LOCALISATION OP GEO-PERCEPTIVE LAYER 487 



value. The excitatory eH'ect then declines till it vanisheii 

 at a depth of 2*4 mm. 



The depth of layer at which maximum excitation takes 

 place varies to some extent, according to the thickness of 

 the shoot. Thus while in a thin specimen of BryophyUtim 

 3't'» mm. in diameter the geo-perceptive layer was found at 

 a depth of 0*0 mm., it occurred at the greater depth of 

 0"'S mm. in a thicker specimen, f) mm. in diameter. In 

 Nympfuea also the perceptive layer was found at a depth 

 of OvS mm. in a thin and at a depth of 1 "4 mm. in a 

 thick specimen. 



Having thus succeeded in localising the geo-perceptive 

 layer by experimental means, it was now possible to examine 

 the anatomical characteristics of the layer by examining 

 it under the miscroscope, I also wished to find out from 

 microscopic examination, the cause of certain differences 

 uoticed in the determinations of the perceptive layer in 

 BryophyUum and in Nymj^Jicea. In the former the probe 

 always encountered the maximally excited geo-perceptive 

 layer from whichever point of the surface it entered the 

 organ ; this indicated thai the sensitive layer in Bryophyl- 

 him was continuous round the axis. In Nymphcea^ however, 

 the probe occasionally missed the sensitive layer; but a 

 new point of entry led to successful localisation of the 

 perceptive layer ; this was probably due to the particular 

 layer not being continuous but interrupted by certain gaps. 



MICROSCOPIC EXAMINATION OF THE MAXIMALLY 

 EXCITED LAYER. 



The specimens were taken out after the electric test, and 

 the transverse sections made at the radial line of the 



36 



