SOURCE OF LIGHT 27 



raised, and the heat absorbed by the plant raises its tem- 

 perature to an unknown degree. 



The condition of a satisfactory light of uniform intensity 

 is amply fulfilled by the ' Pointolite ' lamp, in which the 

 source may be regarded as practically a point ; the intensity 

 of light under the same voltage is constant day after day. 

 The light is ' cold,' and the small proportion of heat-rays 

 may be completely absorbed by passage through a thick 

 stratum of alum-solution. When the light falls on the 

 water in the plant-vessel, it produces no variation of tem- 

 perature, as tested by a thermometer placed in the vessel. 

 The only precaution which need be taken in the use of 

 Pointolite is to place the resistance-coil which regulates 

 the current outside the experimental room, so that the 

 heat produced may not cause any variation of temperature 

 of the room. 



It is especially necessary to study the effect of light 

 from the sky on photosynthesis, as the plant is subjected 

 in nature to this light. In employing this light, however, 

 we have (i) to determine the period of the day when it is 

 most constant, and (2) to have a sensitive photometer for 

 testing its uniformity during the period of the experiment. 

 An Electric Photometer has been devised for this purpose 

 which will be fully described in a subsequent chapter. 



We may next refer briefly to the favourable physio- 

 logical condition of the plant on which the constancy of all 

 modes of response depends. I have shown in my previous 

 works that a vigorous plant exhibits uniform responses, 

 both mechanical and electrical, for a considerable length 

 of time. Plants in a depressed condition exhibit, on the 

 other hand, a feeble or irregular response. A plant may be 

 in a sluggish condition at the beginning, in which case 

 stimulus renders it more active ; but prolonged and exces- 

 sive stimulation causes fatigue and depression of response. 



I have thought it necessary to enter into some detail 

 as to the preliminary adjustments for securing quantitative 

 accuracy in the results. It takes some little time to master 



