432 CHAP. XXXVII. GENERAL REVIEW 



increased rate of growth. The method of geo-electric 

 response is more sensitive in the detection and quantitative 

 determination of the effect of stimulation by gravity than 

 that of mechanical response. 



Localisation of geo-perceptive layer. — The induced gal- 

 vanometric negativity of the upper side of the shoot when 

 stimulated by gravity is not uniform in the different tissues 

 of the organ. The excitatory reaction attains a maximum 

 value at a definite layer, beyond which there is a decline. 

 This is the geo-perceptive layer which has been localised 

 by measuring the depth of intrusion of the exploring Electric 

 Probe at which maximum deflection of galvanometric 

 negativity is detected. The geo-perceptive layer thus 

 localised is found to be at or near the starch-sheath which 

 contains a number of large-sized starch-grains (p. 341). In 

 the stem of certain plants the distribution of excitability ex- 

 hibits two maxima, the focus of excitation being not single 

 but double. Microscopic examination showed that the 

 starch-sheath in these is double and that the positions of 

 the two electric maxima coincide with those of the two 

 starch-sheaths (p. 345). These results afford strong support 

 to the statolith-theory that it is the weight of heavy particles 

 which induces geotropic excitation in the higher plants. 



Critical angle .for immediate geotropic excitation. — The 

 critical angle has been found in a large number of plants 

 to be about 31 "8°. This is additional evidence in favour 

 of the statolith-theory, for the abrupt reaction beyond the 

 critical angle can be most satisfactorily attributed to the 

 sudden fall of particles from the base to the side of the 

 sensitive cells (p. 360). 



Geotropism of root. — On subjecting the tip of the root 

 to the stimulus of gravity, the upper side exhibits excitatory 

 reaction of galvanometric negativity (p. 368). This shows 

 that the root-tip, which contains starch-grains, becomes 

 directly stimulated. The electric response in the growing 

 region, above the stimulated point at the root-tip, is positive, 

 indicative of increase of turgor and expansion, showing that 



