DEVELOPMENT OF THE HORMONE CONCEPT 7 



Figure 1). So much stress was laid on the root formation 

 that the sweUings and callus were considered as ''being 

 much of the nature of roots" (Bk. IV, Ch. V). During the 

 next hundred years the physiological concept of correlation 

 seems to have been lost, the emphasis being placed on 

 morphology, i.e. upon the inherent nature of the tissues 

 themselves. The discovery of sieve tubes by Hartig and 

 von Mohl led them to the opinion that there was indeed a 

 downward-moving sap, and this was later proven by Han- 

 stein (1860). The content of this sap, however, was studied 

 from the \aewpoint of organic food materials rather than 

 that of its correlating functions. 



At this time began the period of rapid development in 

 plant physiology. The phenomenon of correlation was 

 studied in greater detail and Sachs (1880, 1882, 1893) 

 brought forward a complete theory, a modern version of 

 Duhamel's views, which covered most of the known facts 

 of morphogenesis and correlation, and can still be regarded 

 as a modern treatment of the subject. Sachs' great achieve- 

 ment was that he applied the laws of causality to morphol- 

 ogy. His starting point was the thesis that ''morphological 

 differences between plant organs are due to corresponding 

 differences in their material composition, which must be 

 already present at the time of initiation, even though at this 

 stage chemical reactions and other crude methods fail to 

 show any differences." To account for these differences he 

 assumed the existence of root-forming, flower-forming, and 

 other substances, which move in different directions through 

 the plant. For example, the former would be formed in 

 leaves, and would move towards the base of stems. If a cut 

 be made in a twig, this will be "an obstacle for further down- 

 ward movement," and roots will be formed above the cut. 

 Light and gravity were assumed to affect the distribution of 

 these special substances. With only two assumptions: 1, the 

 existence of organ-forming substances which, in minute 

 amounts, direct development, and 2, polar distribution of 

 these substances, — a distribution which may be modified by 



