272 CHAP. XXVIH. GENERAL REVIEW 



this particular increase being independent of the C0 2 - 

 concentration of the liquid in which the plant is immersed. 

 The maximum under sunlight (0-48) was found to be nearly 

 double that under 3000 lux (p. 43). 



The Hydrilla plant may be used as a photometer, since its 

 sensitiveness to light is very great. The physical and the 

 physiological determinations of intensity of light agree with 

 each other (p. 45). Photosynthesis increases with (1) the 

 duration of exposure, and with (2) the intensity of light. It 

 also increases with the sine of the directive angle. This may 

 be summarised in the general statement that the amount 

 of photosynthesis is proportional to the quantity of the 

 incident light (p. 48). 



Relative Efficiency of the Different Rays 



The relative effectiveness of light of different colours has 

 been ascertained by the new method of Flotation (see p. 168), 

 in which a piece of Hydrilla is suspended from the pan of a 

 Torsion Balance. 



The essential conditions for investigating the relative 

 efficiency of light of different wave-lengths are (1) the 

 securing of a pure spectrum, (2) a sensitive device for the 

 quantitative determination of photosynthesis, and (3) the 

 simultaneous measurement of the energy of the acting rays. 

 Pure spectrum of high dispersion was obtained by a carbon 

 disulphide prism. The different rays were successively 

 thrown on the stationary plant-vessel and on the adjoining 

 strip of the Magnetic Radiometer which detected a rise 

 of about to oi) fo ° C., so that the determination of the 

 photosynthetic activity and that of the energy of the rays 

 were practically simultaneous. 



The special difficulty arising from the enfeebled intensity 

 of radiation in a highly dispersed spectrum was overcome 

 by the Heterostatic Method. By means of an Auxiliary 

 Photic-stimulator, the plant is brought to the verge of 

 photosynthesis. Exposure to the feeble rays of the spectrum 



