I36 CHAP. XVII. EFFECT OF VARIATION OF TEMPERATURE 



about 35° C. Partial opening of the stop- cock s 2 , attached 

 to c, allows a stream of water to circulate in the metallic 

 outer vessel, and the temperature of water in the inner plant- 

 vessel is gradually raised. With a little practice it is possible 

 by proper manipulation of the stop-cock s 2 to control the 

 rate of rise of temperature. The plant-vessel is at first 

 filled with water at 8°C, at which temperature photo- 

 synthesis is completely arrested. The temperature is now 

 gradually raised, by adjusting the flow of water in the outer 

 vessel, at a rate of about i° C. per minute. This uniform 

 rate of rise causes no shock to the plant. The temperature 

 of the plant is thus gradually raised by, say, 3 C. The 

 temperature has to be maintained absolutely steady for 

 about two minutes for the completion of the observation of 

 the rate of photosynthesis at the given temperature. A 

 thermometer placed in the outer vessel gives the temperature 

 of the water outside ; the steadiness of the internal tempera- 

 ture is assured when the temperature indicated by the two 

 thermometers in the outer and the inner vessels is the 

 same. When the temperature of the inner vessel is lower 

 than the temperature of the room, heat enters it ; when, 

 on the other hand, it is higher, the flow of heat is outwards. 

 By partial opening or closure of the stop-cock s 2 the tem- 

 perature of the water in the plant-vessel can be so adjusted 

 that it remains absolutely constant during the period of 

 two minutes for an observation. The successive bubbling 

 periods are now found to be very uniform. The importance 

 of adjustment of temperature for securing accurate results 

 will appear from the following occurrence in an experiment 

 in which uniformity of the rate of bubbling was being 

 observed. The uniformity was suddenly disturbed : the 

 cause was afterwards traced to the opening of a door which 

 allowed a draught of air to enter the closed room in which 

 the experiment was being conducted. The change of the 

 rate of photosynthesis produced by such a slight variation of 

 temperature is not very great ; it is, however, necessary to 

 avoid anything of the kind in experiments for the accurate 



