746 



GENERAL CONDITIONS OF PLANT-LIFE, 



If these values are erected as ordinates upon the solar spectrum, taking its 

 corresponding parts as abscissae, the result, as shown in Fig. 475, is that the curve of 

 the different powers of light for causing evolution of gas corresponds in the main 

 with the curve of subjective brightness of the same regions of the spectrum ; but 

 does not coincide with the curve of heating power. 



Pfeffer's experiments had shown that the method first employed by me for 

 determining the intensity of the action of light on water-plants, viz. counting the 

 number of the bubbles of gas given off in a unit of time, gave nearly the same 

 results as actual ineasurement of the gas, the result being in fact somewhat too 

 great, and inexact in inverse proportion to the amount of gas given off. He then 

 applied this method to determine the amount of oxygen given off from a small 

 water-plant {Elodea canadensis) when exposed to a portion 1 3 mm. in breadth of a 

 very intense solar spectrum 2 3 cm. long. In this experiment he had the advantage 



A. B. C. 



D. 



E. I. r. 



a. 



H. 



Fig. 473.— Graphic representation of the efficacy of rays of different refrangibility in causing the evolution of oxygen, com- 

 pared with their brightness and heating power. The solar spectrum A—H serves as a base, on which lines to represent the 

 three different effects are erected as ordinates ; the three curves are thus obtained which represent asbimilation, brightness, 

 and heat. 



of being able to determine the amount of gas given off by the same plant in all 

 the regions of the spectrum in successive very short spaces of time, and of thus 

 avoiding various errors of observation which inevitably accompany eudiometric ob- 

 servations, or at least are very difficult to get rid of. A number of observations 

 conducted in this manner gave the following result as the mean capacities for 

 decomposition possessed by the different regions of the solar spectrum, yellow 

 being placed at 100: — 



Red 



