156 PLANT-LIFE ON LAND [OH. 



But far the most convincing evidence is that derived 

 from the details of propagation. The simpler denizens 

 of the water commonly show a phase where detached 

 cells are motile in water, as we have seen in the case 

 of Ulva and Ulothrix (Chapter II). Such an 

 incident figures in the life-cycle of all the simpler 

 Land plants. It is seen in Mosses, Liverworts, Ferns 

 (Chapter III), Club-Mosses, and Horsetails. None 

 of these live through their normal life without 

 dependence at one critical point upon external fluid 

 water as the medium for that motility. In this they 

 are believed to show traces of their ultimate aquatic 

 origin still preserved. It is not till we reach the 

 Higher Flowering plants that freedom from that 

 embarrassing condition of life is seen : for in them 

 fertilisation is by a pollen-tube, and external fluid 

 water is no longer necessary for the process. In 

 this respect the Flowering plants have become in 

 the fullest sense Plants of the Land. 



Further than this it has been suggested (Chapter 

 II) that the motile stage of Algae may itself have 

 been actually the primitive state for them, and have 

 preceded that non-motile phase which is usually 

 styled the "plant." The ultimate ancestors of the Sea- 

 weeds were probably detached and motile organisms, 

 like the simple Flagellates of the present day. It is 

 a very reasonable, and even physiologically a probable, 

 view that primitive life was like them naked and 



