THE ALG^E 97 



(Phacotus, Pteromonas, etc.), distinguished by having the cell- membrane firm, 

 and often composed of two parts. 



Volvocese. The more striking Volvocacese belong to the third family. These 

 are always multicellular. The simpler forms (Gonium, Pandorina) consist of 

 (usually) sixteen similar cells (Fig. 65) inclosed in a common envelope. By 

 repeated bipartition of these cells new cell-families arise which are set free by 

 the softening of the gelatinous matrix of the mother-family. In the genera 

 Pleodorina (Fig. 65, C) and Volvox two kinds of cells are present, small ones 

 incapable of division, and much larger ones (gonidia) which by division give 

 rise to the new plants. In Pleodorina the gonidia are ciliated, and except in 

 size closely resemble the small vegetative cells. In Volvox the number of goni- 

 dia is small (rarely over 10-12), and they are many times larger than the other 

 cells, which may be several thousand in number. The multiplication of the Vol- 

 vocacese is sometimes extremely rapid, so that the water of small ponds may be 

 colored green by the multitude of these swimming colonies. 



In Pleodorina and Volvox the repeated division of the gonidium soon results 

 in the formation of a hollow sphere in which the cells are at first in contact, but 

 separate more and more with the development of the mucilaginous cell-walls. 

 The interior of the fully developed spherical plant is filled with soft mucilage. 

 In Volvox the protoplasts of the cells remain connected by extensions of proto- 

 plasm, but no such communication can be demonstrated in Pleodorina. 



Sexual Reproduction. The Volvocacese show very beautifully the gradual 

 evolution of the sexual cells. 



In such forms as Sphserella and Pandorina there is very little difference 

 between the sexual and non-sexual cells, and the gametes are alike (Fig. 67, B). 

 In the genus Eudorina certain cells assume the function of eggs, while others 

 undergo division into numerous much smaller elongated cells, the male cells or 

 spermatozoids. The latter penetrate into the gelatinous envelope of the female 

 plant, where they come in contact with the egg-cells and effect their fertilization. 

 In Volvox (Fig. 67, D, E) the egg-cells, which exactly resemble the non-sexual 

 gonidia, are quite destitute of cilia, and very much larger than the spermato- 

 zoids, which are largely composed of nuclear substance. 



In all cases the product of fertilization is a resting-spore which becomes 

 invested with a heavy membrane, and usually assumes a brown or red color. 

 This spore on germination produces either single zoospores (Chlamydomonas, 

 Pandorina), or its contents, by repeated bipartition, give rise to a multicellular 

 individual like those produced from the gonidia (Eudorina, Volvox). 



Affinities of Volvocaceae 



The Volvocaceae are probably directly related to the Flagellata, 

 and through these show affinity with the lower animal forms. In- 

 deed, they are actually claimed by some zoologists as animals. 

 Their very evident relationship with the Protococcoideae, and through 

 these with the higher plants, however, indicates that although they 

 have certain resemblances to animals, they are, in all essential 

 respects, true plants, and probably represent the starting-point for 

 the line of development leading up to the higher green plants. It 

 is, however, among the simpler forms, like Chlamydomonas, that we 

 are to look for the connection with the Protococcoideae, and not 

 among such highly specialized forms as Volvox. 



