40 
ON THE DISEASES OF PLANTS. 
others,, but confine themselves to one part. The leaves of apple and pear trees are 
very subject to this affection. 
The treatment of such is not as for the aphis, in consequence of the protection 
they receive from the cuticle. The most powerful remedy is boiling water. 
Wounds of leaves with oviposition. — The most curious effects produced by 
the wounds of insects, are the various kinds of galls. These are tumours produced 
on various succulent surfaces, varying according to the insects which affect them, or 
the plants in which they occur. They are caused by insects of the order Hymenop- 
tera and genus Cynips, and occur in a variety of plants, but most frequently in the 
oak and willow. It is very evident that these curious excrescences must materially 
injure the health of the plants on which they occur, from the inordinate consump- 
tion of substance required for their nourishment ; and this is found to be the case, 
for when they occur in great numbers, from the excess of nourishment drawn off, 
the branch, or even the whole plant suffers, the leaves become distorted and small, 
and the whole plant stunted in growth. 
They scarcely admit of a cure in many plants. The method, however, in the 
early stage, is the application of boiling water ; afterwards, when of considerable 
size, nothing but their removal will be of service. 
Parasitical Plants or Leaves. — Leaves are liable to become attacked by various 
cryptogamic plants, from circumstances not well ascertained. The most familiar of 
these diseases is known by the name of " mildew," ( Sporotrichum macrosporum, ) 
which is at once observable by the white appearance it presents on the leaves. 
Certain circumstances seem particularly favourable to its appearance, as cold dry 
weather, and particular exposures ; plants under the shade of others, or otherwise 
shaded, apparently suffering more than those fully exposed. 
From microscopical observation, this parasitical plant, constituting " mildew," 
seems to be composed of globular semi-transparent masses, apparently sometimes 
attached to a stalk, sometimes to hairs on the plant, or collected into heaps on the 
surface of leaves and stems. 
Amongst the remedies proposed, perhaps the best is thinning around the affected 
part, or removing to more light and air. Delicate and rare plants may have their 
leaves washed with water. Lime water has been found of service on apple trees. 
In hardy plants slightly affected, remove all the diseased leaves or shoots, and attend 
to the other circumstances mentioned. An apple tree annually affected was per- 
fectly cured by a free washing of what is termed the u cream of lime " during the 
winter, and the tree appeared very little the worse, having next spring thrown out 
healthy shoots in abundance, except at the termination of one branch, which passed 
partly through a currant bush next to it on the wall, and which suffered the usual 
attack from mildew. From this case we are inclined to think that the seeds of the 
parasite remain about the buds and stems until a fit period for their evolution. 
Parasitical plants, apparently of a somewhat similar structure, occur of different 
colours. There is the red, termed " rubigo, or rust," (Uredo salicis,) occurs on the 
leaves and stems of many plants. There is also a black kind, termed "smut," 
(Uvedo segetum). It is most destructive to wheat, oats, &c, which are also very 
often affected with the red kind. 
