24 PLANT DISEASES AND FUNGI.
instance, which is adduced, of the analogy above alluded to : "The
likenesses between the inflammations in plants and in animals are
best shown in their visible structural changes, and these have been
admirably traced by Waldenburg. He has applied various irritants
to leaves, fruits, and stems, such as foreign bodies, setons, crushings,
cauteries, and others. The results, speaking very generally, are that,
as in ordinary wounds, the cell structures actually involved in the
injury perish and dry up; that those most nearly adjacent suffer
degeneration, indicated by their protoplasmic contents becoming
turbid, and their chlorophyll becoming yellowish or brownish, while
in those next to them, and within distances varying according to the
injury and the texture of the part injured, enlargement of cells
ensues, and increase by division and thickening of the cell walls.
In these changes you may study, with comparatively easy experi-
ments, imitations (as near as differences of texture will allow) of the
most constant constituents of inflammation in animals, especially
those of the least acute of the productive interstitial inflammations,
leading to thickening, opacity, induration, and other such changes."
Pursuing this subject of analogy still further, we come face to
face with the results of some of the most recent investigations in
which microbes play an important part. It is only very recently
that a long entertained suspicion has been verified of the presence
of bacteria in plant disease, as well as in that of animals. The
disease known in the United States as "Peach yellows" has con-
stantly evaded all search for mycelium, or trace of fungoid develop-
ment, and yet it is a destructive and insidious foe. Professor Burrill
made investigations in 1888 and 1889, but without any decided
results. Nevertheless he reports that "he had found in the tissues
of the root, and of the old and young stems of diseased trees, an
organism, classed with the bacteria, which is not known to occur
elsewhere. This organism has been frequently obtained by method
of cultures under circumstances which preclude the possibility of its
coming from anything except the inner cells of the tree. He had it
growing in artificial media, and it exhibited all the peculiarities of a
pathogenic rather than a saprophytic microbe. It had peculiarities
which served to distinguish it from all others of its kind, and he
was convinced it had never before been described by any one. He
found it in every set of specimens examined, known to be affected
with this disease, and had thoroughly tried in the same manner to
find it in healthy stock and failed." Still further he says: "If the