294 chap. xxvii. anaesthetics and geotropism 



Experimental Method 



In order to obtain a characteristic response within a short 

 time, it is desirable to choose a specimen which is sensitive 

 and in which unnecessary weight is reduced by cutting off 

 portions which are non-essential. These conditions are 

 fulfilled by an isolated petiole of Tropaeolum from which 

 the lamina has been removed. The cut ends are WTapped 

 in moist cotton, and after a rest of about half an hour the 

 irritability of the specimen is found to be fully restored. 

 The beginning of the geotropic response can now be easily 

 detected by the employment of a magnifying lever. 



Tropaeolum grows in Calcutta during the winter months 

 from November to January, and also during the spring 

 season in February and March ; the plants begin to die 

 off from the beginning of April. The experiments described 

 below were carried out during two years in succession. The 

 records given by the spring- and the winter-specimens 

 exhibit certain characteristic differences. In the spring- 

 specimen the latent period — the time which elapses 

 between the application of the stimulus of gravity and the 

 commencement of the geotropic up-movement — varies from 

 20 seconds to 6 minutes ; the rate of movement afterwards 

 becomes uniform and remains so for about half an hour. 

 The slope of the curve and the distance between the suc- 

 cessive dots indicate the geotropic activity ; any induced 

 enhancement of the normal rate is, as already explained, 

 exhibited by the erection of the curve and greater separation 

 of the successive dots ; depression, on the other hand, is 

 indicated by the opposite change. In the winter-specimen, 

 owing to the general depressed rate of growth, the geotropic 

 response is very sluggish, as seen in the prolonged latent 

 period, which in the particular experiment was found to 

 be 48 minutes ; the sluggish character of the response is 

 also indicated by the gentle slope of the geotropic curve 

 (fig. 171). 



The uniformity of the erectile response cannot be main- 



