I i 2 VARIATION 



against gravity, and with an energy sufficient to maintain it in 

 a fairly upright position, developing stems, branches, and leaves. 



Occasionally this latter geotropism fails, and the branches 

 hang downward, forming a "weeping" variety. This is espe- 

 cially common in the willow and the birch, though by no means 

 unknown in other trees, notably the elm, maple, and beech. 



" Cut-leaved " varieties and "fan tops" 1 occasionally arise 

 suddenly, all of which may be preserved by grafting or by bud- 

 ding, so that with proper attention we may have weeping, cut- 

 leaved, and fan-top varieties, not of a few but of many species 

 of trees and shrubs, although the readiness with which a partic- 

 ular mutation may appear in one species is no guaranty of its 

 appearance in another. 



A tree which has always before borne peaches may suddenly 

 bear nectarines, or more likely a single branch may make the 

 departure, the remainder of the tree continuing to bear peaches 

 as before. In any event the mutation may be propagated by bud 

 or possibly by seed, in which latter case a nectarine-bearing 

 tree results. This tree may bear nectarines all its life, or it may 

 occasionally bear peaches on the whole or a portion of its top. 

 The significant fact is that there is no intermediate between the 

 peach and the nectarine, and yet the one may arise at any time 

 from the other. 2 The mutation from peach to nectarine is clear- 

 cut and distinct, and the reversion from nectarine to peach 

 when it occurs is equally complete. 



The apricot appears to be related to the plum much as 

 the nectarine is to the peach. In both cases the main stocks 

 (peaches and plums) exist in many varieties, and the mutations 

 (nectarines and apricots) in but few. In the case of the former 

 (the peach) the main stock is downy, while the mutant is gla- 

 brous, or destitute of downy covering. In the latter, however, 

 the conditions are reversed, for the main stock (the plum) is 

 glabrous, while it is the mutant that is downy. 



1 A fan-top tree is one in which the branches are borne on opposite sides, after 

 the fashion of corn. In a small forest plantation belonging to the writer is a fan- 

 top linden, now grown to considerable proportions. 



2 Inasmuch as the peach is considered as the main race, the nectarine is said 

 to arise from the peach by mutation. Therefore when peaches are borne upon 

 nectarine trees the case is considered to be one of reversion. 



