ON THE DISEASES OF PLANTS. 
47 
and this, in some cases^ seems to answer tolerably well, while in others it entirely 
fails. 
Gangre?ie of Stems. — The disease just described is frequently the cause of 
extensive gangrene, by which we mean, that the part loses entirely its vitality, and 
is, in many cases, changed in its consistency, having its texture completely broken 
up. In all cases the colour is changed, and generally to a reddish brown or black ; 
but the texture is not always broken up. Gangrene seems to occur, for the most 
part, in consequence of wounds, parasitical plants, great heat or cold, excess or want 
of moisture, lightning, &c. 
Extremes of heat and cold, and sudden alteration of temperature, are frequent 
causes of gangrene in succulent stems and other parts of plants, as the leaves, 
flowers, &c. ; and the hopes of the gardener are often blasted by their influence, 
especially in spring. 
The cherry, plum, and other stone-fruit trees, are subject to a species of 
gangrene, which is generally called the " Gum," from the gummy effusion which 
frequently issues from the aflfected stems. The affected branches must be removed 
as soon as possible. The disease is very rapid in its progress. 
Stems sometimes become bark-bound, and this is supposed to arise from the 
cuticle not giving way as it ought to do ; trees in consequence become stunted in 
growth. It is sometimes cured by longitudinal incisions along the stems down to 
the alburnum. 
Natural Decay of Plants. — Plants, like all other organised bodies, are destined 
to exist but for a time ; some for less than a day, others for more than a thousand 
years ; but all are subject at last to decay. Each having certain functions to 
perform, that the continuation of the species may be insured, natural decay never 
comes on till this process is completed, and the period specified varies in almost 
every diflPerent species. 
Many plants, after having produced seeds but once, die ; others again are 
permitted to perform this process from year to year ; but in these there is, at 
particular periods, a decay of the temporary organs, and after a time they, too, come 
under the influence of that general law which all organised bodies are forced 
to obey. 
In considering the methods to be pursued to counteract natural decay, we must 
keep in mind, that plants, for the most part, decay in the same ratio that they come 
to perfection ; and, therefore, whatever means are to be employed, they must be 
practised with a constant reference to that fact, that it is not until after the plant 
has become fit to re-produce its species that natural decay appears ; from which it 
is obvious, that the only means to be employed, are such as will prevent the plant 
from attaining that particular period. 
