DIMORPHISM AND POLYMORPHISM 381 



The assumption, which is in accordance with de Vries's ideas, 

 that all the hereditary tendencies of the species are contained 

 in every cell of the ivy-shoot, and that those which concern the 

 leaves only undergo development in response to the stimulus 

 caused by the light, and those corresponding to roots only to 

 that produced by the shade, does not materially help us in the 

 solution of the problem. In point of fact, the same cells are 

 not capable of forming roots and leaves ; the leaves are much 

 less numerous than the short and closely aggregated roots, and 

 therefore a large number of cells, or groups of cells, which give 

 rise to roots when shaded, do not develop leaves when exposed 

 to the light; they consequently contain no 'leaf-determinants.' 

 Hence the same idioplasm cannot produce roots when shaded 

 from, and leaves when exposed to, the light ; but the deter- 

 minants for the roots or leaves respectively must be distributed 

 very differently in the cells. 



The predisposition to form either of these two structures is 

 obviously determined in the growing point or apex of the shoot. 

 The cells which are continually being produced by the apical 

 cells are destined at a very early stage to form the rudiments of 

 roots or leaves ; only a certain number of cells on the illumi- 

 nated side are provided with leaf-determinants, while much 

 more numerous cells on the shaded side are furnished with 

 root-determinants. The determination therefore takes place at 

 a very early stage, when the shoot is actually in an embryonic 

 condition : and the degree of illumination determines which side 

 is to be provided with leaf- and which with root-determinants, 

 — each kind being distributed in accordance with a different 

 law, — as well as the side to which the groups of each kind of 

 determinants are to pass during the nuclear divisions in the 

 cells arising from the apical cells. This case is analogous to 

 that of the inversion of the viscera in man, except that we do 

 not know the cause of this change of position. Some influence, 

 however, must in this case also be exerted during the early 

 embryonic stages, and cause the liver to take up a position on 

 the left side, and the spleen and heart on the right, long before 

 these parts are actually formed. No subsequent influences 

 could cause the liver to become shifted from the right to the 

 left side, or could transform the liver into the spleen ; just 

 as in the ivy-shoot no influences can lead to the formation of 

 leaves on the shaded side when it is once covered witli roots, 



