56 PLANT PHYSIOLOGY 



absorption bands of the chlorophyll spectrum, the largest 

 and darkest being in the most luminous part of the spec- 

 trum, the next toward the actinic violet end. Sachs's 

 classical experiment of cultivating plants behind colored 

 screens — the one consisting of a layer 1.2-1.5 cm. thick of a 

 saturated solution of potassic dichromate, which absorbs 

 the blue and violet rays whoUy, but allows the others to 

 pass; and the other of a layer 1 cm. thick of a dark 

 solution of cuprammonia, which absorbs only the red, 

 orange, and yellow rays — shows that the luminous rays 

 absorbed are used to elaborate about 90J? (starch serving 

 as the indicator) of the food first formed by the plant, 

 while the more actinic rays yield only 5-7^. Thus the en- 

 ergy absorbed by chlorophyll as indicated by the two main 

 absorption bands in the spectrum is made to produce 95- 

 91% of all the carbon food compounds first formed in the 

 plant. The remaining 5-3% are formed by the other visi- 

 ble rays absorbed. The thermal ultra-red rays and the 

 actinic ultra-violet rays, although absorbed by the plant, 

 are not used in manufacturing non-nitrogenous foods. 

 Engelmann's bacteria method,t now almost as famous as 

 Sachs's colored screens, though applicable only to small 

 plants, affords much more delicate and exact evidence of 

 the statements just made. The method consists in the 

 employment of motile aerobic bacteria in conjunction with 

 small green aquatic plants. Beneath the stage of the 

 microscope a prism is so placed as to throw a short spec- 

 trum upon the object on the slide. Upon the slide, in a 

 drop of water containing motile bacteria, is laid a filament 

 of (Edogonium, Spirogyra, or some similar plant. Around 

 those cells illuminated by the red, orange, and yellow rays, 

 the bacteria will congregate in greatest numbers, there 

 moving with the utmost activity. Around other illumi- 

 nated cells also the bacteria will collect, but in much smaller 



* Sachs, J. von. Wlrkungen larbigen Lichts auf Pflanzen. Gesammelte 

 Abhandlungen, Bd. I., p. 261 et seq. 



t Engelmann. Th. W. Botanieche Zeitung, 1882, 1883, 1884, 1886, 

 1887. Also Die Erscheinungsweise der Sauerstofiausscheidung chlorophyll- 

 haltiger Zellen. Verhandl. d. Amsterdamer Academie, 1894. 



