432 BOTANICAL SOCIETY OF EDINBURGH [Sess. lxxix. 



A New Method of Continuous Automatic Registration 

 of Transpiration. By R. A. Robertson, M.A., B.Sc, 

 and S. J. Wilkie, B.Sc. (Carnegie Research Scholar, 

 St. Andrews). (Plate LVIII.) 



(Read 10th June 1915.) 



For a research in hand on Transpiration Periodicity it 

 was found necessary to obtain continuous automatic records 

 extending over periods of some duration. The ordinary 

 automatic method of weighing the plant and making the 

 necessary correction was inconvenient, while other methods 

 entailing interruption for direct observation at short inter- 

 vals were obviously of little use in this case. The illus- 

 tration shows the apparatus devised which has. yielded 

 satisfactory results. 



The principle of the method is that the vapour transpired 

 is automatically trapped, weighed, and a record taken on 

 a rotating drum ; from the data thereby available con- 

 tinuous graphs can be constructed for lengthy periods. 

 Dry air is passed into a receiver containing the transpiring 

 material, the outgoing air is passed over calcium chloride, 

 and the aqueous vapour is measured. The air is drawn 

 continuously through the closed system by an aspirator 

 pump, the pressure of which is regulated by manometer. 

 The air is drawn through (A), a tube of chloride, where it 

 is dried ; thence through (B), a glass receiver, in which is 

 the transpiring material. The humid air from the receiver 

 next passes through (C), the index tube of chloride. This ■ 

 tube is counterpoised on the arm of the lever (D), the other 

 arm of which makes a tracing on the smoked paper on the 

 drum (E) which makes one revolution per twenty-four 

 hours. The counterpoising is made as delicate as possible ; 

 the chloride is renewed once per day, with the minimum of 

 interruption. The tubing used to link up the system is of 

 very flexible rubber, and every care is taken to prevent 

 it becoming hard. Condensation of vapour on the glass 

 receiver is obviated by accurate regulation of the rate of 

 aspiration and by keeping the temperature constant. 



Satisfactory continuous records extending over twelve 

 to thirty days have been made for herbaceous plants, suc- 

 culents, and needle-leaved Gymnosperms without appreci- 

 ably damaging the experimental material. 



