Some Bacterial Diseases oe Plants. 7 
subsequent blackened appearance of the young leaf tufts and flower 
clusters; some of the leaves may be only partly blackened, while the 
greater part of the blade remains green; the young twigs show a 
blackened, shrivelled bark, resembling very much green brush which 
lias been only partially burned. The blight makes its appearance 
early in the spring shortly after the blossoms have fallen and works 
rapidly back from the blossom clusters an inch or more a day. It 
soon involves the tender, succulent twigs and may ultimately destroy 
the whole limb. If the diseased wood be cut with a sharp knife, a 
dark ring between the bark and the wood will usually be seen. This 
is a further indication that the tree is affected. This ring of tissue, 
now blackened, is known as the cambiuin layer. It is here that a 
rich and abundant supply of food is to be had and, quite naturally, 
what is good for the growth and development of the tree is equally 
as nourishing for the germs. So we find, upon microscopic exam¬ 
ination, that this blackened cambium tissue has lost its normal color 
and appearance and instead of the cells being almost white in color 
with a clear, colorless liquid within, they are brownish, more or less 
Ijroken, and all through this mass of broken down tissue is a straw- 
colored liquid, clouded with millions upon millions of germs. These 
germs is growing here have dissolved out the starchy constituents 
of the plant tissue and literally eaten up the cells to such an extent 
that the circulation of the plant is destroyed. The rapid progress 
^vhich the disease makes is to be accounted for, in a measure at 
least, by the fact that the causal microorganisms have the power of 
very rapid movement; that is, we say they are motile. Thus en¬ 
dowed, they are persistently swimming about in the protoplasm of 
the cambium, dissolving and boring their way into new, healthy 
tissue where a fresh supply of food can be obtained. As the dis¬ 
ease progresses and the smaller limbs show the infection, the tender 
bark may crack and a thick, black, sticky gum, alive with germs, 
may exude; soon afterwards the bark becomes dark colored, har¬ 
dened and shrunken. 
The disease makes its greatest progress during the actively 
growing period in the spring when the tree is putting forth a mul¬ 
titude of young, tender twigs and the new tissues are gorged with 
sap. Just such conditions as these favor the growth of the blight 
microorganisms, so it is easy to see why the disease is worst at this 
time of the year. As the season advances, the plant tissues harden, 
less sap is flowing, and conditions for germ life become less favor¬ 
able. As a result, by the middle of summer the active progress of 
tlie blight is checked by natural causes, but the disease is still pres¬ 
ent in what may be termed a latent form. In this stage it some¬ 
times remains through the balance of the season and, protected by 
the bark of the twig or limb, it may live through the winter, giving 
