THE EVOLUTION OF SEX IN PLANTS. 



33 



Sphaerella a large number of gametes (32-64) are formed in a mother 

 cell. These sexual cells are quite similar in size and form, and when 

 they conjugate no one could assert a difference in sex. Most species 

 of Chlamydomonas resemble Sphaerella in having gametes essentially 



Fig. 2. Gametes and gametangia of the Volvocaceae; a, Chlamydomonas Steinii; b, 

 Pandorina ; c, Eudorina ; d, Volvox. (a, after Goroschankin ; b, Pringsheim ; c, Goebel.) 



similar (see Fig. 2, a), but there are forms in this genus presenting 

 a marked advance. In Chlamydomonas Braunii* the gametes are of 

 two sizes and the smaller always unite with the larger. However, 

 both are ciliate and consequently motile, so that morphologically they 

 are similar, although there can be no doubt of the sexual differentia- 

 tion. It is not customary to call the female cell an egg until it has 

 lost its free swimming possibilities and as a passive cell awaits the 

 specialized motile sperm. 



Pandorina, like Sphaerella, produces a large number of gametes, 

 16-32 in the mother cell, but here there is a considerable range of 

 variation in the size of the sexual elements, although the form is always 

 the same. Sometimes these gametes will pair, a small one with a 

 larger (see Fig. 2, b), as in Chlamydomonas Braunii, thus showing 

 the tendency towards sexual differentiation. However, there is no 

 rule in this habit of Pandorina, for frequently gametes of equal size 

 conjugate, and one can not assert that the larger cells are always 

 destined to be female sought by smaller male elements. What de- 

 termines the variation in size of the gametes in Chlamydomonas 

 Braunii and Pandorina, and the consequent differentiation in sex? 

 It. depends entirely on the number of gametes formed in the mother 

 cells and on the size of the latter. The larger female gametes are 

 formed less numerously and general^ in larger mother cells and conse- 



* Figured in Popular Science Monthly, November, 1901, p. 67. 



