566 MOSSES AND FERNS ' chap. 



cable-like strands, which simulate true roots, but are less 

 efficient than these. 



The size to which the gametophyte may grow depends 

 largely upon the water supply, which must be regarded as the 

 most potent factor governing the development of the plant 

 body. It is evident that the delicate rhizoids alone are insuf- 

 ficient to supply with water a plant of any but the most modest 

 dimensions. Indeed, in many Bryophytes, the rhizoids play 

 but a minor part in supplying water, as the whole plant may 

 absorb water much as an Alga does. So also we find very few 

 Bryophytes in which the development of mechanical tissues is 

 sufficient to make the plants (except small ones) stand firmly 

 upright. Either the plant is prostrate, or it maintains its up- 

 right position by virtue of the mutual support offered by its 

 neighbours, most of the large Mosses growing in dense tufts 

 or mats. 



It is evident that the size to which a terrestrial gametophytic 

 structure can grow is necessarily limited, owing to its inade- 

 quate means of obtaining water. Either the plant must grow 

 where there is a permanent and abundant water supply, or else 

 it must dry up and completely cease its activity during periods 

 of drought. It would seem as if the originally aquatic gameto- 

 phyte could never adapt itself perfectly to terrestrial conditions, 

 and upon the sporophyte devolved the development of a differ- 

 ent plant-type adapted from the first to life in the air. As the 

 sporophyte assumed the character of an independent plant, it 

 gradually replaced the gametophyte as the predominant struc- 

 ture of the higher plants. 



/ The origin of the sporophyte of the Archegoniates, as we 

 have seen, is to be sought in the zygote of some Green Alga. 



1 This in its simplest form is a single thick walled resting spore, 

 adapted to resisting drought, and changes of temperature which 

 are fatal to the growing plant. From its very nature, it is 

 primarily the terrestrial phase, so to speak, of these typically 

 aquatic organisms. The embryo-like cell mass developed in 

 ColeochcEte may very properly be compared to the embryo- 

 sporophyte of Riccia, or of any Liverwort. However, each 

 cell of the rudimentary sporophyte of Coleochcete produces but 

 a single spore, and this is a zoospore like those of other Algae, 

 and is-clearly associated with the normally aquatic habit of these 

 plants. 



