GRAFT-HYBRIDS 389 



the parents. Whether it resembles one or other parent, or is 

 roughly intermediate between them, will depend on whether 

 one parent contributes a greater proportion of dominant, or 

 more conspicuous, characters than the other. Common examples 

 of such hybrids in the wild state are afforded by various Willows, 

 Geum intermedium (Fig. 224, B), Quercus intermedia, etc. 



It has long been familiar to gardeners that shoots, which show 

 a mingling of the characters of scion and stock, are sometimes 

 produced as a result of grafting. The intermediate nature of 

 these so-called graft-hybrids appears to be due to the fact that 

 both scion and stock contribute to their development, the tissues 

 of the one forming a skin over those of the other. The character 

 of the seeds, and of the resulting offspring, is determined by the 

 plant responsible for the formation of the subepidermal layer 

 from which the archesporial tissue arises. This explains the 

 fact that the seeds of graft-hybrids always breed true to the 

 characters of either scion or stock. The commonest example of 

 such a graft-hybrid is that known as Cytisus adami, which is due 

 to the grafting of the Purple Broom (Cytisus purpureus) on the 

 Laburnum (C. laburnum). 



It has been repeatedly noted that the fusion of the nuclei 

 of the gametes appears to be the most important step in sexual 

 reproduction. This is supported by the fact that, in all the 

 higher plants, the male cell consists of little else than the nucleus 

 (cf. pp. 350, 368), and that, in hybrid-experiments, it is im- 

 material whether the one or the other parent is employed as 

 the male. 1 The nucleus has also been seen to play a very im- 

 portant part in the activity of the cell, and, when dividing, to 

 pass through a very complex series of changes. These have as 

 their outcome an equal distribution of the chromatic material, 

 which indeed is the only part of the nucleus that remains 

 recognisable throughout all the phases of division. It may 

 therefore be reasonably supposed that, in some way or other, 

 the chromatin is the carrier of the hereditary qualities of the 

 organism. 



Sexual fusion may then be regarded as operating in two 

 ways, firstly as a stimulus leading to further development, 



1 Some forms of CEnothera and Epilobium appear to constitute an 

 exception to this generalisation, 



