THE ORIGIN OF SEX IN PLANTS 13 



themselves, growing and dividing. But sooner or later 

 the protoplasmic contents of some of its cells, usually after 

 division into two or more parts, may escape through a hole 

 in the cell-wall, as motile naked bodies, into the water in 

 which the plant lives. In fact they pass from the stationary 

 encysted state, where each formed part of the fixed plant- 

 body, to that corresponding to the motile Euglena. Some- 

 times the whole protoplasm of a cell escapes without 

 division, but more commonly it divides into two or more 

 parts. According to the number of divisions, the motile 

 bodies differ in their size. If the protoplasm is un- 

 divided, or has divided into a small number of parts such 

 as two or four, these escape into the water through an 

 opening in the wall, as relatively large, pear-shaped zoo- 

 spores ; this name is given to them because they show 

 active movements, due to the lashing of four delicate 

 cilia attached to the transparent pointed end (Fig. 6, C). 

 Each zoo-spore consists of a small mass of living proto- 

 plasm, with a nucleus, a green chlorophyll-body, and a 

 red eye-spot ; features which it shares with Euglena, 

 though differing in the number of the cilia. After a 

 period of movement in water it settles, draws in its cilia, 

 and forms a cell-wall. This encysted cell may then 

 germinate, and divide to form a new multicellular filament 

 like the parent. This is a vegetative or non-sexual mode 

 of increase, which merely secures multiplication, together 

 with dispersal of the new plants by the movement of the 

 zoo-spores. 



But in other cases the division of the protoplasm of 

 the cells of the Ulothrix-fLlameut may be carried further ; 

 consequently the parts will themselves be smaller, though 

 more numerous ; and when these escape as before, they 

 appear similar in form to the zoo-spores, but only with 



