ACTION OF LIGHT ON VEGETATION. 



679 



ordinarily examined, into a single continuous absorption-band occupying the whole 

 of the more refrangible half of the spectrum. 



The bands /, //, ///, and /rare situated in the red, orange, yellow, and yellow-green. 

 The deep black band /, sharply defined on both sides, lies between Fraunhofer's lines 

 B and C; the three others, shaded off on both sides, diminish in strength in the order of 

 their numbers. Between these bands the illumination is dim, and progressively in the 

 order of the numbers ; i. e. is less dim between // and III than between / and //, &c. 

 To the left of /the light is undiminished. 



The bands T, VI, and FIl in the more refrangible half of the spectrum are shaded 

 on both sides; Vis situated to the right of Fraunhofer's line F; FI, which is dark in the 



Fig. 447. — Absorption-spectra of the colouring matter of chlororophj-II (after Kraus). A the spectrum 01 the alcohohc 

 extract of green leaves ; B that of the blue-green constituent soluble in benzol ; C that of the yellow constituent. The 

 absorption-bands of A and A' are indicated in the less refrangible (left-hand) portion as they would be produced by a 

 more concentrated, in the more refrangible (right-hand) portion of the spectrum as they would be produced by a less 

 concentrated solution ; the letters B—G indicate the well-known Fraunhofer lines of the spectrum ; the figs. / — /V/ Kraus's 

 absorption-bands in succession from the red to the violet end ; the spectra are divided into twenty equal parts. 



middle, to the left of and on the line G ; VII may be regarded as the total absorption of 

 the violet end. 



The Spectrum of li'ving leaves agrees with that of the solution in its main character- 

 istics ^ The bands / — F ?ire, according to Kraus, easily made out in all ordinary leaves 

 of Dicotyledons, Monocotyledons, and Ferns. But this spectrum differs constantly from 

 that of the solution in all its bands being ahvays nearer the red end ; a point which was 

 determined by Kraus by the use of Browning's micro-spectroscopic apparatus. This 

 difference in position of the absorption-bands of the spectrum is, as he shows, an 

 illustration of the universal rule that the absorption-bands approach nearer to the 

 red end in proportion to the specific gravity of the solvent of the colouring substance. 

 It follows from this that the green colouring matter is distributed in such a manner 

 in the colourless matrix of the chlorophyll-grains that it must be considered in a state 

 of solution. In no case can the colouring matter of chlorophyll in living cells be in a 

 solid state, or equivalent to the residue left behind when the solution is evaporated. 



* For further evidence of this very remarkable fact see Geiland und Rauwcnhoff, I.e., p 

 It is not easy to understand how any physicist could maintain the contrary. 



604. 



