143 
lourable matter, more or less favour or modify the 
operation of this agent. 
397. The foregoing observations are farther ap- 
plicable to those chequered or party-coloured leaves, 
many of which draw so much regard from their 
elegance and beauty. In many instances^ these ap- 
pearances proceed from age, from injury, or disease : 
but the regularity and uniformity of the colours, in 
other cases, indicate a natural state of the plant. 1ft 
the per/oliata pic t is joins, says M. Senebier, the 
green leaves are spotted with yellow. The green 
leaves of the pimpernel of the mountain have yellow 
stripes. In a species of aloe, the middle part df 
the leaf is . green, and the borders are yellow, 
while others are green and yellow, or yellow and 
green *. Now, in green plants, the yellow colour 
is formed by reducing the green, as in the leaves" 
which fall in autumn, and this is effected by the 
development of acid matter. If, therefore, acid na- 
turally prevail in one part of a leaf more than in 
another, a proportional degradation of colour will 
be produced, and this, too, in particular parts or 
places, according, probably, to variations in the 
structure of the leaf, or some other accidental cir- 
cumstance j so as to give rise to that mixture of 
green and yellow which has been above stated. 
Should the acid abound still more, it may, as in cer- 
tain red leaves of autumn, give rise to a red colour. 
Thus in the tri-coloured amaranth, the leaves are, at 
the same time, green, yellow, and red ; but this is 
most frequent in autumn, as the power of vegeta- 
Jtion declines f. 
