyte is a cylindrical body which 

 later develops an enlarged upper portion, the capsule or 

 theca, borne upon a long stalk, or seta (Fig. 29, D, sp). 

 The latter is usually traversed by a strand of conduct- 

 ing tissue, possibly homologous with the columella in 

 the sporophyte of Anthoceros, or the vascular bundles 

 in the young fern-sporophyte. 



The assimilative tissue in the sporophyte of the higher 

 Musci is very perfect. The basal part of the capsule 



