42 



DIMORPHIC LEAVES IX RELATION TO HEREDITY, 



more space is lost between the rows of small leaves and the large ones 

 than Ijctween tlie leaves in the rows. 



When a definite dimorphism exists the (Ufferences in the leaves 

 or branches are not merely physiological, but moiphological. There 

 are two kinds of leaves or of branches, not merely two conditions of 

 the same kind. Anisophylly presupposes only one kind of leaves, 

 but with a wide range of accommodation to external conditions. 



Inequalities of leaves due 

 to differences of exposure 

 to sunhght may be con- 

 sidered as a weaker form 

 of the same kind of accom- 

 modation shown in am- 

 phibious plants that pro- 

 duce either an aquatic or 

 an aerial form of foliage, 

 depending on the medium 

 in which they happen to 



grow. 



A term that has a more 



definite signification from 



the standpoint of heredity 



is dichogeny, defined by 



De Vries as follows: 



I mean [by dichogeny] all those 

 cases where the nature of an organ 

 is not yet decided during the 

 early stages of its development, 

 l)ut may yet be determined by 

 external influences. Thus, under 

 normal conditions the runners of 

 the potato plant form at their 

 tips the tubers, but on being ex- 

 posed to light, nr when the main 

 stem lias been cut off, they de- 

 volop into green slioots. * * * 

 In such cases it is clear that 

 the possibility of developing in 

 either of two different direc- 

 tions is dormant in the young primordia. For this very reason I should like to 

 api)ly the name dichogeny to this phenomenon. And it e\-idently depends upon 

 external influences what direction is taken. Therefore, a selection must take place 

 from among the availalde hereditary characters of the species, and this selection may 

 bo iufluence<l by artificial interference. For the theory of hereditary characters such 

 experiments are therefore of the highest interest.' 



» De Vries, II. Intracellular Pangenesis (translated from the German by C. Stuart Gager, Chicago), 

 1910, pp. l.'l-lf.. 

 221 



Yui. is.— Lateral l)ranch of tlic paper iiiulborry (lirou.sso- 

 netia), with leaves unequal in size (anisophylly) and diverse 

 in form (heterophylly). (Reduced.) 



