Il8 DYNAMICS OF LIVING MATTER 



spectrum are more effective heliotropically than the less refractive red 

 and yellow rays. There exists thus apparently a division of labor, 

 the longer light waves accelerating assimilation, the shorter waves ac- 

 celerating heliotropism. This can be demonstrated with the aid of 

 screens, inasmuch as behind red screens the plants assimilate well, 

 while they do not bend or bend only slowly, toward the source of light ; 

 while behind a blue screen they bend actively toward the light, their 

 assimilation being diminished. 



We call organisms which bend or grow toward the source of light 

 positively heliotropic or phototropic, and those that bend or grow away 

 from it negatively heliotropic. 



As far as the mechanism of the heliotropic bending is concerned, 

 we must remember that in most cases it occurs most effectually in the 

 tips of branches or roots. As this region is also the growing region, 

 botanists frequently state that the process of heliotropic bending is a 

 function of growth. This, however, is certainly not true for grasses, 

 in which the bending occurs in the nodes which are flexible, while in the 

 less flexible internodes no bending occurs. It seems therefore as though 

 the phenomena of growth were not essential in the heliotropic reaction, 

 and that the reason that the tips react better to light than the older 

 parts is perhaps due in part to the fact that the latter are not so soft 

 and flexible. 



How can light bring about heliotropic curvatures? Let us suppose 

 that light strikes a plant on one side only, or more strongly on one side 

 than on the opposite side, and that it be absorbed in the superficial 

 layers of tissue of that side. In this case we assume that on that side 

 certain chemical reactions occur with greater velocity than on the 

 opposite side. What these reactions are is unknown; we may think 

 provisionally of oxidations. This change in the velocity of chemical 

 reactions either produces a tendency of the soft elements on that side 

 to contract a little more than on the opposite side, or creates otherwise 

 a greater resistance to those forces which have a tendency to elongate 

 or stretch the plant, e.g. hydrostatic pressure inside the cells, or 

 imbibition of certain tissue elements. The outcome will be that one 

 side of the stem will be stretched more than the opposite side, and this 

 will bring about a curvature of the stem. Where the latter is soft at 

 the tip, the bending will occur only, or chiefly, in that region; and as 

 the degree of softness decreases rapidly from the tip downward, the 

 result will be that the tip will bend toward the source of light. This 

 result may possibly be aided by a greater photosensitiveness of the 

 extreme tip of the stem, although I am not aware that this is an estab- 

 lished fact. 



