338 CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



of the stem of Impatiens made shortly after it had been 

 placed in a horizontal position. The starch-grains are 

 seen to have fallen on to the inner tangential wall of the 

 statocysts which abut on the phloem. Geotropic curvature 

 has been explained to be due to the joint effect of con- 

 traction of the upper and expansion of the lower side. 

 What is the difference in the stimulating mechanism of 

 the geo-perceptive cells of the upper and lower halves ? 

 Taking a seedling of Eclipta which is highly sensitive to 

 geotropic stimulation, it is laid horizontally, and a vertical 

 longitudinal section made after the production of upward 

 curvature. The illustration (fig. 201) shows the physio- 

 logical changes induced in the two halves. The cortex in 

 the upper half shows a relative contraction, while that 

 in the lower half a relative expansion. The starch-grains 

 in the upper statocysts are seen to press against the inner 

 tangential wall of the cells nearest the phloem ; in the lower 

 half of the stem, the starch-grains in the cells are, on the 

 other hand, pressing on the outer tangential wall furthest 

 from the phloem. It is important to bear in mind this 

 characteristic difference in the stimulation of the two halves 

 of the organ. 



• 



Method of Transverse Perforation 



Experiment 181. — I next carried out a complete explora- 

 tion of the interior of the peduncle of Nymphaea along its 

 diameter. The probe started from the upper surface, and 

 came out at the lower by successive steps of 0-2 mm., the 

 corresponding geo-electric response being observed at each 

 step. The successive readings were taken after right- 

 handed rotation from the vertical to 90° ; the rotation was 

 never carried out in the left-handed direction to — 90°. 

 The probe, entering from above, passed through a region 

 giving negative electric variation ; and then beyond the 

 central axis entered a region where the galvanometric 

 indication was positive. 



