Chap. VIII. CONSTITUTIONAL VIGOUR. 2S5 



CHAPTER VIII. 



Difference between Crossed and Self-fertiliscd Plants in 

 Constitutional Vigour and in other respects. 



Greater constitutional vigour of crossed plants The effects of great 

 crowding Competition with other kinds of plants Self-fertilised 

 plants more liable to premature death Crossed plants generally 

 flower before the self-fertilised Negative effects of intercrossing 

 flowers on the same plant Cases described Transmission of the 

 good effects of a cross to later generations Effects of crossing 

 plants of closely related parentage Uniform colour of the flowers 

 on plants self-fertilised during several generations and cultivated 

 under similar conditions. 



Greater constitutional Vigour of crossed Plants. As in 

 almost all my experiments an equal number of crossed 

 and self-fertilised seeds, or more commonly seedlings 

 just beginning to sprout, were planted on the oppo- 

 site sides of the same pots, they had to compete 

 with one another ; and the greater height, weight, and 

 fertility of the crossed plants may be attributed to 

 their possessing greater innate constitutional vigour. 

 Generally the plants of the two lots whilst very young 

 were of equal height; but afterwards the crossed 

 gained insensibly on their opponents, and this shows 

 that they possessed some inherent superiority, though 

 not displayed at a very early period of life. There 

 were, however, some conspicuous exceptions to the 

 rule of the two lots being at first equal in height ; thus 

 the crossed seedlings of the broom (Sarothamnus 

 scojmrius) when under three inches in height were more 

 than twice as tall as the self-fertilised plants, 



