[V.] LYTHRUM, CUCURBITACEjE. 97 



but it can be so, though imperfectly, by its own 

 two sets of stamens, and by the shorter stamens of 

 the two other forms ; it can, therefore, be fertilised, to 

 use Mr. Darwin's expression, " legitimately " in two 

 ways, and " illegitimately " in four ways. The same 

 is the case with the other two forms, so that eighteen 

 modes of union are possible, of which six are natural 

 or "legitimate," twelve are illegitimate, and more 

 or less sterile. This case is therefore indeed most 

 complex. 



Mr. Darwin suggests {Jour. Linn. Soc, v. viii. 

 1864, p. 193) that the trimorphous condition of this 

 plant may be advantageous, because if it were dimor- 

 phous only there would be but an equal chance in 

 favour of any two plants being of different forms, and 

 therefore capable of self-fertilisation ; whereas, being 

 trimorphous, the chances are two to one. In the 

 cowslip and primrose, where large numbers of plants 

 grow together, this, he thinks, would not be so 

 material. 



The genus Lythrum is also remarkable for the 

 great difference* existing between different species. 

 For instance, L. grmfferi, like L. salicaria, is tri- 

 morphous ; while L. thymifolia is dimorphous, and 

 L. hyssophifolia is homomorphous. 



CUCURBITACE^. 



Of this order we have only a single species, the common Bryony 

 (Bryonia dioica). The flowers are dioecious, the males in small clusters, 

 pale yellow, about half an inch in diameter ; the females much smaller. 

 IBoth secrete honey. 



11 



