380 THE GERM-PLASM 



haps therefore suppose that an increase of temperature prevents 

 the ' winter ' halves of the double-determinants in question from 

 developing, while it is beneficial to the 'summer' halves; and 

 that conversely, the development of the 'summer' halves 

 remains stationary when the temperature is lowered, w'hile the 

 'winter' halves continue to develop. This may be illustrated, 

 as in the case of ordinary sexual dimorphism, by supposing a 

 determinant of the germ-plasm to be spherical, and to consist 

 of a 'summer' and a 'winter' half. This determinant would 

 remain unchanged throughout embryogeny, and even during 

 the entire caterpillar stage ; and the increased or diminished 

 temperature would only determine which half should outgrow 

 the other and prevent it from controlling the cell, at the 

 beginning of pupation, when the wings are formed. 



In Vanessa levana the males and females resemble each 

 other so closely in the pattern of the wings that they cannot be 

 distinguished from one another with any degree of certainty ; 

 but in many other seasonally dimorphic butterflies, sexual dimor- 

 phism is exhibited in the pattern and coloration of the wings, 

 and we must therefore assume that they possess double-deter- 

 minants consisting of ' male ' and ' female ' halves, each of 

 which is again subdivided into a ' summer ' and a ' winter ' half. 

 We do not know what factors determine the sex of butterflies, 

 but in many cases the determination is efl"ected early, for ovaries 

 and spermaries can be distinguished from one another in the 

 full-grown caterpillar. Thus, as in the case of Termites, the 

 decision concerning the subdivisions of the double-determinants 

 takes place subsequently to that with regard to the primary 

 halves. 



4. DiCHOGENY IN PLANTS 



De Vries has made use of the term ' dichogeny ' to describe 

 that form of dimorphism which becomes manifest when a 

 young vegetable tissue, under normal conditions, is capable of 

 developing in different ways according to the external influences 

 to which it is exposed. Shoots of ivy bear leaves on the side 

 which is exposed to the light, and roots on the opposite side ; 

 but if the plant is rotated, the same shoot will grow leaves on 

 the side which previously bore roots, and vice versa. The 

 stimulus due to light therefore apparently causes a group of 

 cells, which would have formed roots if they had been in the 

 shade, to give rise to leaves. 



