EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 379 



the duration of application is often less than a minute, and 

 seldom exceeds five minutes. It should also be borne in 

 mind that individually and collectively the effects of the dis- 

 turbing causes enumerated would, if anything, be towards 

 expulsion from the plant-vessel. We shall see, however, 

 that the typical responsive effect is movement in the opposite 

 direction, indicative of an enhancement of suction. With 

 regard to the second cause of misgiving, as to whether plant- 

 tissues may or may not be made to exhibit excitatory varia- 

 tion by means of induction-shocks, I have found that some 

 specimens, notably those of Croton, are sufficiently suscep- 

 tible to this form of stimulation. For this purpose it is 

 necessary to use very strong induction-shocks from a large 

 coil. This necessity is further increased by the fact that 

 much of the induction-current is uselessly and unavoidably 

 shunted by the water in the vessel. 



This electrical stimulation may be applied to the cut end 

 of the branch, immersed in the water of the plant-vessel, in 

 either of two different ways. The first of these may be 

 described as the Terminal Mode of Application, its object 

 being to localise the excitation more or less at the lower 

 end of the specimen. This is done by tying two small 

 pieces of platinum, in connection with the electrodes, to 

 diametrically opposite sides of the base of the stem by means 

 of a thread. Or two pins, in connection with the electrodes, 

 may be pricked into the lower section of the specimen near 

 its circumference. This transverse mode of stimulation, 

 across the diameter of the stem, is not, theoretically, so 

 effective as longitudinal stimulation would be, but under 

 the particular experimental conditions nothing better could 

 be devised (fig. 224). The second mode of applying this 

 electrical stimulus I shall distinguish as Sub-terminal. Here, 

 two pins in connection with the electrodes pierce through the 

 stem, one above the other, in planes at right angles to each 

 other (fig. 225). After arranging the electrical connections 

 in any one of the ways enumerated, the specimen is adjusted 

 in the Shoshungraph, and allowed a period of rest for the 



