72 PLANT RESPONSE 



air, bubbling through water, is made to enter the chamber 

 through an entrance-pipe. The exit-pipe is connected with 

 an aspirator. By proper manipulation of the stop-cocks of 

 the air-bag and the aspirator, a gentle stream of humid air is 

 kept in constant circulation through the chamber. A modi- 

 fication of this arrangement enables us to study the effect of 

 various gases and vapours on the excitability of the organ. 

 A series of responses is first taken, under normal conditions, 

 that is to say, when the plant is surrounded simply by a 

 moist atmosphere. By now turning a three-way tap in 

 a given direction, the water-vapour can be made to pass 

 through a vessel filled with a given gas, before reaching the 

 plant chamber. Or the pure gas can be introduced alone. 

 The series of responses now obtained shows both the pre- 

 liminary and the permanent effects of the gas. For it is now 

 easy, by means of the three-way cock, to shut off the gas, 

 and to re-establish the first or normal condition. The re- 

 sponses next given afford an indication of the after-effect of 

 the gaseous reagent. It will be seen that by this means the 

 effects of various gases and vapours can be studied with the 

 greatest ease (fig. 35). 



The next point to be dealt with is that of the mode of 

 application of a uniform or graduated stimulus. For electrical 

 stimulation we may use induction currents. For this purpose, 

 a sliding induction coil of Du Bois-Reymond's pattern is 

 used. The intensity of the shock is here graduated, by 

 bringing the primary coil nearer and nearer to the secondary. 

 Non-polarisable electrodes make suitable connections with 

 the upper and lower ends of the organ. Electrical stimu- 

 lation is often preferable on the whole, but unless vigorous 

 specimens are used, it is apt when strong to induce fatigue. 

 This may be avoided by the use of moderate stimulus and 

 high magnification. Or we may, if desired, use thermal 

 stimulus, which is effected by means of heat generated 

 in a continuous length of thin German-silver wire which 

 surrounds the whole length of the specimen, in the form of 

 a cylindrical cage. This wire cylinder is fixed appropriately 



