358 
PHYSIOLOGY. 
BOOK II. 
The red colouring matter obtained from leaves forms infusions 
which, like those from flowers, become more intense when 
acted upon by acids. Yellow leaves act in this manner like 
yellow flowers. It is supposed by some, that while red is 
owing to the development of acid, other colours may be 
ascribed to the presence of an alkali. This is, however, far 
from proved. 
" The same colours which niark 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 spe- 
cies of Amaranth which, in an apparently healthy and natu- 
ral state, have leaves banded with both yellow and red. 
Macaire has determined that the red chromule of leaves 
which are thus discoloured is chemically the same as that 
produced in the autumn. It is worthy of note, that in regu- 
lar 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. Macaire assures us that 
the chromule of the bracts of Salvia splendens presents the 
