24 PLANT-LIFE ON LAND [CH. 



movement they are apt to be overlooked. But there 

 is no doubt that there is a regularly recurrent phase 

 of motility in every completed life-cycle. It may 

 then be enquired which of these two phases, the 

 stationary plant or the motile zooid, was probably 

 the prior condition in evolution. 



Among certain simple aquatic organisms called the 

 Flagellates minute forms exist which are always freely 

 motile, and have no fixed and encysted stage. Each 

 individual consists of a naked protoplast similar in 

 essentials to the zooid of Ulva or Ulothrix. This fact 

 suggests very strongly that the free and motile 

 condition was primitive, and that the encysted state 

 with a cell-wall surrounding each stationary protoplast 

 was later and derivative. The individual cells of the 

 Ulva plant are so minute that they are not seen 

 individually with the naked eye. But the obvious 

 Ulva plant is a large congeries of them. The cells 

 of the Flagellates are also individually small, but 

 being isolated they are invisible to the naked eye. 

 And so it is that while an ordinary visitor to the 

 sea-shore may be familiar with so obvious a plant as 

 Ulva, he will be unaware of the existence of its 

 propagative cells, as well as of its simpler unicellular 

 prototypes the Flagellates. This was for long the 

 position of Algologists also, and the view above 

 stated has emerged only in comparatively recent 

 times. The suggestion thus made for Ulva or 



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