54 



PLANT PHYSIOLOGY 



the red, orange, and yellow, and in the green and blue parts 

 respectively. In the normal solar spectrum are certain dark 

 narrow lines, named after their discoverer, Fraunhofer's 

 lines. These indicate parts of the spectrum in which there 

 is no light or other known form of energy. When a chloro- 

 phyll screen is interposed in the path of a beam of sunlight 

 the spectrum differs from that of normal light in that be- 

 tween the Fraunhofer lines B and C, and E and F, energy 

 in the form of light has been absorbed by the chlorophyll. 

 This energy, absorbed by the lifeless chlorophyll pigment, is 

 what is employed by the living chloroplastid in elaborating 

 a nutritious compound from carbon-dioxide and water. The 



Blue 



2 Leaves 



B C D E b 



FIG. 1. CHLOROPHYLL SPECTRA. (AFTER REINKE.) 



accompanying figure indicates the absorption, as reported 

 by Reinke,* of an alcoholic extract of green leaves, and of 

 two leaves and of seven leaves of Iinpatiens parvitiorn 

 interposed in the path of a beam of light. 



The figure shows plainly that light which has passed 

 through one layer of chloroplastids is essentially and obvi- 

 ously poorer in energy than that which falls upon the first 

 layer. The chromatophores of a single-celled green plant, 

 and of plants consisting of filaments or single layered films 

 of cells, receive beams of light containing the maximum of 

 energy assuming that none is absorbed by the medium in 

 which they live. In plants composed of masses of cells, 

 only the outermost layer of chlorophyll-containing cells re- 

 ceives the maximum amount of light and energy. The 

 deeper layers evidently receive less and less. A moment's 

 thought will lead one to conclude that only the peripheral 

 la.yers of cells in a massive plant or organ will receive much 



* Reinke. J. Untersuchungen iiber die Einwirkung des Lichtes auf die 

 Sauerstoffausscheidung der Pflanzen. 2te Mittheilung: Wirkung der 

 einzelnen Strahlengattungen des Sonnenlichtee. Bot. Zeitung. 1884. 



