THE ALGJE 



95 



are unquestionably closely related, and probably lead up to the higher 

 green plants. The others are more specialized forms, probably de- 

 rived from the other Chlorophyceae, but not showing such obvious 

 relationships. The Conjugate are sometimes removed from the Chlo- 

 rophyceae, but this seems hardly warranted. As stated before, the 

 Characese differ much from the other forms, and might with propriety 

 be considered as a class, coordinate with all the other Chlorophyceae. 



Order I. Volvocaceae 



The Volvocaceae are at once distinguished from the other Green 

 Algae by the fact that their vegetative cells are ciliated, and the 

 plants are therefore actively motile. They may be either unicellu- 

 lar, or they are cell-families more or less intimately united. In the 

 genus Volvox the cells are congenitally united by protoplasmic 

 threads, and the whole should be considered as a single multicellular 

 organism, and not a colony of unicellular individuals. 



The cells of most Volvocaceae resemble closely the zoospores of 

 many of the higher Chlorophyceae. They are oval or globular in 

 form, surrounded by a membrane which may be of unmodified cellu- 

 lose, but is more often more or less 

 gelatinous and very thick (Fig. 65, 

 A, B). This membrane or envelope 

 is perforated to allow the two long 

 cilia to protrude. There is usuayy 

 a single large chromatophore present. 

 This is somewhat cup-shaped, and 

 vivid green in color. Imbedded in 

 the chromatophore is a large roundish 

 body, the pyrenoid, a structure fre- 

 quently found in the chromatophores 

 of the Algae, but whose nature is 

 still somewhat obscure. The pyre- 

 noid is of albuminous nature, and is 

 probably associated with the assimi- 

 lation of C0 2 by the chromatophore. 

 The pyrenoids are sometimes re- 

 garded as similar to the protein- 

 crystals, found as reserve-food in the 

 cells of many plants. The nucleus 

 lies in the cytoplasm within the 

 cavity of the cup-shaped chromato- 

 phore, and in the cytoplasm, near 

 the base of the two long cilia, are usually two small contractile 

 vacuoles. There is also present at the forward end a red pigment- 



FIG. 66. Successive stages in the 

 division of a gonidium of Pleodorina 

 Californica, seen from above at 

 the following hours: b, 3.20 P.M.; 

 c,4.15; rf,5.05; e, 5.40; /, the same 

 at 10.15 A.M. of the following day. 

 (After SHAW.) 



