342 CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



the ' starch-sheath.' The statohth-theory thus obtains 

 strong support from an independent hne of experimental 

 investigation. The statohth-theory has been adversely 

 criticised because in simpler organs geotropic action takes 

 place in the absence of statoliths. There is no doubt that 

 the weight of other cell-contents may in certain cases be 

 effective in geotropic stimulation ; it may nevertheless be 

 true that ' at a higher level of adaptation the geotropically 

 sensitive members of the plant-body are furnished with 

 special geotropic sense-organs — a striking instance of 

 anatomico-physiological division of labour.' ^ 



As previously stated, the electric response in different 

 layers can be successfully detected in vigorous specimens at 

 the proper season. Under less favourable conditions the 

 sensitivity may be found to have disappeared. 



Evidence of Insensitive Specimens 



I describe the various physico-chemical concomitants 

 which accompany the condition of relative insensibility. 

 I employed three different tests — the electric, the geotropic, 

 and the microscopic — by which the sensitive could be dis- 

 tinguished from the insensitive condition. 



Experiment 183. Electric test. — Later in the season the 

 geo-electric indications given by the various plants were 

 found to have almost disappeared. That the tonic con- 

 dition of the specimen was below par was independently 

 revealed by the response to the prick of the probe ; this, in 

 vigorous specimens, evokes an electric response of galvano- 

 metric negativity. But the response to prick in subtonic 

 specimens is quite different, being one of galvanometric 

 positivity. The prick-effect, in fact, often gave me an 

 indication as to the suitability of the particular specimen for 

 the observation of geo-electric response. 



Experiment 184. Test of geotropic reaction.- — I took 

 four different insensitive specimens of Bryophyllum and 



1 Haberlandtj ibid. p. ^gy. 



