86 Plant Life and Evolution 



ever, that the gap between the algse and the mosses 

 is a very wide one. 



Occasionally liverworts are found which are true 

 water plants, such, for example, as Ricciocarpus 

 (Fig. 8, A), which grows ordinarily as a floating 

 aquatic. If the water dries up, however, the liver- 

 wort settles upon the mud and grows very luxuri- 

 antly, the contact with the earth acting apparently 

 as a stimulus. Roots are developed penetrating the 

 mud and the plant assumes quite a different form 

 from that of the floating condition. The behavior 

 of this liverwort may perhaps illustrate the first 

 step in the development of the higher plants from 

 alga-like aquatic ancestors. These water plants 

 stranded upon the mud by the subsidence of the 

 water may have developed roots in response to con- 

 tact stimulus of the solid earth, and prolonged 

 their growing period, and thus may have inaug- 

 urated the line of land plants which was destined 

 to become the dominant plant type of the future. 



Amphibious Nature of the Archegoniates. The 

 essentially amphibious nature of the mosses and 

 ferns is best shown in their method of fertilization. 

 If we examine a liverwort like that shown in the 

 figure, we shall find the sexual cells, eggs, and 

 sperms borne in organs of characteristic structure. 

 The female organ (Fig. 9, C) is multicellular and 

 usually has the form of a long-necked flask, which 

 contains the egg-cell. This structure is called the 

 archegonium and is remarkably uniform in struc- 



