34 DIMORPHIC LEAVES IN RELATION TO HEREDITY. 



from the standpoint of heredity, as the variation toward simple leaves 

 in an individual plant of Triumph cotton growing under field condi- 

 tions, as already described on page 17. In the latter case the simple 

 leaves were not induced by the external conditions, or the effect was 

 limited to a single individual that must be supposed to represent an 

 unusually susceptible condition. And in such cases of individual 

 variation the change of expression is much more definite and perma- 

 nent than when the change is shared by a whole series of plants or by 

 plants of different kinds. The production of simple leaves on fruiting 

 branches of Egyptian cotton in the greenhouse represents a general 

 tendency to reduction of lobes manifested in many kinds of cotton 

 under such conditions.^ 



This distinction does not turn, primarily, on the amount of differ- 

 ence or the extent of the change of expression, but upon the manner 

 and permanence of the change, and the same is true of the changes of 

 expression that constitute the phenomena of dimorpliism. The result 

 in both cases is the production of entire leaves; but one case probably 

 represents a definite mutative variation, the other a readily reversible 

 environmental accommodation. Dimorphism and bud mutations 

 may also appear to accomplish the same result, in that two definitely 

 different kinds of branches are produced on the same plant, but in 

 the mutation the change is permanent, whereas the dimorphic changes 

 belong to the series of regular alternations, though maintained by the 

 plants themselves instead of being induced by changes of external 

 conditions. 



The substitution of vegetative limbs for fertile branches, as often 

 occurs in the cotton plant, inchcates that the external environment 

 is a factor in modifying expression even in distinctly dimorpliic char- 

 acters, though it is not definitely known whether increase of vegeta- 

 tive branches results from the formation of a different kind of buds 

 in the first place or represents a transformation of buds that had a 

 previous tendency to prothicc fruiting branches. There are indica- 

 tions that both kinds of changes occur, depending on the time when 

 the external conditions are changed. Although the normal course of 

 development follows regular steps it is often influenced profoundly 

 by external conditions, even with respect to characters that are known 

 to be subject also to mutative changes of expression, such as the 

 "cluster" character in cotton. The occurrence of mutative variation 

 has also been fountl to be influenced by external conditions, mutations 

 beinof much more numerous in some localities than others, in fields 

 planted with the same selected stock of seed. 



> Attention has been culled by Mr. T. 11. Kearney to the fact that similar modifications in oak leaves 

 growinR in shaded positions have been pointed out by Brenner in a paper on "Climate and Leaf in the 

 Genus Quercus." Flora, vol. 90, 19ff2, pp. U4-160. 



221 



