34 



Mr. W. L. Balls. 



[Dec. 22, 



magnitude of the temperature-changes produced by other causes,* such 

 even as clouds by night. 



The horn hygroscope and the cobalt-paper method were incapable of 

 adaptation to automatic performance, but just as it seemed that there could 

 be no alternative to personal observation, Mr. Francis Darwin showed the 

 writer his Porometer. This simple appliance solved the difficulty. 



The writer's first recording porometer consisted of a constant pressure aspirator 

 attached to the porometer chamber ; the out-flowing water from this was collected 

 in a syphon-bucket, supported on a spring and carrying a stylus. The slow descent 

 of the bucket, followed by* its sudden rise when emptied by the syphon, marked on a 

 clock drum the time elapsing during the passage of a certain volume of air. Apart 

 from some irregularity in the discharge of the syphon, this appliance was useless for 

 field-work, since the aspirator acted as an air-thermometer, and the discharge of 

 water was consequently, irregular, even though the porosity of the inlet were 

 constant. Certain data were obtained with it in the laboratory, and during limited 

 periods of time in the field, which indicated the details to be considered. 



A suggestion made by Mr. F. Hughesf led at this stage to the trial of a method 

 in which a gas-holder was continually being charged with air-bubbles from a small 

 water pump. The altitude of the gas-holder varied with the aperture of the exit, 

 viz., the stomata, and an integrated pressure-graph was thus obtained. This method 

 was not developed further, owing to the satisfactory performances of the appliance 

 next described, which was more compact and portable. 



§ 2. The Stomatograph. 



Eetaining the principle of the porometer, with modifications in the method 

 of attaching the chamber to the leaf, an electrically-driven air-pump was 

 constructed, which forced a fixed volume of air at constant pressure through 

 the leaf at each stroke, and recorded the time taken in so doing. It was 

 then easy to record the changes which were taking place in the porosity of 

 the leaf which sealed the exit from the pump. 



By arranging the pressure and pump capacity suitably, the stroke of the 

 pump was made of sufficient frequency to prevent any notable error being 

 caused by variations in temperature or barometric pressure during the 

 occupation of the pump by any one charge of air. 



The cotton plant possesses from 200 to 300 stomata on its lower leaf- 

 surface, and about 100 on the upper, to each square millimetre. The aperture 

 of these stomata, which are of moderate dimensions, varies from complete 

 closure to widest distension during the twenty-four hours. Consequently, 

 the flow of air from one side of the leaf to the other is extremely facile at 

 certain times, aud only a very low pressure is required. It was found that 

 a pump discharging 5 c.c. of air at each stroke, under a pressure of - 5 mm. 



* Balls, W. Lawrence, 'Cairo Sci. Journ.,' 1911. 

 t Chemist to the Department of Agriculture, Cairo. 



