564 MOSSES AND FERNS CHAP. 



ancestors of the lower land plants to the terrestrial mode of life 

 was probably very gradual. We may still find forms among 

 the simpler Algae which are to a greater or less degree adapted to 

 a terrestrial life. Such types as Plcurococcus, Botrydium, and 

 species of Vancheria may be cited. In Pleurococcus no special 

 organs for water absorption are developed, and the cells simply 

 vegetate as long as the surrounding atmosphere is sufficiently 

 moist, becoming dried up and dormant when the necessary 

 moisture is lacking. Botrydium, however, is provided with a 

 relatively extensive system of roots, which penetrate the moist 

 earth and enable the plants to live for a considerable time as a 

 genuine land plant, since the loss of water due to transpiration 

 is made good so long as there is an adequate supply of water in 

 the soil. These Algae, however, have no efficient check against 

 the loss of water in the parts exposed to the air, and very quickly 

 die when the supply of water from the earth is suspended. 



Such Schizophycese as Nostoc and similar terrestrial forms, 

 by the development of the massive gelatinous or mucilaginous 

 envelope, are protected against rapid loss of water. The gel- 

 atinous tissues of many sea-weeds, which are exposed for short 

 intervals to the air, no doubt serve a useful purpose in holding 

 water. None of these forms, however, can be considered as 

 very well equipped for a strictly terrestrial existence. 



To judge from the life-history of certain aquatic Liverworts, 

 such as RicciocarpuSj it seems not unlikely that the primitive 

 Archegoniates arose from some aquatic Algae, probably not very 

 unlike Coleochccte. These may have become stranded upon the 

 mud by the subsiding water, and by the development of rhizoids 

 which are often induced by such contact with a solid medium, 

 the activity of the plant would be prolonged until the rhizoids 

 were unable to extract sufficient moisture from the soil to supply 

 the needs of the plant. To judge from the analogy of Riccio- 

 carpus, this contact with the soil is a stimulus to a much more 

 vigorous growth than is the case when the plant is floating, and 

 we can conceive that the vegetative vigour of the Alga might 

 have been enhanced by its new terrestrial mode of life. 



The direct origin of the simple gametophyte of such a Liver- 

 wort as Anenra or Anthoccros, from some confervoid type is 

 readily conceivable, but the very great difference in the com- 

 plexity of the reproductive organs between even the simplest 



