GEEEN ALG^E. 



33 



Of these may be mentioned Zygnema (Fig. 19, A), with two 

 star-shaped chloroplasts in each cell, and Mesocarpus (Fig. 19, 

 B, D), in which the single chloroplast has the form of a thin 

 median plate. (_B shows the 

 appearance from in front, C 

 from the side, showing the 

 thickness of the plate.) Me- 

 socarpus and the allied gen- 

 era have the spore formed 

 between the filaments, the 

 contents of both the uniting 

 cells leaving them. 



Evidently related to the 

 pond scums, but differing in 

 being for the most part 

 strictly unicellular, are the 

 desmids (Fig. 20). They are confined to fresh water, and 

 seldom occur in masses of sufficient size to be seen with the 

 naked eye, usually being 

 found associated with 

 pond scums or other fila- 

 mentous forms. Many of 

 the most beautiful forms 

 may be obtained by ex- 

 amining the matter ad- 

 hering to the leaves and 

 stems of many floating 

 water plants, especially 



FIG. 19. Forms of Zyynemacese. A, 

 Zygnema. B, C, 1), Mesocarpus. 

 All x 150. 



the bladder weed ( Uiricu- 

 laria) and other fine- 

 leaved aquatics. 



FIG. 20. Forms of Desmids. A, B, Clos- 

 teriwn. C, D, D', Cosmarium. D, and 

 D' show the process of division. E, F, 

 Staurastrum ; E seen from the side, F 

 from the end. 



The desmids include the most beautiful examples of unicellular plants 

 to be met with, the cells having extremely elegant outlines. The cell 

 shows a division into two parts, and is often constricted in the middle, 



