No. 522] 



PLANT RESPONSES 



over the implied meaning of "tropism" as applied to 

 animals seems to have arisen mostly through the attempts 

 of Loeb and his followers to simplify the conception of 

 the sensitive processes, to conceive of the stimulus as 

 directly producing the reaction, as one might think of a 

 heliotropic curve in a marine annelid produced directly 

 by the sun's rays shortening the muscles on the sunward 

 side. The controversy among the zoologists wages about 

 the terms direct and indirect. Indeed, the banners of the 

 opposing forces may be said to bear respectively but 

 these two mottoes. On the one side the herald has pro- 

 claimed these terms to the satisfaction of the botanists; 

 that is, all sensitive reactions are indirect; that is. they 

 are complex, the stimulus producing a change in the pro- 

 toplasm, local or wide-spread, and this excitation of pro- 

 toplasm producing an impulse, over a lesser or greater 

 distance, setting in operation, or releasing, energy which 

 brings the visible response. On the other side, the terms 

 are but obscurely defined, and the nearest we come to a 

 definition is that the excitation and the reaction are local. 



Among plants there may be cases in which the excita- 

 tion and reaction are local, and there are certainly eases 

 in which a considerable part of the plant is affected. 

 Rothert, experimenting with the coleoptile of the oat. 

 found that he could induce positive heliotropism when 

 only the basal, motile, part was illuminated, the rest be- 

 ing shaded; or he could obtain the same reaction when 

 the basal part was shaded and only a millimeter of the 

 tip of the leaf was illuminated. In the former case, for 

 aught we know, the whole process was local ; in the latter, 

 it certainly was not local. Yet there is no doubt in our 

 minds that the processes were the same in both cases. 

 In both cases there were excitation, conduction, reaction ; 

 only in the latter case, the excitation was in a different 

 part of the plant, and the conduction was over a greater 

 stretch than in the former. Yet it is over the differences 

 which these two illustrations show that the controversy 

 has arisen over direct and indirect response. A case 



