222 THE STRUCTUKE OF FLOWERS. 



diclinism. Mr. Darwin tliouglit " a very dry station 

 apparently favours the presence of the female form,"* i.e. 

 a lessened vegetative vigour tends to check the development 

 of the corolla and stamens, especially if a low temperature 

 accompanies it ; just as, conversely, we have seen how a 

 high temperature enhances it. Mr. Hart thus found that, 

 with Nepeta Glechoma, all the plants which he examined near 

 Kilkenny were females ; while all near Bath were hermaph- 

 rodites, and near Hertford both forms were present, but 

 with a preponderance of hermaphrodites. f 



Both Miiller and Mr. Darwin offer theories to account for 

 the origin of these gynodioecious plants. 



Miiller, after quoting Hildebrand's view, which he rejects, J 

 says,§ " Of the flowers of the same species growing together, 

 the most conspicuous are first visited by insects, and if the 

 flowers on some plants are smaller than on others, perhaps 

 owing to scanty nourishment, they will generally be visited 

 last. If the plant is so much visited by insects that cross- 

 fertilisation is fully insured by means of protandrous dicho- 

 gamy, and self-fertilisation is thus rendered quite needless, 

 then the stamens of the last-visited small-flowered plants 

 are iiseless, and Natural Selection Avill tend to make them 

 disappear, because the loss of useless organs is manifestly 

 advantageous for every organism. 



" This explanation rests upon the hypotheses, (1) that 

 the flowers of those sjiecies in which small-flowered female 

 plants occur together with large-flowered hermaphrodite 

 plants are plentifully visited by insects and are markedly 



* Forms of Floivers, p. 301 



t Mature, 1873, p. 1G2; and see below, p. 239. 

 X FertiUsatiiw., etc., p. 473. 



§ L.c, p. 484. Compare his remarks on Scahiona arvensis, I.e., pp. 

 310, 311. 



