624 Mutations 



broom-like variety of the Populus nigra, from 

 which it is distinguished by its erect branches 

 and other characters of minor importance. It 

 is often called the pyramidal or fastigiate 

 poplar. Its origin is absolutely unknown and 

 it occurs only in the cultivated state. In Italy 

 it seems to have been cultivated from the 

 earliest historical times, but it was not intro- 

 duced into other countries till the eighteenth 

 century. In 1749 it was brought into France, 

 and in 1758 into England, and to-day it 

 may be seen along roads throughout cen- 

 tral Europe and in a large part of Asia. But 

 the most curious fact is that it is only observed 

 in staminate specimens ; pistillate trees have not 

 been found, although often sought for. This 

 circTunstance makes it very probable that the 

 origin of the broom-like poplar was a sudden 

 mutation, producing only one individual. This 

 being staminate, it has been propagated ex- 

 clusively by cuttings. It is to be admitted, 

 however, that no material evidence is at hand 

 to prove that it is not an original wild species, 

 the pistillate form of which has been lost by 

 vegetative multiplication. One form only of 

 many dioecious plants is to be found in cultiva- 

 tion, as for instance some South American 

 species of Ribes. 



Total lack of historical evidence concerning 



