CHAPTER IV. — SPECIAL PHYSIOLOGY OF REPRODUCTION. 775 



a process is known as hyhrldisation, and the progeny as lnjhrids, 

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

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



Hybridisation hfts been but imperfectly investigated in tbe Thallophyta, but 

 it is known to take place between Fucus serratm S »*nd F. vesiculosu* ? : in 

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

 the allied genera Physcomitrium and Fnuaria ; F. hytirometrica $ x F. 

 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 Aspeiula ; 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, Dujitalis purpureo-bitea, the hybrid resulting 

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

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

 as Verbascum Lychnitii $ x phoeniceam J is used. 



Hybridisation is by no means equally common in all families ; 

 thus, amortg" Phanerogams, whilst it is common in such orders as 

 the CompositoD, Scrophulariacese, Polygonaceae, Salicaceae, and Or- 

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

 and quite exceptional in the Umbelliferee 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 Geraniaceae, 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 Iridacece the same 

 conti*ast 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 



1 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 bo duly fertilised by the male 

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



