58 MR JOHN AITKEN ON DEW. 



the sap forced the mercury. These gauges did their work well enough, but 

 they were somewhat slow in action, as it takes some time for sufficient liquid 

 to be exuded to displace the mercury and force it up the tube. Where high 

 pressures were required to be measured, the U tubes had to be abandoned, on 

 account of their inconvenient height, as well as the length of time required for 

 making an observation with them, and the pressures were measured by means 

 of air-pressure gauges. These gauges were made of a short length of wide 

 thermometer tube, having a bore of less than 1 mm. diameter, a short 

 column of mercury being put in to form an index. The pressures were 

 calculated from the volumes, and corrections made when necessary for 

 temperature. The gauges were occasionally compared with a column of 

 mercury to see that everything was correct. The pressures given cannot be 

 considered correct to more than 10 mm. of mercury, but for the present purpose 

 this degree of accuracy is sufficient, as the pressures are very indefinite, 

 varying' with so many conditions that anything like an exact figure cannot be 

 looked for in experiments of this kind. 



I shall now describe in detail an experiment made on a cauliflower, as 

 it is similar to those made on other plants, of which it will only be necessary 

 here to give the results. The pot containing the plant was placed on a sheet 

 of metal, and a glass receiver got ready large enough to cover it entirely. The 

 stalk of one of the blades was selected for making the connection between 

 the pressure gauge and the plant. This stalk, while the leaf was still on it, was 

 prepared for making a water-tight joint by filling up the longitudinal groove in 

 its upper surface with beeswax, laid on with a slightly heated iron. The blade 

 was now cut off, and the gauge attached by means of a short length of soft 

 india-rubber tube ; the stalk having been made round by means of the beeswax, 

 a tight joint was easily made. With the exception of this one leaf cut off, all the 

 others were left untouched. The receiver was now put over the plant to stop 

 evaporation from the leaves, and everything left at rest. After a short time 

 the pressure was seen beginning to rise in the gauge, and drops also began to 

 show themselves all round the edges of the leaves. As time went on the drops 

 increased in size, and the pressure went up to 290 mm. of mercury, at which 

 point it stopped. This pressure can be considered correct only for the par- 

 ticular plant under the particular conditions. It simply meant that when the 

 pressure rose to 290 mm., the whole of the supply of sap sent up by the root 

 could find an exit by exudation. If the supply had been greater, or the 

 exuding pores fewer or smaller, the pressure would no doubt have gone higher, 

 and vice versa. 



The plant in the above experiment was in the same condition as it would be 

 on a dewy night. All evaporation from the leaves was stopped, and transpira- 

 tion having ceased, the root continued to send up its supplies of sap, first filling 



