286 REPRODUCTION 



In some instances the cause of the failure of the pollen of one 

 plant to fertilise the ovules of another may possibly be due to 

 the want of power of the pollen-grain to develop pollen-tubes 

 long enough to reach from the stigma to the ovules within the 

 ovary ; or the tissues of the style may offer some mechanical 

 obstruction to the advancing pollen-tubes. In most cases, how- 

 ever, it would appear that there is some other quite unknown 

 cause at work which prevents the living substance, composing 

 the reproductive cells of certain plants, from exercising a fertilis- 

 ing influence on each other. 



When the relationship between the male and female repro- 

 ductive cells is too close, and also when it is too remote, fertility 

 is reduced. For the production of the most vigorous and the 

 most prolific progeny there must be a certain degree of difference 

 between the reproductive cells which unite. 



As pointed out previously (p. 279) the most fertile sexual 

 union takes place between the reproductive cells of flowers 

 which arise on diiferent individual plants of the same species. 

 The progeny resulting from such cross - fertilisation grow 

 luxuriantly and produce numbers of seeds capable of giving 

 rise to equally robust offspring. 



It is also found that well-marked, wild and cultivated varieties 

 and races of the same species of plant generally cross readily : 

 thus, the cross-pollination of diff"erent varieties of wheat, barley, 

 turnips, apples, carnations, roses and other plants, results in the 

 production of offspring. The progeny arising from cross-fertilisa- 

 tion between two varieties or races of the same species are termed 

 cross-breeds, or variety-hybrids. 



Variety-hybrids usually possess the following characters : — 



(i) They are often more luxuriant and robust in constitution 

 than their parents ; their roots are frequently extensive and the 

 shoots and leaves large. 



(ii) They usually grow more rapidly, flower earlier, and 

 produce a larger number of flowers than the parents. 



