180 



THE ALG^E 



FIG. 175. Pleurococcus vul- 

 garis, a one-celled green 



Several cells illustrating the 

 method of cell division and 

 their association in small 

 groups 



a pyrenoid (Sec. 196). The nucleus can sometimes be seen in 

 the center of the cell. The cells are exceedingly resistant to 

 cold and drought, but under very severe conditions they protect 

 themselves by forming a heavy cell wall, thus becoming resting 

 cells. Sometimes the contents of a cell 

 break up into several daughter proto- 

 plasts, but as a rule the only method 

 of reproduction is by simple cell divi- 

 sion. Other forms of one-celled algae, 

 with more complicated methods of re- 

 production (by zoospores and gametes), 

 are frequently found associated with 

 Pleurococcus, but should be carefully 

 distinguished from this simple alga. 



Pleurococcus may seem almost as 

 simple as the one-celled blue-green alga 

 Gloeocapsa, but its cell structure with 

 a chromatophore and well-defined nucleus is far in advance of 

 the Cyanopliycece. 



215. Sphaerella and Volvox.* These forms are representatives 

 of one of the most interesting families of the green algse, the 

 Volvox family 1 (Volvocacece). The lowest members are one- 

 celled and resemble the flagellates (Sec. 204), but the higher 

 forms are cell colonies of remarkable structure and life histories. 

 The vegetative cells are motile, being always provided with two 



* To THE INSTRUCTOR : It is rather difficult to obtain material of the 

 Volvox family, and it cannot be depended upon for type study. Therefore 

 laboratory work on the reproductive processes in the algse can much better 

 be arranged with such types as Ulothrix, or Ulva, or some form of the Chce- 

 tophoracece, or Cladophora, CEdogonium, or Vaucheria and Fucus. But the 

 Volvocacece and Flagellata are so important to a conception of certain primi- 

 tive conditions of algal life that they should be discussed in any extended 

 general course. The fact that zoologists have found Volvox and its relatives 

 of interest should not deter botanists from making use of their own. 



1 For a detailed account of the Volvocacece see Goebel, Outlines of Classi- 

 fication and Special Morphology of Plants, and Engler and Prantl, Die Natiir- 

 lichen Pflanzenfamilien. 



