44 
ON THE DISEASES OF PLANTS. 
f Transverse section of the straw, highly magnified, g The fungus, highly magnified. 
Discoloration of Leaves. —Upon certain laws which regulate the proportion of 
acid and alkaline matter depend the colours of the leaves of plants, and these we 
know to be as various as the shades in the rainbow. There appears to be a certain 
proportion of these matters, however, allotted to each, a deviation from which 
generally more or less injures the health of the plant. 
The most usual colour of leaves is green in various shades, but they exist also of 
different shades of purple, &c. The most usual morbid change of colour in leaves 
is from green to white, either of the whole or only of part of a leaf, the white por- 
tions generally running in longitudinal stripes, sometimes dotted or quite irregular 
in distribution, at other times around the edges of its leaves only. 
Mr. Knight has observed that plants with white leaves cannot survive long, and 
that variegated plants bear the deprivation of light ill, but he believes there are 
many such which are neither in a state of disease nor debility. This indeed seems 
to be the case with some, but there are very few which will not be found to suffer 
in various degrees from the variegation. 
Morbid discoloration of leaves happens from various causes, which counteract 
the natural influence of light on the leaves, and it may ultimately be in consequence 
of the obliteration of the pores, or " culs de sac," according to Mr. Todd Thomson, 
by which respiration is performed. In such cases the blanched portions may obtain 
a quantity of carbonic acid, from that absorbed by the green portion of the leaf; 
but when all is white, the acid, it is possible, may be obtained by the roots or stems. 
However, it is agreed that it is from the accumulation of carbonic acid which takes 
place, such portions of leaves being unable to decompose it, that the change of 
colour happens, and which, indeed, seems to vary according to the excess of acid 
present. Plants, in such a state, are feeble in proportion to the extent of change 
produced. They lose their odour, taste and inflammability, and the juices are said 
to be nearly the same in all ; but these particular properties may often be restored 
by a removal of the causes when known. 
A want of proper nourishment is a frequent cause of discoloration in leaves ; 
also absence of light, and attacks of insects, as in the white and red tumours of the 
aphis ; again, from the destruction of roots or other vital parts • and, indeed, any 
thing which tends to in jure the health of the plant generally, is apt to produce dis- 
coloration of the leaves. 
