342 
NATURE 
[ Aug. 31, 1871 

or less brown colours, insoluble in bisulphide of carbon, 
and of very variable solubility in water or alcohol. The 
spectra show strong absorption at the blue end, extend- 
ing over the green, often the red is very dull, and some- 
times there are definite absorption bands, when the solu- 
tion is acid, neutral, or alkaline. On the whole they are 
in that state of oxidisation which has a maximum inten- 
sity of colour, and are simply decolourised by further 
oxidisation. 
The very numerous tints of foliage depend almost 
entirely on the relative and absolute amount of the various 
colours of these different groups, but much remains to be 
learned before we can explain all their relationships. The 
colour of green leaves is mainly due to a mixture of 
chlorophyll and xanthophyll and the variation in the 
relative and absolute amount of these easily accounts for 
the darker and brighter greens. The tints are also much 
modified by the pressure of colours of the erythrophyll 
group, which, according to circumstances, may give rise 
to lighter or darker browns, approaching to black, or to 
reds. Healthy unchanged leaves also contain various 
substances belonging to the chrysotannin group, but in 
many cases when these belong to the more typical kinds 
of tannic acid, their colour is so faint that they have 
little or no influence on the general appearance of the 
leaves. . 
The relation of these groups to one another is still 
somewhat obscure. There are facts which seem to indi- 
cate that chlorophyll may in some cases pass into xantho- 
phyll by oxidisation, and xanthophyll into chlorophyll 
by deoxidisation, but neither point can be considered 
to be established. There is manifestly some connec- 
tion between the formation of chlorophyll and ery- 
throphyll ; and those conditions which are favourable 
to the production of one are unfavourable to the develop- 
ment of the other. In the present state of our knowledge 
it seems most probable that chlorophyll is formed when 
the vital functions of the leaves are very active, and ery- 
thropbyll when they are less active but not destroyed. 
Exposure to light also appears to be necessary, and we 
often see rough natural photographs of superjacent 
leaves produced in this manner. As I have already said 
there are several different kinds of erythrophyll, giving 
very different spectra, but the most prevalent are two 
which are related to each other in an interesting manner. 
One of these is more especially found in very young 
leaves, and when slightly oxidised artificially it passes 
into the other. This more oxidised kind is that found in 
the greater number of leaves which are red in autumn. 
Both are completely decolourised by further oxidisation, 
and most probably this occurs in leaves themselves when 
their red colour-is lost. Since many contain erythrophyll 
in early spring and lose it as the season advances, whilst 
it still continues to be present in the leaf-stalks, I am 
much inclined to believe that its disappearance is due to 
the ozonised oxygen given off from the chlorophyll, which 
is developed to so much greater an extent in the leaves 
than in the stalks, and that its reappearance in autumn 
in many leaves is characteristic of the period when they 
are not dead but have more or less ceased to give off 
ozone, 
On the approach of autumn, before the leaves have 
withered, we have thus in the foliage of different plants 
an exceedingly variable mixture of chlorophyll, xantho- 
phyll, and erythrophyll, with the different members of the 
chrysotannin group; and it isto the changes which occur 
in some or all of these substances that the very variable 
tints of autumn are due. The most striking of these de- 
pend on the alteration of the chlorophyll. So long as it 
remains green the production of bright reds and yellows 
is impossible, but when it disappears the yellow colour of 
the xanthophyll is made apparent ; and, if much erythro- 
phyll is present, or contemporaneously developed, its colour, 
combined with this yellow, gives rise to scarlet or red. 
In many cases, however, the chlorophyll does not disappear, 
but is changed into the dark olive modification, easily 
prepared artificially by the action of acids on the more 
green, and when this is present, only dull and unattractive 
tints can be produced. We may thus easily understand 
why the special tints of early autumn are yellows and reds, 
or dull and dark greens. In these changes the various pale 
yellow substances of the chrysotannin group remain com- 
paratively unaltered, and even sometimes increase in 
quantity ; but they soon pass into the much darker red- 
browns of the phaiophyll group, whilst the erythrophyll 
fades ; and thus later in the autumnithe most striking tints 
are the brighter or duller browns, characteristic of the 
different kinds of plants or trees. 
As already named there are many different species of 
colouring matters belonging to the chrysotannin group, 
both of those which are, and of those which are not, closely 
related to the more typical kinds of tannic acid. So far I 
have not been able to ascertain whether there is any one 
particular artificial oxidising process which will in each 
case give rise to the exact products naturally formed in 
the leaves themselves ; but on the whole there is such a 
close correspondence between them that we cannot hesi- 
tate in concluding that the rich brown tints of autumn 
are mainly due to the oxidisation of the previously- 
existing more or less pale yellow colour of the chrysotan- 
nin group—a conclusion fully borne out by various 
independent facts. The difference in kind of tannic 
acid, and the absence or presence of any consider- 
able amount of a chrysophyll substance, explains in a 
very satisfactory manner the difference in the tint of the 
leaves of different trees. Thus, for example, the quino- 
tannic acid found in a comparatively pure state in the yellow 
leaves of the beech is changed by oxidisation into the fine 
red-brown colour of those leaves at a later period. This 
kind of tannic acid also occurs in the elm, but is there 
mixed with more or less of a chrysophyll, which turns to 
a duller brown ; and thus we find the leaves of different 
elm-trees vary in tint, and are often of very dull brown 
colour. The leaves of the oak and Spanish chesnut con- 
tain gallo-tannic acid, and this, when oxidised, gives rise 
to a dull tint, like that seen in the faded leaves of those 
trees ; and similar principles hold good in other cases, 
As far as we are able to judge from the various facts 
described above, we must look upon the more characteristic 
tints of the foliage of early spring as evidence of the not yet 
matured vital powers of the plant. In summer the deeper 
and clearer greens are evidence of full vigour and high vita- 
lity, which not only resists but also actually overcomes the 
powerful affinity of oxygen. Later on the vital powers are 
diminished, and partial changes occur, but the affinity of 
the oxygen of the atmosphere is nearly balanced by the 
aon thease 

