418 The Physiology of Plants BOOK in 



It can hardly be spoken of as opposing Pfliiger's position, 

 as it was not of the same broad general character. It took 

 into account only the vegetable organism, and dealt with 

 hypothetical substances for whose existence there was no 

 chemical evidence. In its final form in 1893 Sachs' sug- 

 gestion was that special stimulating and constructive sub- 

 stances are formed in the metabolic processes in the leaves. 

 He was inclined to hold that one of such special stimulating 

 substances is necessary for, or operative in, the formation 

 of stem, root, and leaf respectively, a particular one indeed 

 for every form of growth. To produce the differentiation 

 of stem and root he considered that two such stimulatory 

 substances, unlike each other, are to be regarded as existing 

 in the axis of the plant, and that the differentiation is 

 brought about owing to their becoming separated by the 

 action of gravity. 



Sachs' generalization has no chemical support ; he could 

 say nothing of the nature of his hypothetical stimulatory 

 substances, but based his view upon a few specially striking 

 instances of response to chemical stimulation. He originally 

 attributed a great share in the formation of the different 

 stimulating materials to the character of the food. 



Vochting opposed Sachs' position in papers published in 

 1885 and 1899-1900, claiming that his observations do not 

 bear the interpretation he gave them. The same view was 

 held by Reinke in 1897. Pfeffer also was an opponent of 

 Sachs' hypothesis ; in his Pflanzenphysiologie he took the 

 ground that the theory involves a disregard of the mechan- 

 ism of auto-regulation, whereby any definite result is always 

 due to the co-operation of various factors; and further, 

 that different combinations including the same factor may 

 lead to dissimilar results. 



It will be seen that these speculations on the part of 

 the botanists do not carry the question beyond the point 

 at which Pfliiger left it in 1875. 



