ADAPTATIONS FOR INTERCROSSING. 



237 



Salicaria. This was indicated by Vaucher in 1841, more par- 

 ticularly described by Wirtgen in 1848, but was interpreted by 

 Darwin, and the more recondite differences brought to notice, in 

 1864. 1 " The three forms may be conveniently called, from the 

 unequal length of their pistils, the Iong-st3'led, mid-styled, and 

 short-styled. The stamens also are of unequal lengths, and 

 these may be called the longest, mid-length, and shortest." 

 The pollen of the different classes of stamens is of two sorts as 

 to color, and of three as to size, the largest grains from the 

 largest stamens. " The pistil in each form differs from that in 

 either of the other forms, 

 and in each there are two 

 sets of stamens, different 

 in appearance and func- 

 tion. But one set of 

 stamens in each form 

 corresponds with a set 

 in one of the other two 

 forms. Altogether, this 

 one species includes three 

 females or female organs, 

 and three sets of male 

 organs, all as distinct 

 from one another as if 

 they belonged to different 

 species ; and. if smaller 

 functional differences are 

 considered, there are five 

 distinct sets of males. Two of the three hermaphrodites must 

 coexist, and pollen must be carried by insects reciprocally 

 from one to the other, in order that either of the two should be 

 fully fertile ; but, unless all three forms coexist, two sets of 

 stamens will be wasted, and the organization of the species as 

 a whole will be incomplete. On the other hand, when all three 

 hermaphrodites coexist, and pollen is carried from one to the 

 other, the scheme is perfect : there is no waste of pollen and no 



1 In an article On the Sexual Relations of the Three Forms of Ly thrum 

 Salicaria, in Jour. Linn. Soc. viii. 169. Also on the Character and Hybrid- 

 like Nature of the Offspring of the Illegitimate Unions of Dimorphic and 

 Trimorphic Plants. Ibid. x. 393, 1868. Reproduced and extended in his 

 volume entitled " Forms of Flowers," 1877. 



FIG. 470. Diagram of the flowers of the three forms of Lythrum Salicaria, in their 

 natural position, with the petals and calyx removed on the near side. The dotted 

 lines with the arrows show the directions in which pollen must be carried to each 

 stigma to ensure full fertility. (From Darwin.) 



