382 THE GERM-PL.\SM 



It therefore seems to me that the same cells cannot give rise 

 to two kinds of structures in this case, but that some groups 

 form roots, and others leaves ; and that, moreover, the idio- 

 plasms of these groups of cells differ from, and cannot be trans- 

 formed into, one another, but that the sides of the shoot which 

 produce roots and leaves respectively, are determined by the 

 influence of the light as soon as the first embr3-onic rudiments of 

 the shoot is formed. 



The fact that rudiments of roots can actually be proved by 

 the aid of the microscope to exist in the tissue of the stem in some 

 plants, seems to me substantially to support this view. This is 

 the case in the willow, in which the power of producing roots 

 from cuttings exists in a high degree. But numerous other 

 plant: are also capable of multiplying by means of cuttings, and 

 we may suppose that in them, in spite of tlie absence of visible 

 root-germs, the determinants for the tissue of the roots are 

 present, and are ready to develop into germs whenever external 

 influences leading to the formation of roots come into play. 



All cases of dichogeny, however, cannot be explained in this 

 manner. In some instances the same cells may actually develop 

 in different ways, and the idioplasm may therefore become 

 transformed by external influences. A transformation as re- 

 gards the dorso-ventral arrangement of the parts in a young 

 shoot of Tluija takes place when it is turned round so that the 

 relative positions of the upper and under surfaces are reversed.* 

 The cells which, under ordinary conditions, take on the form 

 of palisade cells, then assume the structure of the cells on the 

 lower surface, and vice versa. 



It appears to me that this fact is to be explained by suppos- 

 ing that the determinants of these two forms of cells are both 

 present in each cell, but that only one or the other of them 

 becomes active, according to the degree of illumination. I can, 

 however, form no idea as to why such an arrangement is met 

 with in this case. 



* Compare the statements made on this subject by Detmer, Biolog. 

 Centralblatt, Vol. VII., No. 23. 



