34 GENERAL BOTANY 



result in curvature. If, therefore, seedlings are rotated so that 

 opposite sides of the stem and root are alternately placed in a 

 position of stimulation for shorter periods than are required for a 

 reaction, the organs will fail to respond to gravity. The time dur- 

 ing which an organ like a root must be continuously stimulated 

 on one side in order to secure a reaction is called presentation 

 time. In Sachs's experiment the presentation time to gravity was 

 too short in any given position of the growing stem and root tip 



to secure curvature, and these 

 c~~^ } a organs therefore grew in any 

 (/* i direction in which they were 



placed at the beginning of 

 the rotation. This proved 

 definitely the effect of grav- 



Fio.19. An experiment by Charles Dar- ^ aS a direc t iv ^ force in 



win, designed to locate the sensitive the growth of roots and 



portion of the root to gravity stems in the normal vertical 



a, uninjured roots of the bean extended position. It will thus be Seen 

 horizontally for twenty-three hours and , T ^ . , , , 

 thirty minutes; b, root tips of the bean that Knight and Sachs em- 

 touched with caustic and extended hori- ployed quite different metll- 

 zontally for the same length of time as , . , . 



those in a. After Charles Darwin ods 111 their experiments. 



Knight aimed to substitute 



another force for gravity in order to note its effect on root and 

 stem growth, while Sachs sought to neutralize and so eliminate 

 the effects of gravity. Knight's experiment proved that roots 

 and stems respond to an external force which can be controlled 

 and its effects therefore proved, while Sachs's experiment showed 

 conclusively that gravity was necessary for the downward 

 growth of roots and the upward growth of the stem. 



Darwin's work entitled " Tho Power of Movement in Plants " 

 greatly extended the observations already made on the sensitive- 

 ness of the root to gravity and other stimuli. His most important 

 contribution to the subject was the elaboration of the idea, 

 already discovered by Sachs, that the sensitive zone is located 

 in the very apex of the root. Fig. 19 illustrates Darwin's method 

 of locating the sensitive, or perceptive, zone of the root in the 

 root tip of the common garden bean ( Vicia faba). Darwin's 



