THE PHEXOMEXA OF REVERSION 339 



ids do not reach both the daughter-nuclei ; but that both the 

 halves of some of them which result from the division pass into 

 the same daughter-nucleus. On the other hand, it might even 

 perhaps be possible for an entire idant to be transmitted to only 

 (^w^ of the daughter-nuclei. It must be possible for some such 

 irregularity to occur in the distribution of ids during nuclear 

 division, for otherwise the occurrence of a different combination 

 of the primary constituents in the course of growth — such as, 

 indeed, actually occurs in cases of bud-reversion — would be 

 inexplicable. 



Let us distinguish between those ids which correspond to 

 the original form, and those which have become modified, by 

 describing them respectively as 'green' and 'variegated' ids. 

 The reversion of a shoot must then be due to the unequal 

 distribution of the ids amongst the daughter- nuclei during the 

 divisions of the apical cells of the shoot, so that a majority of 



• green ' ids were distributed to a cambium cell containing 



• blastogenic ' germ-plasm, or even also to an apical cell of 

 a young lateral bud. Conversely, a variegated shoot might 

 subsequently originate from a green one, as in fact actually 

 occurs. 



I should be inclined to offer a similar explanation of the re- 

 versions to the ancestral form which so frequently occur in all 

 the numerous varieties of our trees and shrubs, such as, for 

 example, the oak-leaved heterophyllous hornbeam, the fern- 

 leaved oak, the varieties of the maple and birch which possess 

 greatly subdivided leaves, and the copper-beech and copper- 

 hazel. The tendency to revert is very varied even in the 

 different varieties of the same species. The golden-striped 

 variety of Euonyinus japonica, for instance, is very liable to 

 revert, while in the silver-striped variety of this plant reversion 

 rarely occurs. 



This difference would simply depend on the relative minority 

 of ancestral ids in the variety in cjuestion. Reversion will never, 

 or only rarely, occur when the idioplasm only contains a few- 

 ancestral ids ; if. however, there are so many of these that the 

 unmodified ids only form a small majority, reversion can easily 

 take place. 



In this connection I must mention another instance, which 

 has very frequently been discussed since Darwin's time, viz.. 

 that of the peculiar • graft-hybrid ' Cytisiis adami. The com- 



