336 CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



and galvanometric positivity with expansion, the geotropic 

 curvature of the stem or the petiole is thus due to the joint 

 effect of contraction in the upper and expansion in the lower 

 half. Some difficulty may be encountered in finding a satis- 

 factory explanation of the two opposite reactions, but the 

 following considerations will help to remove it. The facts 

 established are : 



1. That the upper half of the horizontal shoot under- 



goes contraction under geotropic stimulation. 



2. That the lower half undergoes expansion. 



3. That the pressure on the protoplasm in the cells of 



the starch-sheath, exerted by the heavy particles, is 

 the cause of stimulation. 



4. That the particles press on the inner tangential 



ectoplasmic layer of the sensitive cells in the upper, 

 and on the outer tangential ectoplasmic layer in 

 the lower, half. A reasonable explanation of the 

 opposite reactions of the upper and lower halves 

 may probably be found in the unequal excitability 

 of the protoplasmic layer on different sides of the 

 sensitive cell. It will be shown (p. 356) that the 

 excitability of the ectoplasmic layer at the apical 

 end of the geo-perceptive cells is greater than at 

 the basal end. It will therefore be natural to 

 expect that a similar difference of excitability exists 

 between the ectoplasmic layers lining the inner and 

 the outer tangential walls of the ceUs (see fig. 189), 

 so that pressure of particles will induce maximal 

 stimulation in the former and subminimal stimula- 

 tion in the latter. 

 5. That while maximal stimulation induces retardation 

 of growth culminating in actual contraction, sub- 

 minimal stimulation causes expansion and enhance- 

 ment of the rate of growth (p. 83). 



Hence the opposite electric responses given by the upper 

 and the lower sides of the organ can be explained on the not 



