CHAP. VIII. FLOWERS ON THE SAME PLANT. 299 



like those on distinct plants during the develop- 

 ment of species. 



It was therefore necessary to ascertain by experiment 

 what would be the effect of intercrossing flowers on 

 the same plant, in comparison with fertilising them 

 with their own pollen or crossing them with pollen 

 from a distinct plant. Trials were carefully made on 

 five genera belonging to four families ; and in only 

 one case, namely, Digitalis, did the offspring from a 

 cross between the flowers on the same plant receive 

 any benefit, and the benefit here was small compared 

 with that derived from a cross between distinct plants. 

 In the chapter on Fertility, when we consider the 

 effects of cross-fertilisation and self-fertilisation on the 

 productiveness of the parent-plants we shall arrive at 

 nearly the same result, namely, that a cross between 

 the flowers on the same plant does not at all increase 

 the number of the seeds, or only occasionally and to a 

 slight degree. I will now give an abstract of the 

 results of the five trials which were made. 



(1.) Digitalis purpurea. Seedlings raised from inter- 

 crossed flowers on the same plant, and others from 

 flowers fertilised with their own pollen, were grown in 

 the usual manner in competition with one another on 

 the opposite sides of ten pots. In this and the four 

 following cases, the details may be found under the 

 head of each species. In eight pots, in which the 

 plants did not grow much crowded, the flower-stems 

 on sixteen intercrossed plants were in height to those 

 on sixteen self-fertilised plants, as 100 to 94. In the 

 two other pots in which the plants grew much crowded, 

 the flower-stems on nine intercrossed plants were in 

 height to those on nine self-fertilised plants, as 100 

 to 90. That the intercrossed plants in these two latter 

 pots had a real advantage over their self-fertilised 



