ANIMAL AND PLANT REPRODUCTION 27 



Those of lesser size also germinate and produce new 

 individuals, but these are small and their growth slow. 

 Only the smallest are incapable of carrying on their vege- 

 tative functions. These come together in pairs and fuse. 

 Two individuals become one as a prerequisite to renewed 

 vigor. Vegetative spores become gametes. Something 

 valuable — ^speed of multiplication — is given up that some- 

 thing more valuable in the general scheme of evolution 

 may be attained. 



This is indeed an alluring genesis of sex. It is rather a 

 genesis of sex, however, than the genesis of sex. Various 

 manifestations of sex are present in other widely sepa- 

 rated groups of unicellular or simple filamentous plants, 

 the Peredmeee, the Conjugatce and the Diatomece — the 

 ConjugatcB being indeed the only great group of plants in 

 which there is no long continued asexual reproduction. 

 In these forms' one cannot make out such a good case of 

 actual gametic origin, but the circumstantial evidence of 

 sex development in parallel lines is witness of its para- 

 mount importance. 



After the origin of sex, many changes in reproductive 

 mechanisms occurred in plants, but most of them resulted 

 merely in better protection for the gametes, in increased 

 assurance of fertilization, in provision for better distri- 

 bution, or in greater security for the young plant. 



First, perhaps, there was physiological differentiation 

 of the gametes. At least such an interpretation may be 

 given to the form of conjugation found in Spirogyra and 

 other ConjugatcB, where, either by solution of the wall 

 separating them, or by the formation of a tube-like out- 

 growth of one or both cells so that the ends touch, the 

 contents of one cell pass over to the other. We may 



