208 EEV. G. HENSLOW — FERTILISATION OF PLANTS. 



on that account ; but the pea and others may be regarded as 

 exceptions which have become self- fertilising instead. 7. Highly- 

 self- fertile forms may arise under cultivation. In cultivating the 

 Ipomcea purpurea for three years, not one single plant of the self- 

 fei-tilised appeared taller than the crossed ; in every indi\-idual case 

 the intercrossed was taller than the self-fertilised ; but in the 

 third generation one plant grew to a greater height, and Mr. Darwin 

 says he was so much surprised that he saved the seed to see what 

 would happen. However, it only beat its competitor by '6 p.c, 

 that is, the ratio of the intercrossed to the self-fertilised was 

 100 to 100-6 ; which is practically one of equality. He called that 

 plant " Hero," and saved the seed, to see whether the seedlings 

 would show any tendency to be taller than the intercrossed. The 

 descendants did exert a power of growth quite equal to the ordinaiy 

 intercrossed, and became more fertile than is usually the case, for 

 they had a higher average of seeds per capsule than in any other 

 cases. No benefit followed from intercrossing, and not even any 

 benefit from crossing with new stock ! So that this was a very 

 remarkable individual, for its descendants had great self-fertilising 

 powers and always beat their competitors. When one studies the 

 details of the experiments with the different species, there is 

 scarcely a single table where there are not three or more self- 

 fertilised seedlings which beat their competitors. So that Hero 

 was not by any means an exceptional case of a self-fertilised 

 plant growing taller and being more vigorous than the others. 

 Indeed it seems to have been a very common thing indeed. 

 Hero was the first, and therefore Mr. Darwin studied that in- 

 dividual. 8. Special adaptations occur for self- fertilisation. We 

 have had so much literature on the subject of intercrossing pub- 

 lished of late years, that the peculiarities of self-fertilisation 

 have been rather neglected. Take, for instance, the two common 

 mallows, Malva rotundifolia and sylvestris; the latter is a con- 

 spicuous large flowering species, and is " proterandrous," i.e. the 

 stamens all shed their pollen long before the styles and stigma 

 rise up and are ready to receive it ; so that it cannot possibly be 

 self-fertilising. Malra rotundifolia, on the other hand, is self-fertile : 

 the styles rise up and are mature at the same time as the stamens. 

 They curl backwards, inserting the stigmas amongst the anthers, 

 and thus secure the pollen on their own stigmas, and the plant is 

 self-fertilised. The pansy ( Viola tricolor) does not usually set seed 

 of itself, but requires to be intercrossed. It is a self-sterile plant. 

 Hermann Miillor, who has especially studied these conditions, 

 found some very small and inconspicuous varieties, in wliich 

 the stigma was turned towards the stamens, and received the 

 pollen directly from them. It will be found to be a general 

 rule that small inconspicuous forms are self-fertilising, while 

 larger and brightly-colounnl flowers require insects. Inconspicuous 

 flowers are therefore pr()bal)ly always self-fertile, without, how- 

 ever, precluding tlie possi])ility of their being crossed. I have 

 examined a very large number of our common weeds, and I 



