128 SCIENCE PROGRESS 



characters of both parents, the purple broom and the laburnum. 

 This was not confined to the flowers ; leaves and stem also 

 showed an intermediate structure. There could be no doubt 

 that this Cytisus Adami, as the new plant came to be styled, was 

 of hybrid origin ; and if the account of Adam were correct it 

 would seem that there could be equally little doubt that it had 

 arisen during, or subsequent to, the process of grafting. That 

 doubt has for various reasons been thrown on Adam's story is 

 no reflection on the individual, but merely results from the 

 general fact that practical men keep no exact records of the 

 plants passing through their hands, and in particular do not 

 carry out their operations under the carefully controlled con- 

 ditions which modern science demands. 



Apart from actual proof there are strong reasons supporting 

 the "graft" origin of the hybrid. Perhaps the most important 

 is the fact that no attempt to raise seed from the laburnum 

 pollinated by the purple broom, or from the purple broom 

 pollinated by the laburnum, has ever succeeded : the two plants 

 are mutually absolutely sterile. And then we have the very 

 remarkable behaviour of the hybrid itself. Cytisus Adami is 

 now a common ornamental shrub ; it has been possible to 

 multiply the original shoot indefinitely by grafting, and its 

 appearance and characteristics are widely known and may be 

 studied in numerous private gardens. Its most striking feature 

 is that it does not maintain the intermediate character in all its 

 branches. From a shoot of typical Adami will arise, apparently 

 without reason, a branch of pure laburnum, or one which 

 possesses the characters of the purple broom alone. Not only 

 this, but even a single flower will show one half hybrid, and 

 the other belonging to one of the parents : a single petal may 

 have the same mixed constitution, or a single leaf. This 

 production of " vegetative throwbacks " is extremely rare in 

 sexual hybrids. 



Striking as these facts are, so eminent an authority as Prof. 

 De Vries wrote in his "Species and Varieties" that there was 

 no evidence to show that Cytisus Adami had not arisen as a 

 sexual hybrid. Against the fact that the two parents are 

 mutually sterile he points out that although, since 1829 C. pur- 

 pureas has been grafted on C. Laburnum many thousands of 

 times, yet in no single case has this resulted in the formation of 

 a hybrid. That such had occurred on a single occasion is just as 



