ON THE DISEASES OF PLANTS. 
39 
consists of a pole of wood from thirty to fifty feet long, fixed perpendicularly in the 
earthj having a brass wire^ sharpened at the pointy, running from two to three inches 
above the summit down into the ground. These should be planted at the distance 
of from one to two thousand feet from each other. By their influence on the electric 
state of the atmosphere^ the hail is said to be melted down to snow or rain, the 
descent of which does comparatively little mischief. It is chiefly in southern climes 
that these are required. 
Plants receiving wounds in their leaves must be protected from extremes of heat 
and cold, from too much moisture, and altogether treated more carefully than usual, 
according to the habits of the plants affected. 
Punctured wounds of leaves. — Leaves are punctured by various insects for two 
purposes, viz. for the purposes of procuring food, and for oviposition. The most 
remarkable insects of the first kind is the aphis, which attack the under surface of 
the leaves and young stems, sucking the sap of the plants by inserting their curious 
proboscides. From these wounds the leaves become variously distorted in shape, 
generally assuming a blistered appearance, in consequence of the irritation from the 
punctures at particular parts ; and sometimes several of these diseased portions may 
he seen rising nearly half-an-inch on the surface of a leaf otherwise healthy ; the 
concavity of these are found to contain numerous aphides. Such distortions are 
generally of a whitish or reddish colour. 
On the leaves of the maple tree, round purple tumours, with narrow necks, are 
often found projecting from their upper surface, having a small opening leading into 
them from the under surface, nearly closed, however, by a number of morbidly 
enlarged hairs : these we believe to be the efl:ects of punctures by the aphis. Small 
white tufts are often seen attached to the leaves and stems of the larch : these are 
are produced by a brown aphis, and when examined by the microscope are seen to 
consist of a bundle of fine filaments, generally twisted around the animal, and 
-attached to the extremities of many : oval-shaped eggs are also frequently seen. 
Various methods have been proposed and practised for the destruction of the 
aphis. The most eflfectual appears to be fumigation with tobacco-smoke, followed 
up by washing with lime-water, and finishing by digging the ground around the 
plants. 
Leaves are frequently found traversed by white lines, twisting and running in 
all directions ; they are caused by the larvae of certain insects, which eat their way 
through, leaving their excrements behind them in their paths ; and so neatly do they 
perform their work of destruction, that by far the finest dissections we have ever 
seen were the works of some of these animals. They burrow under the cuticle, and 
generally confine themselves to the upper surface of the leaf, sometimes to the lower ; 
occasionally, however, both are attacked. 
Some insects lay their eggs close to each other on the back of the leaves of 
certain plants. After a time these become hatched, and the larvee, each for itself, 
pierces a small hole immediately above the attachment of the egg from which it 
came, and, passing through the leaf, arrives at the upper surface, when it commences 
its mining, covered only with the cuticle : these do not traverse the leaves like the 
