ACTION OF LIGHT ON VEGETATION. 
759 
of Dicotyledons, Monocotyledons, and Ferns. But this spectrum differs constantly from 
that of the solution in all its bands being always 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-granules 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. 
If an alcoholic solution of chlorophyll is agitated with any quantity of benzol (say 
double its volume) two very sharply separated strata are formed after the fluid comes 
to rest, a lower alcoholic stratum of a pure yellow colour, and an upper blue-green 
stratum of benzol. Kraus considers this process to be a dialytic one ; there are, ac- 
FlG. 476.— Absorption-spectra of the colouring mattef of chlorophyll (after. Kraus). A the spectrum of the alcoholic 
extract of green leaves; /> that of the blue-green constituent soluble in benzol ; C that of the yellow constituent. The 
absorption- bands of A and B are indicated in the less refrangible (left-hand) portion as they would be produced by a 
more concentrated, in the more refrangible (riglit-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. l—VII Kraus's 
absorption-bands in succession ftom the red to the violet end ; the spectra are divided into twenty equal parts 
cording to him, two colouring substances in the ordinary chlorophyll-solution, a blue- 
green and a yellow one, soluble in very different degrees in alcohol and benzol. 
Kraus therefore holds the spectrum of chlorophyll to be a combination-spectrum, 
/. e. that it arises from the superposition of the two spectra of the blue-green and 
the yellow colouring-matter. The blue-green substance gives the four narrow absorp- 
tion-bands in the less refrangible half of the spectrum (Fig. 476, B), and part of the 
band T/ which is situated at G in the more refrangible half. The band ^(Fig. 476 C) 
results from the yellow colouring matter which has absorption-bands in the more re- 
frangible half of the spectrum. The band VI of the chlorophyll-spectrum is the result 
of partial superposition of corresponding bands in the spectra of the yellow and the 
blue-grcen substances, which however do not perfectly coincide. Both colouring sub- 
stances alike produce the absorption-band VII at the violet end. 
According to Kraus' statements it is possible to decompose chlorophyll into two 
out (Ueb. Lichtwiikimg und Chlorophyllfunction, Jahrb. f. vviss. Bot. XII, 1881) that the chloiophyll- 
spectrum is the j^a.ne in leaves which are assimilating as in those which are not.] 
