ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 27 



Pscudoplatamis was taken, and its cut end fixed into a glass 

 tube by a piece of india-rubber tubing. Into the other end of 

 this glass tube a long capillary tube was fixed by means of a 

 perforated cork. The capillary tube was led through a small 

 aperture in the bottom of the chamber into a small beaker of 

 water. Upon the lower part of the capillary tube two engraved lines 

 indicated a measured length. The transpiring shoot absorbs water 

 from the broad glass tube, and this draws up water from the 

 beaker, through the capillary tube. If the beaker be lowered for 

 a few seconds an air-bubble is drawn into the capillary tube ; 

 after the beaker is replaced this bubble slowly ascends the capil- 

 lary tube. The time which it occupies in traversing the measured 

 length bears a direct relation to the rate of transpiration. By 

 taking the reciprocals of such readings, a series of figures propor- 

 tional to the amounts of water absorbed by the branch in a given 

 time are obtained. Below I simply give the readings in seconds, 

 an increase in the time of course indicating that absorption has 

 been slowed, and conversely. 



The shoot was fitted up as described, and allowed to transpire 

 in the chamber till a fairly constant rate had been attained. A 

 number of readings were made to ascertain that this was so. 

 Precautions were also taken, so that the vapour transpired should 

 not collect in the chamber, making the air humid, and gradually 

 lower the rate of transpiration. 



Number of seconds occupied 

 Time by bubble in traversing 



the measured length. 



12.0 (noon) 94 (constant). 



S0 2 was now introduced into chamber 

 till the concentration was *G milli- 

 gramme per cubic foot. 



12.5 (p.m.) 125 



12.15 „ 133 



The glass roof of the chamber was 

 cleaned outside, causing the increase 

 in rate indicated by the next reading. 



12.30 „ 112 



12.40 „ 140 



The concentration of SO._, was now in- 

 creased to 12 milligrammes per cubic foot 



12.48 „ 6G0 



1.10 „ 705 



2.5 ,, 700 (approximately). 



The atmosphere was changed so that 

 only traces of SO., remained. 



2.20 „ 127 



