CHAPTER IV. SPECIAL PHYSIOLOGY OF REPRODUCTION". 773 



It must not, however, be concluded that because there is no 

 observable difference between a gamete and an asexually-produced 

 spore, there is no difference whatever between them ; on the 

 contrary it is clear that they differ widely, since the former 

 cannot (except in the rare cases of parthenogenesis), whilst the 

 latter can, develope into a new organism. 



The second question is as to the nature of sex : what is the 

 difference, if any, between a male and a female gamete ? In 

 some cases there is a marked external difference ; for instance, in 

 the Pteridophyta, Bryophyta, and many Algae, the female gamete 

 is a large motionless oosphere, whilst the male gamete is a small 

 actively-swimming spermatozoid. But this marked difference is 

 not essential, it is merely adaptive ; it is an adaptation to a more 

 or less aquatic mode of life or, at least, of fertilisation. Moreover, 

 it is obviously inapplicable in explanation of the many cases in 

 which the two conjugating gametes are externally quite similar 

 (isogamous plants, see p. 80). Nor has minute microscopic in- 

 vestigation brought to light any distinguishing criterion. Bat it 

 must not be concluded on this account that there is no difference 

 between a male and a female gamete; it is obvious that there is 

 an essential physiological difference between them. For, were it 

 otherwise, it would be impossible to account for such a fact, for 

 instance, as that even where, as in many Algae, the gametes are 

 all extruded into the water, fusion never takes place between two 

 male or two female gametes, but only between a male and a 

 female. 



Brief allusion may be made to the means by which the sexual 

 process is ensured. It might be thought that the most effectual 

 means would be the development of the male and female organs 

 in close propinquity on the same individual. No doubt this is 

 the case, but the result is to ensure the less advantageous mode 

 of the process, the mode of self-fertilisation ; in fact, in many 

 cases in which the male and female organs are thus developed close 

 together as in monoecious plants (p. 86), self-fertilisation is pre- 

 vented by dichogamy (for instance, the prothallia of Ferns, p. 

 399). The real problem is, then, to ensure a sexual process 

 between two gametes derived from distinct individuals. The end 

 is attained either directly, by bringing the diverse gametes 

 together; or indirectly, by bringing the spores together, and 

 consequently also the gametophytes. 



The method of bringing the two gametes together is essenti- 



