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, extending over the green ; 
often the red is very dull, and sometimes they show definite 
absorption-bands in the central part of the spectrum, when 
the solution is acid, neutral, or alkaline. On the whole, they 
are in that state of oxidization which has a maximum in- 
tensity of colour, and are simply decolourised by further 
oxidization. This, however, they resist strongly ; and since, 
moreover, they are often very insoluble in water, leaves con- 
taining them retain their colour unchanged for a long time, 
even when lying on the damp ground. 
It may be convenient to separate the dark colours produced 
by more complete decay from the group just described, and 
to designate them by the term humus ; but, at the same time, 
it is difficult to point out any very decided physical distinc- 
tion. We might, indeed, say that humus is a phaiophyll 
colour, but could not say that all phaiophyll colours are 
humus. : 
Having thus given a general account of the leading groups 
of coloured substances occurring in leaves, I will proceed to 
describe the manner in which they are related to one another, 
since the very numerous tints of foliage depend almost entirely 
on their relative and absolute amount. At the same time I 
must say that much more must be learned before we shall be 
able to explain all their relationships, and to understand why 
they are produced in some cases and not in others. 
The colour of green leaves is mainly due to a mixture of 
chlorophyll and xanthophyll, and the variation in the rela- 
tive and absolute amount of these completely accounts for the 
darker or brighter greens. ‘The tints are also much modi- 
fied by the presence of colours of the erythrophyll group, 
which, according to circumstances, may give rise to reds, or 
lighter or darker browns, approximating in tint to some of 
the colour of the phaiophyll group, but really due to a very 
different cause—to mixtures, and not to simple colours. 
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 peculiar blue 
tinge seen on some mosses appears to be due to interference 
of light, and not to the presence of any blue colouring 
matter. 
The relation of these groups to one another is still some- 
what obscure. There are facts which seem to indicate that 
