474 The Physiology of Plants BOOK in 



by Czapek in 1898 ; he found that the geotropic bending 

 begins just behind the percipient apex. A similar localiza- 

 tion for response to contact stimulation was determined 

 by McDougal in 1897. 



In adult organs whose growth has ceased other mechanisms 

 were found. Sachs showed in 1865 that the adult pulvini 

 or motile organs of periodically motile compound leaves 

 can execute geotropic curvatures under appropriate condi- 

 tions, the mechanism being modifications of turgor and 

 not of growth. Sachs found also that when haulms of 

 grasses in which growth has ceased, are placed horizontally, 

 growth recommences under the stimulus of gravitation, 

 being measurable along the under sides of the swollen 

 nodes. 



The researches so far discussed left untouched a point 

 of considerable importance, viz. the nature of the stimulus. 

 We have no conception of the manner in which gravitation 

 produces its effect, nor what its attraction really is, but it 

 seems probable that in some way the effect of weight upon 

 the living substance of the sensitive cells may bring about 

 the stimulation. From 1886 onwards, researches were 

 directed to this question. In that year Berthold suggested 

 that the primary effect of gravity, as regards stimulation, 

 may depend on the passive sinking of the heavier parts. 

 Noll, in 1892, argued in a similar way that Knight's experi- 

 ment on the action of centrifugal force involves the presence 

 somewhere within the sensitive structure of bodies possess- 

 ing weight, or at any rate that the protoplasm must be able 

 to distinguish between varying pressures on different sides 

 of the cell. In 1900 Noll suggested more definitely that 

 a perceptive apparatus of some such nature must exist in 

 the ectoplasm. He thought it might consist of vesicles of 

 sensitive plasma filled with cell sap and containing a small 

 heavy particle in the fluid. In the same year Nmec and 

 Haberlandt, at about the same time, called attention to the 



