226 PART III. THE CLASSIFICATION OF PLANTS. 



,tinm ; process, fertilisation; product, a fructification termed a 

 .cvstocarp (Rhodophycese). 



The evolution of sexuality can be well traced in the Algae. Thus 

 in Ulothrix (Confervoideae) two kinds of zoogonidia are produced, 

 microzoogonidia which are small, macrozoogonidia which are larger; 

 the latter always germinate independently, but the microzoogonidia 

 conjugate in pairs to form a zygospore, though, if they fail to 

 conjugate, they are capable of independent germination ; they 

 are, in fact, imperfectly sexual gametes. In Ectocarpus (Pheeo- 

 sporeae) also the sexuality of the gametes is imperfect. In most 

 cases the microzoogonidia are incapable of independent germina- 

 tion, and are therefore completely sexual cells (planogametes). 



The evolution of sex can also be well traced in the Algse by a 

 comparison of the sexual cells of the higher and lower forms. In 

 Ulothrix, for instance, the gametes are similar both in their form 

 and in the part which they take in the sexual process. In 

 Ectocarpus the gametes are externally similar, but they do not 

 behave alike in the process of conjugation, for some come to rest, 

 whilst others remain motile, as a preliminary to that process 

 which consists in the fusion of a motile with a resting gamete ; 

 hence the former is considered to be male, the latter to be female. 

 In Cutjeria the gametes differ in size, and, though they are all mo- 

 tile at first, the larger gametes soon come to rest when conjugation 

 follows; in this case difference of sex is indicated from the first by 

 the larger size of the female gametes. InFucus the differentiation 

 is carried still further, in that the female gamete (now called an 

 oosphere) is not ciliated nor motile, and is much larger than the 

 motile male gamete (now called a spermatozoid) ; but both cells 

 are extruded, from the organs in which they are formed, into the 

 water. In the typical oogamous forms (e.g. Vaucheria, Coleo- 

 chaete, Volvox, Chara) there is the further and final stage of 

 sexual differentiation, that the oosphere is not extruded, but re- 

 mains in the female organ (oogonium), whereas the spermatozoids 

 are set free, and, still retaining the character of planogametes, 

 swim by means of cilia, ultimately entering the oogonium and 

 fertilising the oosphere. 



Sexual difference may, however, exist without being accom- 

 panied by any external differentiation. For instance, the sexual 

 cells of Acetabularia, Ulothrix, and Dasycladus, are in each case 

 externally similar planogametes ; but conjugation only takes 

 place in Acetabularia and Ulothrix, between planogametes de- 



