298 A TEXTBOOK OF BOTANY [Ch. VI, 6 



so much simpler, as cleistogamous flowers attest? There 

 is good evidence, based on direct experiments by Darwin, 

 that more and stronger offspring are produced by cross- 

 pollination than by close-, meaning a cross between different 

 plants, for a cross between different flowers on the same 

 plant is little if any better than close-pollination. Thus an 

 advantage must inhere in cross- over close-pollination, which 

 means of course in cross- over close-fertilization, pollination 

 being only a preliminary mechanical step to fertilization. 

 This superiority, in light of the well-known evolutionary 

 principle of the survival of the fittest, amply explains the 

 prevalence of cross-fertilization in plants, and its exclusive 

 use in the higher animals. But why is cross- superior to close- 

 fertilization ? As to this, there is still much doubt, but we 

 can gain some light by translating the matter into terms 

 of the chromosome mechanism. Close-fertilization must 

 necessarily give to the offspring two sets of chromosomes 

 identically alike, precisely as in case of asexual reproduction 

 (page 300), while cross -pollination gives to the offspring two 

 sets of chromosomes which differ somewhat. Hence cross- 

 fertilization has the same superiority over close- that close- 

 has over none ; or to put the matter in another way, cross- 

 fertilization is apparently necessary in order to realize the 

 full benefits of fertilization. As to the advantages of fer- 

 tilization over asexual reproduction we have already spoken 

 (page 286). One must regret that such fundamentally im- 

 portant matters cannot as yet be satisfactorily explained, 

 but they cannot. The future, however, will surely yield 

 their interpretation. 



6. Methods of Asexual Reproduction 



While most reproduction in plants involves fertilization 

 and sex, there is much that is purely asexual. Sexual and 

 asexual methods are by no means mutually exclusive, but 

 commonly exist together in the same plants, either side by 

 side or in a form of alternation. 



