102 HYBRIDISING AND CROSS-BREEDING. 



or have descended from a common origin, and in their entire 

 system of growth are quite as normal and often as fertile 

 as well-authenticated species. Sterile hybrids that is, mules 

 which are incapable of bearing fertile seeds when fertilised with 

 pollen from a nearly-related species are very rare ; and some 

 of those which are reputed sterile produce fertile pollen, which 

 may be used to fertilise other species or varieties. Forsythia 

 viridissima, which rarely produces seed from its own pollen, 

 fruits freely when fecundated with that of F. suspensa ; while 

 the common garden Balsams require cross-fertilisation in order 

 to secure a prolific supply of fertile seeds. 



M. Nageli has pointed out that conditions of sexual vig- 

 our, or even absolute sterility, are no proof of hybridity, since 

 many individual plants of direct descent are sometimes sterile ; 

 indeed the whole question of fertility seems to hinge on con- 

 stitutional vigour and a proper supply of nutriment. While 

 many hybrids seem to have weakened reproductive powers, 

 crosses between different species of Calceolaria, Erica, Di- 

 anthus, and Pisum are more fertile than the original species 

 from which they have descended ; so that we also in some 

 cases obtain additional sexual vigour or fertility in hybrids 

 and their subsequent progeny, notwithstanding that sterility 

 and additional vegetative vigour or growth is in many cases 

 the rule. 



The late Professor Henslow wrote a most valuable treatise 

 on one of the hybrid Foxgloves (Digitalis), with a view to dis- 

 cover, if possible, the distinction between species and hybrids. 

 In this object, however, he failed, not from want of accuracy, 

 but simply because no such difference exists. 



Immediate Effects of Foreign Pollen. It is generally supposed 

 that the effect of crossing or hybridising is confined to the 

 embryo of the seed which has been vivified or quickened by 

 foreign pollen ; but there are at least two or three well-authenti- 

 cated cases in which the impression or influence of the strange 

 pollen has immediately altered the fruit or seed in general 

 appearance. 



Some interesting experiments bearing on the influence of 

 strange pollen on the form of fruit have been made by Maxi- 

 mowicz. (See Jour. Royal Hort. Soc., vol. iii. (new series), p. 

 161 et seq.} The species experimented on were Lilium davuri- 

 cum and Lilium bulbiferum. The pollen of each species was 

 applied to the stigmas of the other species, the process being 

 repeated upon several individual plants. The result was that 

 the capsules borne by the several plants were found to have 

 the form characteristic of the pollen parent ; while the form of 



