ON THE DISEASES OP PLANTS. 
45 
Gangrene of Leaves. — All the diseases of leaves described are apt to cause gan- 
grene, either in the whole, or a portion of the leaf. When such is the case, the leaf 
first becomes yellow, then brown, sometimes black ; and if the plant possesses 
sufficient vital energy, the affected portion or leaf is thrown off, a separation having 
taken place between the dead and living parts. 
Wounds of the Stem. — The stem is very subject to wounds of various kinds, 
according- to the nature of the instrument by which they are inflicted, and the par- 
ticular texture involved. Wounds which penetrate or remove a portion of the bark 
are very common, either from accident or intentionally, as for the cure of some 
diseases, or as in pruning. Such wounds, and indeed all wounds of the stem, heal 
the quicker the more vigorous the branch or plant is in which they occur, and of 
course according to their dimensions. When of great extent, many years may be 
required for the process. 
All wounds heal more rapidly from above downwards than in any other way, no 
doubt stimulated by the returning juices. The most serious wounds of the bark 
are those which detach it to a great extent all round the stem ; for the consequence 
is, that all branches beyond the wound are forced to become fruitful^ and thus 
premature decay is brought on. 
Simple incised wounds, and even those in which the bark is raised from the 
stem a considerable extent, may unite^ if the edges be accurately adapted to each 
other, and the whole covered with some plaster to exclude the air, keep out too 
much moisture, and prevent insects from ovipositing in the wounded part. 
Forsyth's preparation for this purpose is perhaps one of the best. 
In pruning, numerous wounds are necessarily inflicted ; these, however, when 
smoothly and properly made, produce no mischief. Such wounds heal in the same 
way as those simply of the bark. From experiment, very little injury appears to 
occur from cutting down and removing the pith in twigs of any age short of the 
most recent. They, however, cannot bear the mid-day sun so well afterwards. 
Stems bruised or fractured even more than half way through, in many cases, 
will recover ; but when more than that it is generally better to remove them at 
once, especially in old plants, as in such cases there is almost no chance of recovery. 
There are many plants which never recover even slight fractures, but gradually 
decay. 
Many succulent stems will suffer themselves to be bruised and broken to a great 
extent, and yet, by proper management, will recover. The most proper treatment 
for fractured or bruised stems is to tie them up, bringing the edges of the wound 
neatly together to promote a union, and covering the whole with some plaster. In 
this way many fractures unite, and bruises become consolidated. 
/F" ounds of Stems from Insects. — There is a minute insect, which frequently infests 
the stems of apple and pear trees ; it is probably the Coccus arhorum of Reaumur. 
It attacks all the stems indiscriminately, which in consequence become covered with 
numerous muscle-like bodies, varying in length from one-eighth to three-sixteenths 
of an inch. The trees suflTer from these, when numerous, by their inducing disease 
in the bark from the lodgment of water, which they cause, and also by hindering 
the functions of the bark. 
