rETIOLE OF TROPAEOLUM 



333 



Table XXI. — Showing Geo-Electric Response at Different 

 Depths in the Petiole of Tropaeolum. 



Subsequent microscopic examination showed that the 

 maximally excited layer at the depth of 0-20 mm. was that 

 which contains the starch-grains. The geo-perceptive layer 

 is thus found to coincide with the starch-sheath. 



Maximum excitability at the geo-perceptive layer. — The 

 maximum excitation induced at the perceptive layer 

 appears to be due to two factors : first, the direct stimula- 

 tion caused by the fall of the starch-grains ; secondly, the 

 greater general excitability of the geo-perceptive layer as 

 compared with that of the neighbouring ones. The rela- 

 tively greater excitability of the perceptive layer became 

 evident from the effects observed during the passage of 

 the probe with the specimen held vertical, when there was no 

 geotropic stimulation. The insertion of the probe then acts 

 as a mechanical stimulus, and the response of galvano- 

 metric negativity is found to be maximum at or near the 

 starch-sheath, proving that this is relatively the most ex- 

 citable. The response to prick takes place during the thrust 

 of the probe ; the resulting irritation disappears, however, 

 when the probe is left stationary. The normal excitability 

 of the cells is restored, as already stated, after a period of 

 rest of about 10 minutes. 



