216 THE STRUCTURE OF FLOWERS. 



Besides the more obvious differences in the relative 

 lengths of the styles and filaments* of heterostyled flowers, 

 the rule is for the stigmas of the long-styled to be larger or 

 longer than those of the short-styled, f and to have their 

 papillae longer and broader. 



Thus in the nine species of Primula described by Mr. 

 Darwin, in two only were the stigmas nearly alike in both. 

 Of three species of Linum, L. flaviini alone had an appre- 

 ciable difference in the stigmas. In Pulmonai'ia qffi.cinalis 

 and Polygonum fagojpyrum, Forsythia suspensa and ^giphila 

 elata, it was not, or scarcely appreciable. 



Again, besides those mentioned there were twenty species 

 in which the stigmas of the long-styled were markedly 

 superior to those of the short-styled. 



of plants in a wild state being constantly self-fertilised, as Miiller, 

 and, indeed, Mr. Darwin himself has shown to be the case. Thus, 

 he gives two lists, of forty-nine species in each, (Cross and Self Fert., 

 etc., pp. 3 57 and 365), one of self-sterile, the other of self-fertile plants, and 

 adds, " I do not, however, believe that if all known plants were tried in 

 tlie same manner, half would be found to be sterile within the specified 

 limits; for many flowers were selected for experiment which presented 

 some remarkable structure ; and such flowers often require insect aid " 

 (I.e., p. 270). The proportion of self-sterile plants is, in fact, extremely 

 small. Miiller remarks, e.g., of the highly differentiated order 8cro2ohu- 

 larinece, that " in default of insect-visitors, self -fertilisation takes place 

 in most forms ; and in only a few are insect-visits, and consequently 

 cross-fertilisation, so far insured that self-fertilisation is never required 

 and has become impossible." Similarly of Lahiatce he says, " Self- 

 fertilisation seems to be rendered impossible only in the species of 

 Nepeta, Thymus, Mentha, and Salvia described". (Fertilisation, etc., pp. 

 464 and 503). Moreover, while Mr. Darwin includes the Fox-glove and 

 Linaria vulgaris among his sterile plants, Miiller considers them both 

 to be self -fertilising. 



* Exceptions occur, thus Cordia and Linum grandiflorum have little 

 or no difference in the length of the stamens. 



t Leucosmia Burnettiana is remarkable for having the stigma of the 

 short-styled form the more papillose (Forms of Flowers, p. 114). 



