136 Chlorophycede 



The planogametes are usually very similar to the zoogonidia, although they 

 are generally somewhat smaller and there may be slight differences in the 

 ciliation. 



In those Chlorophycese with dissimilar sexual cells the female organ 

 consists of an oogonium, which is usually more or less spherical (CEdogoniacese), 

 sometimes attenuated into a beak (Vaucheria}, or more rarely produced at 

 the apex into a long, narrow tube, the trichogyne (Coleochsetaceas). At the 

 time of fertilization the oogonium contains only one female cell (the egg-cell 

 or oosphere), except in the Sphseropleacea?, in which there are numerous 

 oospheres, the oogonium having in this case been originally a coenocyte. The 

 oosphere is generally a spherical cell containing chloroplasts, and often with a 

 clear, colourless area at one side, known as the receptive spot. This spot 

 is, as a rule, directly opposite the opening of the ripe oogonium and it is the 

 point at which the antherozoid fuses with the oosphere. 



The male organ is known as the antheridium, and is usually unicellular. 

 In Cylindrocapsa and some species of (Edogonium, however, it may consist of 



Fig. 93. Various forms of antherozoids of Green Algae. A, Sph&roplea annulina (Both) Ag. ; 

 B, Volvox aureus Ehrenb. ; C, Aphanochsete repens A. Br. ; D, (Edogonium sp. ; E, Coleoch&te 

 pulvinata A. Br. ; F, Vaucheria sessilis (Vauch.) D.C. All x about 900. 



many cells. Each antheridial cell often gives rise to a number of male cells 

 or antherozoids (=* spermatozoids), but in the (Edogoniacese it gives rise to 

 two and in the Coleochsetacese to only one. The antherozoids may resemble 

 the asexual zoogonidia, but are always much smaller. They are commonly 

 pear-shaped, but may be elongate and almost rod-like. They possess two 

 cilia, which are generally inserted at the pointed end, but in Volvox globator, 

 Vaucheria, etc., the insertion is lateral (fig. 93 F), and in the (Edogoniacese 

 there is a small circlet of cilia around the narrower, anterior end. They 

 are as a rule faintly coloured, either yellow or green, and they often possess 

 a red pigment-spot. 



In all cases, with the exception of Aphanochsete, the oosphere is non- 

 ciliated and remains in situ in the parent plant, being fertilized within the 

 oogonium. The result of fertilization is an oospore. 



It is in the Volvocacese that the evolution of sexuality is so remarkably 

 displayed. There is in the different members of this group a complete series 

 of transition -stages from the simplest form of gamogenesis to the highest 

 type of oogamy, with antherozoids and passive oospheres. Even in the one 



