Unbalanced Crosses 271 



have been able to ascertain, this hybrid never 

 produces seed. 



Another instance of an absolutely sterile 

 hybrid is the often quoted Cytisus adami. It 

 is a cross between the common laburnum 

 (Cytisus Labiirnum) and another species of the 

 same genus, C. purpureus, and has some traits 

 of both. But since the number of differentiat- 

 ing marks is very great in this case, most of the 

 organs have become intermediate. It is abso- 

 lutely sterile. But it has the curious peculiar- 

 ity of splitting in a vegetative way. It has been 

 multiplied on a large scale by grafting, and was 

 widely found in the parks and gardens of 

 Europe during the last century. Nearly all 

 these specimens reverted from time to time to 

 the presumable parents. Not rarely a bud of 

 Adam's laburnum assumed all the qualities of 

 the common laburnum, its larger leaves, richer 

 flowered racemes, large and brightly yellow 

 flowers and its complete fertility. Other buds 

 on the same tree reverted to the purple parent, 

 with its solitary small flowers, its dense shrub- 

 like branches and very small leaves. These too 

 are fertile, though not producing their seeds as 

 abundantly as the C. Laburnum reversions. 

 Many a botanist has sown the seeds of the latter 

 and obtained only pure common C. Laburnum 

 plants. I had a lot of nearly a hundred seed- 



