HISTORICAL REVIEW 13 



7. Further Analysis of Reactions in Plants to Light 



Sachs was the first to point out the inadequacy of the 

 explanation brought forward by De Candolle. He and 

 others found negative as well as positive plant structures in 

 which the rate of growth was retarded by increase of inten- 

 sity of light. The bending from the source of light in these 

 structures could therefore not be due to difference in rate 

 of growth on opposite sides induced by difference in illu- 

 mination. Sachs was already of the opinion that gravita- 

 tion does not control the direction of growth in plants by 

 difference in the direct effect on the upper and lower sur- 

 faces of the reacting organ as Knight had assumed. He 

 says (1887, p. 696)/ " That in geotropic curvatures the 

 important point is only as to the direction in which gravita- 

 tion acts on the part of the plant, and that it is not in any 

 way a matter of a stronger effect on the lower side and a 

 feebler effect on the upper side, requires no proof." He was 

 profoundly impressed by the similarity between the re- 

 actions to light and those to gravity. This together with 

 the inadequacy of the explanations of De Candolle and 

 Knight led him to the conclusion clearly expressed in these 

 words (1887, p. 695): " 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 impressed 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 helio- 

 tropism are to be looked upon as phenomena of irrita- 

 bility. 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 



* The original German edition appeared in 1882. Sachs first announced 

 his views on reactions to Hght in the preface of a paper by H. IMiiller in 

 1876. 



