370 COMPARATIVE ELECTRO-PHYSIOLOGY 



the plant is in no way disturbed. The balance is obtained 

 by allowing water to enter the plant-vessel, from the com- 

 pensator, C, at a rate exactly equal to that of its withdrawal 

 by suction. Thus, under a condition of balance, the record 

 becomes horizontal. An exciting agent now produces an 

 inclination of the record upwards, or a depressing agent a 

 declination downwards. This was the instrument employed 

 by me for the obtaining of records in a simple manner of 

 suctional response and its variations, under different physio- 

 logical modifications. A fuller account of the method and 

 the results obtained by it will be found in my book on 

 ' Plant Response.' As the subject of the ascent of sap is, how- 

 ever, of extreme importance, I thought it desirable to see to 

 what extent the sensitiveness of this instrument could be 

 raised, and also to devise means, in connection with it, for the 

 automatic record of results obtained. For the latter purpose 

 I employed photography. As the water-index, whose ex- 

 cursions in the potometric tube are to be recorded, is trans- 

 parent, it is necessary to provide an additional opaque index, 

 which shall move in and out with it. This consists of a 

 short length of mercury, lying in contact with the end of the 

 water-column. The potometer tube is placed in the field of 

 a magic lantern, and the index is focussed on a moving 

 photographic plate by means of an objective. The sensitive- 

 ness of this method of record may be increased in two ways. 

 First, the bore of the capillary tube may be made finer and 

 finer. But this cannot be carried to an extreme, as the 

 capillary offers great resistance to the free movement of the 

 index. Secondly, the sensitiveness may be increased to any 

 extent by the employment of a highly-magnifying and 

 short- focus objective. By the combination of both these 

 devices, we are able in practice to arrive at an extraordinary 

 degree of sensitiveness, qualifying us to attack some of the 

 most difficult problems with the greatest ease. For our 

 present purpose, however, it is not by any means necessary 

 to approach the limit of this sensitiveness. The photo- 

 graphic records given in the course of the present chapter 



