VARIABLE FACTORS l LIGHT AND C0 2 -CONCENTRAT10N 255 



The employment of an identical specimen introduces, 

 however, certain difficulties. For fatigue might be induced 

 in the course of prolonged experiment, during which the 

 different factors are being brought into various combination. 

 This difficulty is obviated by bringing about the necessary 

 changes in quick succession, celerity in manipulation being 

 acquired from previous practice. The change in light may 

 be produced quickly and with ease by moving the plant- 

 vessel towards or away from the divergent beam from the 

 Pointolite, the various intensities of which are marked on 

 the experimental table. Greater dexterity is necessary in 

 the adjustment of the temperature, for the change has to be 

 brought about gradually to guard against thermal shock ; 

 the definite temperature has, moreover, to be maintained 

 perfectly constant during the period of the experiment. The 

 necessary adjustment is secured, as previously explained, by 

 the proper manipulation of the stop-cocks which regulate 

 the flow of warm water through the water-jacket enclosing 

 the plant-vessel. The introduction of water containing 

 different proportions of C0 2 is accomplished, without 

 causing any disturbance to the plant, by the method which 

 has already been described. 



The variation produced should, as far as possible, be 

 forward and upward, in the direction of increasing intensity. 

 For the plant responds more quickly to an upward change. 

 Should it however be necessary to bring about a change in 

 the opposite direction, it should be done slowly, so as to 

 ensure physiological readjustment of the plant to the lower 

 intensity. 



The experiments are the following : 



I. Two Variable Factors : Light and 

 C0 2 -Concentration 



Successive experiments and results are given in the 

 subjoined account. The temperature was maintained 

 constant at 22° C. 



