DEMONSTRATION OF HELIOTROPISM. 695 



made a long series of investigations during the summers of 1874 and 1875, with 

 much skill and with excellent results. 



The object of his investigation was suggested by theoretical considerations which 

 I then put forward as to the true nature of heliotropism. It was necessary to refute 

 the antiquated and no longer serviceable, though in its time very plausible view of 

 Pyrame de CandoUe. According to this, heliotropic curvature was to be looked 

 upon as the result of one-sided etiolations ; since shoot-axes in the dark or in shade 

 elongate more rapidly than when illuminated strongly on all sides, he assumed that in 

 heliotropic curvature the side turned away from the source of light grows more rapidly 

 than the illuminated side. This explanation seemed quite sufficient for ordinary 

 cases of positive heliotropism, but various other observers had already sought in 

 vain to explain negatively heliotropic curvature also on the basis of De CandoUe's 

 theory ; at all events it was possible among other things to assume that negatively 

 heliotropic organs, the above Mustard root, for example, or the aerial roots of Aroids, 

 grow more slowly in the dark than in the light. But already observations which had 

 been made in my laboratory by Wolkoff, contradicted this assumption, and Hermann 

 Miiller proved that as a matter of fact even negatively heliotropic roots, exactly like 

 positively heliotropic ones, grow more rapidly in the dark than in the Ught, and the 

 same resulted from much older observations by Schmitz on the negatively heliotropic 

 and spontaneously luminous mycelial strands of the Rhizomorphs. All this shows 

 that De CandoUe's theory, although it seems to explain positive heliotropism, can by 

 no means be applied to negative heliotropism. But positive and negative organs 

 agree as to their heliotropism completely, exactly as with respect to their geotropism, 

 only that the effects themselves are in each case opposite, positive or negative. After 

 I had previously established this agreement concerning positive and negative geo- 

 tropism, I could scarcely doubt any longer, from the whole position of affairs (which 

 of course can only be indicated here), that with respect to positive and negative 

 heliotropism, exactly the same would apply. It necessarily followed from this that the 

 standpoint assumed by De Candolle must be abandoned, and that the whole subject 

 of heliotropism must be looked at in an entirely different way — a view which 

 irhpressed me the more, since according to all the facts then known a striking 

 agreement exists between heliotropic and geotropic effects, and at the same time 

 I had even then come to see that geotropism and heliotropism are to be looked upon 

 as phenomena of irritability. In addition to these reflections, also, I came to the 

 conclusion that in heliotropic curvatures the important point is not at all that the one 

 side of the part of the plant is illuminated more strongly than the other, but that 

 it is rather the direction in which the ray of light passes through the substance 

 of the plant. This view became probable to me from the fact that even very thin 

 and in the highest degree transparent organs are able to make heliotropic curvatures 

 — that is, organs in which the side turned towards the source of light is only a little 

 more brightly illuminated than the other. Thus, for example, the root-hairs of the 

 Marchantias are negatively heliotropic, whereas the very thin and translucent sporan- 

 giophores of Mucor are positively heliotropic. Similar examples are to be found 

 also among highly organised plants : the thin stems of the Balsams are extremely 

 translucent, and yet they are at the same time strongly heliotropic, whereas, ac- 

 cording to De CandoUe's theory, it must be premised that heliotropism is the more 



