SELF-STERILE PLANTS. 



11 



see why the pear flower, with its showy petals, its attractive nectar 

 and pollen, and its success in securing insect visits might easily be sus- 

 pected of requiring cross-fertilization. 



The various means by which cross-fertilization is insured and self- 

 fertilization avoided may be stated as follows: (1) By the separation ot 

 the sexes, in which the stamens are borne on one plant and the pistils 

 on another, as in hemp and most species of willows; (2) by special 

 mechanical contrivances which prevent the pollen of a flower from get- 

 ting on its own stigma or which favor the carrying of pollen by insects, 

 as in many orchids, Leguminosse, mints, etc.j (3) by the plants pro- 

 ducing on distinct individuals two and in some species even three forms 

 of flowers, with different lengths of stamens and pistils and different 

 kinds of pollen, as in Primula and Puhnonaria; (4) by a difference in 

 the time of maturity of the pollen and stigma in the same flower, as in 

 Scrophularia nodosa and Gentiana; (5) by more or less complete sterility 

 of the flowers to their own pollen, and a corresponding prepotency of 

 the pollen from another individual, as in the Lobelia, Reseda, Verbascum, 

 etc. 



Of these five means only the fourth and fifth are resorted to by the 

 pear; the fourth, however, is of minor importance, the main depend- 

 ence, as we shall see later, being placed on the fifth. This special 

 phase of cross-fertilization was first fully brought out in Darwin's Vari- 

 ation of Animals and Plants under Domestication, and was further 

 considered in his Cross and Self-Fertilization in the Vegetable King- 

 dom. But a number of observers had previously noted that some 

 plants were self-sterile. Kolreuter at the close of the last century 

 found that Verbascum phceniceum was sterile to its own pollen, but 

 fruited well when pollinated with four other species. In his two works 

 above mentioned Darwin cites a considerable number of cases in which 

 plants had been found to be more or less completely self sterile and 

 adds a number of species observed by himself. In a recently published 

 paper,* Focke has collected from Darwin, Miiller, and others all the self- 

 sterile species, and has made a number of additions, so that the num- 

 ber of such plants is increased to fifty-seven species, which are given in 

 the following list. It should be noted, however, that some in the list 

 are included on circumstantial evidence and not as the result of care- 

 ful experiments. 



* Abhandlungen heraiisgegeben vom naturwissenschaftlichen Vereine zu Bremen, 

 XII, Heft 3, pp. 409-416. Ueber Unfruchtbarkeit bei Bestiiubung mit eigenem Pollen. 



