CHAPTER XXII 



DETERMINATION OF THE PHOTOSYNTHETIC EFFICIENCY 

 OF THE SPECTRAL RAYS 



Difficulties in the quantitative determination of photosynthesis under 

 enfeebled spectral rays — Results with exceptionally sensitive specimens 

 — Enhancement of sensitiveness by rise of temperature — Vacuum- 

 jacket for maintenance of constant temperature— Method of depletion 

 of starch in enhancement of sensitiveness — The Heterostatic Method- — 

 The Auxiliary Stimulator — Additive effect of light — The Absorption- 

 spectrum — Tropic movement under certain rays of the spectrum — 

 Simultaneous determination of energy of rays and of the induced photo- 

 synthesis — Characteristic effects of different rays on photosynthesis 

 ■ — Complementary relation of A and D effects in photosynthesis and 

 in phototropism. 



Having fully explained in the last chapter the method of 

 determining the energy of radiation, we may now pro- 

 ceed to obtain quantitative determinations of photo- 

 synthetic activity under different rays in a pure spectrum. 

 This presents considerable difficulties, owing to the great 

 enfeeblement of the light by the highly dispersing prism 

 such that the intensity of radiation falls below the minimum 

 intensity for photosynthesis. This may be obviated by 

 increasing the intensity of light by widening the slit of the 

 spectrum apparatus ; but this advantage is discounted by 

 the consequent overlapping of the different rays. 



On several occasions, especially in spring, I obtained 

 very sensitive specimens which reacted even under the 

 spectral rays ; but prolonged exposure to the feeble light 

 induced such a depression that the plants soon became 

 insensitive. The only possible way to secure a complete 

 set of observations was to hurry through the determinations 

 as quickly as possible. Out of thirty different attempts, 

 I was fortunate in obtaining four complete determinations 



