IN VARIOUS PLANTS 343 



Nymphaea, and held them horizontal. Shoots of these 

 plants had, earher in the season, exhibited very strong 

 geotropic reaction, the stem curving up through 90° in the 

 course of 10 hours or less. But these specimens, obtained 

 later in the season, exhibited very feeble curvature, which 

 hardly amounted to 10°, even after prolonged exposure to 

 geotropic stimulation for 24 hours. 



Experiment 185. Test of microscopic examination. — I 

 next made sections of the insensitive specimens of Bryo- 

 phyllum and Nymphaea, and on examining them under the 

 microscope discovered a striking difference. A few weeks 

 before, the groups of large starch-grains stained with iodine 

 were the most striking feature of the starch-sheath. But 

 now these starch-grains could not be found in any of the 

 numerous specimens examined. This is evidence that the 

 presence of the starch-grains is associated with the sensitive- 

 ness of the perceptive layer. 



Localisation of Geo-Perceptive Layer in 



Various Plants 



The geo-perceptive layer of a large number of plants 

 was similarly locahsed by the probe, a short account of 

 w^hich is given below. 



Commelina.— The geotropic sensibihty of the stem of 

 this plant is shown by the erectile movement from a hori- 

 zontal position. The geo-electric response at the surface 

 was o. At o-io mm. it was —6, which increased to a 

 maximum of - 18 at a depth of -20. After this the response 

 underwent a rapid decline. The maximally excited layer 

 was subsequently found to contain the starch-grains. 



Myosotis.— The stem of Forget-me-not also gave strong 

 geo-electric response, the maximum excitation occurring 

 at a depth of 0-20 mm. In the microscopic section the 

 starch-containing layer was also found at the depth of 



0-20 mm. 



Centaurea.— The flower-stalk of the Cornflower was 



