DIMORPHISM. 336 



are really sexual forms of one and the same species, 

 ordinarily occurring on different plants, i. e. dioecious, 

 but occasionally formed on the same spike. The same 

 excellent observer has demonstrated J;hat the di- and 

 trimorphic forms of Primnlay of Ldnum, I/ijthmm, and 

 other plants forms differing mainly in the relative 

 length of the stamens and styles, are also connected 

 with striking differences in the number of perfect seeds 

 produced. The most perfect degree of fertility is 

 obtained when the stigma of one form is fertilised by 

 the pollen taken from stamens of a corresponding 

 height. On the other hand, when the union is, as Mr. 

 Darwin states, illegitimate, that is, when the pollen is 

 taken from stamens not corresponding in length to the 

 style, more or less complete sterihty ensues in the 

 progeny, sometimes even utter infertility, such as 

 happens when two distinct species are crossed, so 

 that, in point of fact, the offspring of these illegitimate 

 unions correspond almost precisely to hybrids.^ 



Mere variations of form arising from hybridisation 

 or other causes hardly fall within the limits of this 

 work, though it is quite impossible to say where varia- 

 tions end and malformations begin. There are, how- 

 ever, two or three cases cited by Mr. Darwin^ from 

 Grallesio and Risso to which it is desirable to allude. 

 Gallesio impregnated an orange with pollen from a 

 lemon, and the fruit borne on the mother tree had a 

 raised stripe of peel like that of a lemon both in colour 

 and taste, but the pulp was like that of an orange, and 

 included only imperfect seeds. Risso describes a 

 variety of the common orange which produces 

 " rounded-oval leaves, spotted with yellow, borne on 

 petioles, with heart-shaped wings ; when these leaves 

 fall off they are succeeded by longer and narrower 

 leaves, with undulated margins, of a pale green colour, 

 embroidered with yellow, borne on foot-stalks without 



The reader will find an abstract of Mr. Darwin's views in his work 

 on the * Variation of Animals and Plants,' vol. ii, p. 181. 

 Loc. cit., i, 336. 



