154 VEGETABLE PHYSIOLOGY. 



and deposited on the stigma of the flower a portion of its own pollen. Finally, the 

 corolla of primroses contains many tiny hemipterous insects, of the genus Thrips, 

 which, moving about the flower in all directions, transport from the anthers to the 

 stigma the pollen which adheres to them ; by which means again the plant will be 

 self-fertilized. 



In the fertilization of dimorphic species four operations are thus possible : 1, 

 self-fertilization of the long-styled flower; 2, self-fertilization of the short-styled 

 flower ; 3, fertilization of the short- by the long-styled ; and 4, that of the long- by the 

 short-styled : the two first Mr. Darwin calls homomorphic ; the two others, hetero- 

 morphic. 



Mr. Darwin has artificially fertilized flowers in these different ways, by pro- 

 tecting them from insects, and he has found in the wild primrose (Primula veris) and 

 in the Chinese primrose (Primula Sinensis) that heteromorphic unions produce con- 

 siderably more capsules and good seeds than homomorphic unions. Thus prim- 

 roses present two sets of individuals, which, although belonging to the same species, 

 and both possessing stamens and pistils, are mutually dependent on each other for 

 perfect fertilization. Mr. Darwin concludes that Nature, in establishing dimorphism 

 in primroses, and in distributing the two forms in equal numbers of individuals, has 

 evidently had in view the crossing of distinct individuals ; the relative heights of the 

 anthers and stigmas obliging insects to deposit the pollen of one set on the stigma 

 of the other. Nevertheless it is impossible not to admit that the stigma of the 

 visited flower may receive its own pollen. Now it is a well-known fact, that if the 

 pollen of several varieties fall on the stigma of one individual, that of one of the 

 varieties is prepotent, and its pollen takes effect to the exclusion of that of all 

 the others. Mr. Darwin thinks that it may be inferred from this that in primroses 

 the heteromorphic pollen, which is known to be the most potent, will overcome the 

 action of the homomorphic pollen whenever the two come into collision ; thus, he 

 adds, indicating the efficacy of dimorphism in producing crosses between individuals 

 of the two forms. These two forms, although both bearing stamens and pistils, are 

 in this case truly dioecious ; each of them is fertile, though the pollen of each is less 

 potent on its own stigma than on that of the other form. 



Mr. Darwin has studied dimorphism in the different species of Linum, and he 

 has instituted on L. ymndiflorum and perenne a series of experiments which confirm 

 the preceding conclusions. 



The scarlet-flowered L. cjrandiflorum has also the two types of long and short- 

 styled flowers ; in the short-styled form the five stigmas diverge, project between 

 the filaments, and rest against the tube formed by the petals. In the long-styled 

 form, on the contrary, the stigmas are erect, and alternate with the anthers. Mr. 

 Darwin selected twelve flowers of two long-styled individuals which he fertilized 

 heteromorphically, i.e. with pollen from the short-styled form ; most produced good 

 capsules and seeds; those which were not touched remained absolutely sterile, 

 although their stigmas were covered with a thick layer of their own pollen. He 

 next sought to ascertain the probable cause of this sterility, by placing the pollen of 

 a short- sty led flower on the five stigmas of a long-styled flower, and after thirteen 



