PETIOLE OF TROPAEOLUM 33I 



in their proper seasons, give good results. For the success 

 of the following experiments, it is essential that the plant 

 should be in an exceptionally vigorous condition. 



Distribution of PZxcitabiuty ix the Petiole of 



Tropaeolum 



Detailed results of the experiments on the localisation 

 of the geo-perceptive layer in the petiole of Tropaeolum 

 are given below as typical. The petiole of Tropaeolum 

 has the following special advantages. Geotropically it is 

 very sensitive ; its latent period of response is very short, 

 the horizontally laid petiole beginning to bend upwards 

 in the course of a short time. The leaf may be isolated 

 from the plant, and the cut end of the petiole placed in 

 moist cotton. The normal geotropic irritability of the cut 

 specimen is found to be fully restored in the course of half 

 an hour. The manipulation of a cut specimen, placed 

 alternately in a vertical and in a horizontal position, 

 presents no difficulty. A very large number of specimens 

 can be, moreover, obtained from the same plant. As 

 regards geotropic reaction, the induced electromotive 

 variation of the petiole of Tropaeolum is considerable and 

 attains the maximum value within a few minutes ; the 

 recovery is practically complete after restoration to the 

 vertical. 



The mode of experimental procedure (slightly different 

 from what has already been described) is as follows : the 

 probe is thrust into the petiole by successive steps of 

 0-05 mm., and the electric response observed on displace- 

 ment of the petiole from the vertical to the horizontal 

 position, in which latter position the organ is subjected to 

 geotropic stimulation. The induced electric variation, as 

 already stated, is of considerable intensity. The irritation 

 caused by the prick of the probe is very slight, the immediate 

 effect of the insertion of the probe being a negative deflection 



