154 Regeneration 



piece is excised between the head and the pharynx a 

 new head is formed at the oral end, a new tail at the 

 opposite end, and in the middle of the remaining old 

 tissue a new mouth and pharynx is formed. How does 

 the ** whole" suppress all this formative power in the 

 part before the latter is isolated? It almost seems as 

 if the isolation itself were the emancipation of the part 

 from the tyranny of the whole. The explanation of this 

 tyranny or of the correlation of the parts in the whole 

 is to be found, however, in a different influence. The 

 earlier botanists. Bonnet, Dutrochet, and especially 

 Sachs, ^ pointed out that the phenomena of correlation 

 are determined by the flow of sap in the body of a 

 plant. These authors formulated the idea that the 

 formation of new organs in the plant is determined by 

 the existence of specific substances which are carried 

 by the ascending or descending sap. Specific shoot- 

 producing substances are carried to the apex, while 

 specific root-producing substances are carried to the 

 base of a plant. When a piece is cut from a branch of 

 willow the root-forming substances must continue to 

 flow to the basal end of the piece, and since their further 

 progress is blocked there they induce the formation of 

 roots at the basal end. Goebel^ and de Vries have 



* V* Sachs, J., "Stoff und Form der Pflanzenorgane," Gesammelte 

 Abhandlungen, 1892, ii., 1160. Arbeiten a. d. bot. Inst. Wurzburg, 

 1880-82. 



' Goebel, K., Einleitung in die experimentelle Morphologic der PJlanzen, 

 1908. 



