CHAPTER IV. SPECIAL PHYSIOLOGY OF EEPEODDCTION. 775 



a process is known as hybridisation, and the progeny as hybrids, 

 the hybrid being distinguished as a variety- hybrid, species-hybrid, 

 or genus-hybrid, according to circumstances (see p. 457). 



Hybridisation has been but imperfectly investigated in the Thallophyta, but 

 it is known to take place between Fucus serratus $ and F. vesiculosus $ : in 

 the Mosses some species-hybrids are known, and a few genus-hybrids (between 

 the allied genera Physcomitrium and Fnnaria ; F. hygrometrica $ X P. 

 pyriforme $ ) : in the Ferns variety-hybrids are common, and several species- 

 hybrids are known : in Phanerogams variety-hybrids and species-hybrids are 

 common, and several genus-hybrids are known (e.g. between Verbascum and 

 Celsia ; Brassica and Eaphanus ; Galium and Asperula ; Campanula and 

 Phyteuma; Gymnadenia and Nigritella ; Epiphyllum, Cereus,andPhyllocactus). 

 Species-hybrids are usually designated by a combination of the specific names 

 of their parents ; as, for instance, Digitalis purpureo-lutea, the hybrid resulting 

 from the fertilisation of D. lutea with pollen of D. purpnrea. When it is desired 

 to state accurately which is the male and which the female, such an expression 

 as Verbascum Lychnitix ? x phceniceum $ is used. 



Hybridisation is by no means equally common in all families ; 

 thus, among Phanerogams, whilst it is common in such orders as 

 the CompositjB, Scrophulariaceae, Polygonaceae, Salicaceae, and Or- 

 chidaceae, it is comparatively rare in the Cruciferae and Labiatse, 

 and quite exceptional in the Umbelliferse and Leguminosee. And 

 within the limits of any one natural order the capacity for hybridi- 

 sation is not the same in all genera ; thus, in the Geraniacese, the 

 genus Pelargonium possesses it in a high degree, whilst Geranium 

 and Erodium do not; in the Caryophyllacese, Dianthus readily 

 hybridises, but Silene does not ; and in the Iridaceoe the same 

 contrast exists between Gladiolus and Crocus ; and the same 

 applies to the various species of any given genus. Nor are all 

 hybridisations equally fertile as estimated, in Phanerogams, by 

 the number of sound seeds produced, and by the vigour and 

 fertility of the resulting hybrid-plants ; the general rule being 

 that the more remote the two species hybridised, the more delicate 

 and the less fertile will be the offspring, and in fact many hybrids 

 are altogether sterile ; in any case they are more fertile with 

 pollen from one of the parent-forms than with that of other 

 similar hybrids. It is only when the relation between the parents 

 is such as to ensure the appropriate degree of sexual affinity that 

 the product of the union is a vigorous and fertile progeny. 



It is the rule that hybridisation is reciprocal that is, that if the 

 oospheres of a species A can be duly fertilised by the male 

 gametes of a species B, the oospheres of the species B are equally 



