FEUITS AND SEEDS 243 



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 We can at present only say that hybridisation is more easily 

 e£fect6d the more nearly allied the parent plants are, whether 

 varieties of the same species or species of the same genus. 

 We might explain this by saying that the substances con- 

 tained in the pollen tube, and which have to act upon the 

 egg-cell, will be most similar in the case of kindred forms, 

 and will therefore more easily combine. But we have all 

 sorts of curious exceptions. Thus it has been observed that 

 the pollen of Orchis Morio produces no tubes on the stigma 

 of Orchis fusca, while the pollen tubes of Orchis fusca not 

 only penetrate to the ovules of Orchis Morio, but effect a 

 fertilisation of the egg- cell. 



With regard to the fertility of hybrids, it was formerly 

 held that their vegetative vigour was accompanied by sexual 

 weakness. In some cases the stamens, in others the ovules, 

 would be incompletely developed. Their fertility was, how- 

 ever, said to increase if they were fertilised by one of the 

 parent forms and became again more like it. These state- 

 ments must not, however, be regarded in the light of a fixed 

 rule, for they hold good, too, in the case of the production of 

 new varieties — that is, when flowers of the same species are 

 fertilised with pollen of the same species. 



Our entire method of cultivation tends to increase the steri- 

 lity of our cultivated plants. 



This is caused by the endeavour to grow vigorous plants, 

 in which the vegetative growth is stimulated by abundant 

 water and food supply, until the functioning powers of the 

 various organs are taxed to their utmost limit. Our culti- 

 vated plants exhibit the most luxurious foliage, the most 

 rapid growth, the most succulent tissues, the largest flowers, 

 and the most juicy fruits. But this all entails a retrogression 

 of the sexual reproduction, as is shown by the proliferation 

 and the doubling of flowers, in both of which processes the 

 stamens and carpels are transformed into sterile leaves. 



Such tendencies are inherited, and are manifest in the 

 offspring of all the most luxuriant of our cultivated plants, 

 without having been caused in any way by hybridisation. 



