Volvocacece 185 



hyaline in character and often contains a prominent pigment spot 

 and two (sometimes more) contractile vacuoles. The pulsation of 

 these vacuoles is alternate. There is a single nucleus, usually 

 occupying a central position in the cell. One chloroplast is present 

 containing one or more pyrenoids, and although its form is very 

 variable, it is generally more or less confined to the broader 

 posterior end of the cell. 



Multiplication takes place by the division of the contents of a 

 mother-cell into 2, 4, or 8 daughter-cells, and, as the latter are new 

 motile individuals, the vegetative division in this family is strictly 

 homologous with the asexual reproduction by zoogonidia which is 

 found in the other groups of the Chlorophycese. In all the repro- 

 ductive processes in this family the plane of the first cell-division 

 is usually at right angles to the longitudinal axis of the cell. 



Sexual reproduction occurs in most of the forms by the union 

 of isogamous planogametes formed in a similar manner to the 

 vegetative daughter-cells, but in greater numbers. In some of 

 the higher forms there is a marked heterogamy, sexual reproduc- 

 tion being brought about either by the union of heterogamous 

 planogametes or by the fusion of antherozoids and oospheres. In 

 the zygospores or oospores the pigment spots of the two gametes 

 disappear, but the chloroplasts remain distinct and the cell-wall 

 becomes cuticularized. Germination takes place after a period of 

 repose. 



As one passes from the lower to the higher members of the 

 Volvocacese there is a more striking progressive evolution of forms 

 than is exhibited in any other family of Algae. There is a gradual 

 replacement of isogamy by heterogamy, ultimately reaching the 

 highest condition of oogamy, and associated with this is an increase 

 in the number of cells and size of the coenobium, accompanied by 

 a differentiation of vegetative from reproductive cells. 



The plants of this family are of great interest on account of 

 the fact that they are the connecting links between the lower 

 forms of the green Algse and the Flagellata, a Protozoan group 

 which exhibits a mixture of animal and vegetable characters, and 

 from which it is now generally recognized that the Chlorophyceae 

 have been evolved. There can be no doubt, however, in the mind 

 of anyone who has carefully studied the VolvocaceaB, that their 

 characters are such as to clearly separate them from the Flagellata 

 and place them as a family of the green Algse. 



