ACTION OF LIGHT ON VEGETATION. 
matters. These non-essential substances are far more common in the petals than in the 
leaves, and if of any use to the plant, are only indirectly advantageous, as, for instance, 
in attracting insects. It is doubtful to which of these two divisions certain substances 
should be referred, and perhaps some may not be essential for the healthy performance 
of vital functions, but merely necessary products ; and some may be essential to one 
plant and not to others. 
It has been found convenient to arrange the colouring-matters of plants in the 
following groups, which are as it were of generic value, and include several different 
species. 
Chlorophyll group. — The green substance described as chlorophyll by many writers 
must often have contained two perfectly distinct green substances, and the product of 
the action of acids on one of them, mixed with one, and in some cases with three, 
different species of xanthophyll, and one or two of lichnoxanthine. These two green 
substances are blue chlorophyll and yellow chlorophyll ^ Blue chlorophyll dissolved in 
alcohol is of a splendid blue-green colour, the whole of the green part of the spectrum 
and a considerable part of the contiguous blue being readily transmitted. Yellow 
chlorophyll absorbs the whole of the blue and the blue end of the green, so that the 
general colour is a bright yellow-green. Chlorofucine is of a clear yellow-green colour. 
Red end. Blue end. 
Blue chlorophyll . . . . 
Yellow chlorophyll . . 
Chlorofucine 
"~ _ Fig. 476 Spectra of the chlorophyll group compared. 
It has many properties in common with the above-named two kinds of chlorophyll, 
•being, like both of them, highly fluorescent and easily decomposed into another modifi- 
cation by acids. All three are insoluble in water and soluble in absolute alcohol, but not 
always in carbon bisulphide. 
The difference between their spectra will be better understood by means of the 
figure, 476 b, which represents the absorption-bands as seen in solutions diluted so as 
to show those at the blue end, and only the darkest and most characteristic of those 
in the red. 
Xanthophyll group, — This group includes a number of yellow or orange-coloured 
substances, insoluble in water but soluble in carbon bisulphide, giving spectra with two 
more or less well-marked absorption-bands in different positions, according to the 
particular species. They are not fluorescent, and when dissolved in absolute alcohol, 
after addition of a httle hydrochloric acid, they all gradually become colourless, but two 
of them are first changed into a blue substance. Nearly all green leaves contain three 
perfectly distinct fundamental species, which Mr. Sorby has named orange xanthophyll^ 
xanthophyll, ^Lndyello^iv xanthophyll. The spectrum given in Fig. 476, copied from Kraus, 
^ [The spectrum given by Kraus (Fig. 476 B) is due to a mixture of these with some of 
the products of the action of acids. See Pringsheim, Ueb, natürliche Chlorophyllmodificalionen, 
Monatsber. d. k. Akad. d. wiss. zu Berlin, 1876.] 
