ALG^E. 



23 



of the cell, especially if it has first been treated with iodine, the nucleus 

 may be found. The size of the cells, as well as the number of chloro- 

 plasts, varies a good deal. 



With a little hunting, specimens in various stages of division may be 

 found. The division takes place in two ways. In the first (Fig. 11, .B), 

 known as fission, a wall is formed across the cell, dividing it into two cells, 

 which may separate immediately or may remain united until they have 

 undergone further division. In this case the original cell wall remains as 

 part of the wall of the daughter cells. Fission is the commonest form of 

 cell multiplication throughout the vegetable kingdom. 



The second form of cell division or internal cell division is shown at C. 

 Here the protoplasm and nucleus repeatedly divide until a number of 

 small cells are formed within the old one. These develop cell walls, and 

 escape by the breaking of the old cell wall, which is left behind, and takes 

 no part in the process. The cells thus formed are sometimes provided 

 with two cilia, and are capable of active movement. 



Internal cell division, as we shall see, is found in most plants, but only 

 at special times. 



Closely resembling Protococcus, and answering quite as well for study, 

 are numerous aquatic forms, such as Chlorococcum (Fig. 12). These are 

 for the most part destitute of a firm cell wall, but are imbedded in masses 

 of gelatinous substance like many Cyanophycece. The chloroplasts are 

 smaller and less distinct than in Proto- 

 coccus. The cells are here oval rather 

 than round, and often show a clear 

 space at one end. 



Owing to the absence of a definite 

 membrane, a distinction between fis- 

 sion and internal cell division can 

 scarcely be made here. Often the cells 

 escape from the gelatinous envelope, 

 and swim actively by means of two 

 cilia at the colorless end (Fig. 12, B}. 

 In this stage they closely resemble the 

 individuals of aVolvox colony, or other 

 green Flagellata, to which there is little 

 doubt that they are related. 



There are a number of curious forms common hi fresh water that are 

 probably related to Protococcus, but differ in having the cells united hi 

 colonies of definite form. Among the most striking are the different 

 species of Pediastrum (Fig. 11, Z), E), often met with in company with 



FIG. 12. Chlorococcum, a plant 

 related to Protococcus, but the 

 naked cells are surrounded by a 

 colorless gelatinous envelope. 

 A, motionless cells. B, a cell 

 that has escaped from its enve- 

 lope and is ciliated, x 300. 



