20 PLANT-LIFE ON LAND [ch. 



the surrounding water, each portion forming itself 

 into a separate and actively motile body, or zooid. 

 These minute naked bodies are slightly elongated 

 or pear-shaped, while long cilia are attached to the 

 narrower end of each, the active lashings of which 

 give it rapid movement from place to place (Fig. 2, C). 

 There are in Ulothrix three different types of the 

 zooids which thus escape into the water. Some are 

 relatively large, and may be produced singly without 

 division of the contents of the mother-cell, or more 

 commonly by division of them into two or more parts 

 according to the size of the mother-cell. They escape 

 through an opening of the cell-wall into the water 

 (Fig. 2, B). Each has four motile cilia attached 

 to its narrower end. A period of active movement 

 in water ensues, varying in length according to 

 circumstances. In this way these zoospores may 

 scatter in different directions, and travel a relatively 

 considerable distance. They finally settle upon some 

 solid substratum, forming each a new cell- wall and 

 grow out transversely to their former axis. A 

 rhizoid of attachment is put out in one direction, 

 and in the other there grows a new filament like 

 the original parent. There is thus a means both of 

 increase of numbers and of dispersion of the new 

 individuals. Other similar but rather smaller zooids 

 have been distinguished as micro-zoospores. They are 

 of the same type as the first, but are produced in 



