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the best explanation were, that chlorophyll is formed when 
the vital functions of the leaves are very active, and erythro- 
phyll, like an excess of xanthophyll, when they are less 
active, but not destroyed. 
After I had found that a number of colours belonging to 
the erythrophyll group could be prepared artificially by the 
oxidization of the different kinds of tannic acid, I felt much 
inclined to believe that those found in red leaves might ori- 
ginate in this manner ; but by using equal quantities of the 
green and red parts of the same leaves, I was able to prove 
that the change of colour was not accompanied with any cor- 
responding diminution in the amount of tannic acid. On the 
whole, then, it seems difficult to account for the production 
of erythrophyll by any mere chemical change, and this agrees 
well with the general facts of the case. It is often very 
specially developed in partially broken twigs, in those parts 
of leaves which have been damaged by insects, and in 
detached leaves lying on damp ground with their under sur- 
faces upwards; also when they die, and turn brown in 
patches, or at the edges, a band of erythrophyll is often 
found along the border of the part still green, which is, per- 
haps, not yet actually dead, but certainly would be very soon. 
Exposure to light is also necessary, and we often see rough 
natural photographs of superjacent leaves produced in this 
manner. ‘This result seems to depend more on general in- 
tensity than on any particular rays. All these facts indicate 
an impaired vital action; but, at the same time, it is quite 
clear that in certain plants analogous conditions are met with, 
even when they are growing vigorously. Thus, in early 
spring many young leaves are red or brown, but as their 
chlorophyll-producing powers increase, the erythrophyll 
disappears, or is confined to the stalks and other parts, where 
the true functions of leaves are not actively carried on. 
Sometimes, indeed, as in the case of the red beech, it in- 
creases In amount, and remains during the whole summer, 
but more frequently it is lost, and again makes its appear- 
ance in autumn, either before the chlorophyll disappears, so 
as to give a dark brown, or, whenit disappears, soas at once 
to give a fine red. Its production, therefore, does not 
depend on the weakening of that kind of vitality which mani- 
fests itself in mere growth, no more than the formation of an 
abnormal amount of xanthophyllin plants grown in the dark, 
or in those varieties with very yellow leaves ; and yet we 
know that in this latter case they cannot permanently live 
unless some of the leaves contain chlorophyll. What is 
seen in green leaves variegated with yellow and red, also 
