CHAP. XVI. 
COLOUR. 
431 
other colours may be ascribed to the presence of an alkali. 
This is, however, far from proved. 
“ The same colours which stain leaves in the autumn may 
also be produced by certain accidents. Thus the puncture of 
an insect, the attacks of parasitical fungi, or injury from early 
frosts, produce partially or entirely yellow or red colours ; and, 
what is remarkable, the colours thus accidentally assumed are 
the same as the plant would have taken of itself in the autumn : 
thus accidents turn the leaves of the Poplar and the Lilac 
yellow, of the Sumach or the Pear tree red, as they become 
in the autumn. 
Certain leaves offer naturally, on one or both their sur- 
faces, marks coloured in a particular manner, from the mo- 
ment when they first unfold. Tradescantia discolor, and 
several Begonias, have their under surface red ; certain 
Arums are irregularly blotched with red ; there are species 
of Amaranth which, in an apparently healthy and natural 
state, have leaves banded with both yellow and red. It is 
worthy of note, that in regular and natural colourations red 
is very common, and yellow comparatively rare, although one 
would have thought that the latter, caused, as it seems to be, 
by a slighter kind of change than red, would have been the 
most common. Blue seems altogether excluded from changes 
of the leaves, except in the case of certain Eryngoes. 
In many plants, the leaves which grow in the vicinity of 
flowers are accustomed to offer various tints, which are 
almost uniformly in unison with the colours of the flowers 
they accompany; such floral leaves or bracts are yellow in many 
Euphorbias, scarlet in Sages, violet in Clary, and blue in 
particular states of the Hydrangea. 
“ Why then should it be different with petals and the 
petal-like parts of a flower? These organs are in truth 
nothing but modified leaves ; they are capable in particular 
cases, such as Hcsperis niatronalis, of transforming them- 
selves into genuine leaves, green, and capable of exhaling 
oxygen.” 
With regard to the exact relation that colours really bear 
to one another, and to the causes that are supposed to influ- 
ence them, a memoir upon the colours of flowers, published 
