440 PRACTICAL BOTANY 



the resulting cells tend to round themselves off. Note 

 especially that in groups of four or more cells, a 

 splitting of the cell-wall at the centre of the group 

 is frequently to be observed, which is doubtless the 

 result of the tendency of the cells to assume a 

 spherical form. 



Select a single large cell for examination under the 

 highest power, and observe the following details 



1. The cell- wall is thick, and shows clearly a double 

 contour : staining with Schulze's solution gives a blue 

 coloration of the cell- wall. 



2. The protoplasmic contents, which at first sight 

 appear uniformly green, will show differentiation, 

 especially after staining with iodine solution : thus 

 there may be recognized 



a. A nucleus, which is usually central, though 

 sometimes it is in a lateral position. 



b. Chromatophores, a number of which together 

 form a partial envelope surrounding the nucleus : there 

 are no pyrenoids. 



c. A very scanty colourless protoplasmic basis, in 

 which these bodies are embedded. 



It is held that Pleurococcus is the resting stage of Chlamydo- 

 monas, an organism which differs from other Volvocinese in that 

 its motile cells are separate, and do not form a coenobium : there 

 is, however, still some uncertainty as to the details of the life- 

 history of this organism. Material of Pleurococcus should be 

 kept in water exposed to light, and observations made from time 

 to time to see the motile cells, with their limiting cell-wall and 

 active cilia : the latter are best seen after staining with iodine 

 solution. 



