330 SELF-STERILE PLANTS. CHAP. IX, 



with that from a distinct plant ; but it has been found 

 more convenient to keep them for separate discussion. 

 The present cases must not be confounded with those 

 to be given in the next chapter relatively to flowers 

 which are sterile when insects are excluded ; for such 

 sterility depends not merely on the flowers being 

 incapable of fertilisation with their own pollen, but on 

 mechanical causes, by which their pollen is prevented 

 from reaching the stigma, or on the pollen and stigma 

 of the same flower being matured at different periods. 



In the seventeenth chapter of my 'Variation of 

 Animals and Plants under Domestication 'I had occasion 

 to enter fully on the present subject ; and I will there- 

 fore here give only a brief abstract of the cases there 

 described, but others must be added, as they have an 

 important bearing on the present work. Kolreuter 

 long ago described plants of Verbascum pliceniceum 

 which during two years were sterile with their own 

 pollen, but were easily fertilised by that of four other 

 species ; these plants however afterwards became more 

 or less self-fertile in a strangely fluctuating manner. 

 Mr. Scott also found that this species, as well as two of 

 its varieties, were self-sterile, as did Gartner in the 

 case of Verbascum nigrum. So it was, according to 

 this latter author, with two plants of Lobelia fulgens, 

 though the pollen and ovules of both were in an 

 efficient state in relation to other species. Five species 

 of Passiflora and certain individuals of a sixth species 

 have been found sterile with their own pollen; but slight 

 changes in their conditions, such as being grafted on 

 another stock or a change of temperature, rendered 

 them self-fertile. Flowers on a completely self-im- 

 potent plant of Passiflora alata fertilised with pollen 

 from its own self-impotent seedlings were quite fertile. 

 Mr. Scott, and afterwards Mr. Munro, found that some 





