134 CHAP. XVII. EFFECT OF VARIATION OF TEMPERATURE 



Example : The volume of gas given out under an inten- 

 sity of light of iooo lux was 259 -6 c. mm. per hour at 21 C. 

 and at 751 mm. barometric pressure, the tension of aqueous 

 vapour being 18 -6 mm. 



751 —18 -6 273 



V N.T.P.) = 259-6 X — 7 X -^~ 



u 760 294 



= 232 -3 c.mm. per hour. 



Respiration at 21 ° C. = 7-2 c.mm. per hour. 



Absolute value after correction = 232-3 + 7-2 



— 239 -5 c.mm. per hour. 



Adjustment of Temperature 



The temperature of the leaves of land-plants surrounded 

 by air, as already stated, cannot be accurately determined ; 

 but no such difficulty exists in the case of water-plants, the 

 temperature of which is the same as that of the surround- 

 ing water. Difficulty arises, however, in the production of 

 definite variations of temperature. Addition of ice or of 

 warm water is not merely a crude method, but the sudden 

 change is tantamount to a shock which causes a physio- 

 logical reaction independent of the true effect of change of 

 temperature. The disturbance caused by the shock-effect 

 was completely eliminated by the following device. 



A large-sized test-tube is drawn out so that its upper 

 end B serves as the Bubbler (right-hand illustration, fig. 37). 

 A close-fitting india-rubber cork closes the lower end, to 

 which are fixed a sensitive thermometer, and a glass rod 

 to which the plant is tied. There is also a tube with a 

 stop-cock Si, ending in the funnel f. The plant-vessel is 

 filled with the C0 2 -solution up to a definite mark M in 

 the Bubbler by means of the funnel F, after which the 

 stop-cock s-l is closed. 



For adjusting the temperature, the glass vessel is sur- 

 rounded by an outer rectangular metallic vessel v, one 

 side of which is a sheet of glass for the passage of light. 



