h
AECD-3446(App. )
described in this study with some of those already known.
Plant galls or tumors may be caused by fungi, bacteria, viruses, nematodes,
insects, chemical substances and genetic factors.
They are as varied and numerous
as the number of inciting agents would indicate.
The most intensely studied plant gall is known as Crown-Gall, which is
incited by Agrobacterium tumefaciens.
It has been observed on plants belonging
to widely separated botanical families and it has been described on almost all
organs of susceptible plants (10).
The family Convolvulacese, of which the genus
Ipomoea, is a member is not found on this list, however.
Crown gall bacteria are
widely distributed and are apparently native in many soils where they lead an
independent life or persist in old galls (16).
It is uncertain whether the bac-
teria are intercellular or intracellular, but the bacteria must be introduced
through a wound; and the size of the wound determines to some extent the size of
the gall (9).
Apparently the bacteria produce something which transforms normal
tissue into tumor tissue.
After this, the galls can continue to grow without
the inciting principle, but the nature of the inciting principle and its mode
of action are unknown.
Crown-gall is not a systemic disease, however, and the
relative size of the tumor apparently depends on the amount of transforming principle available at the time of the cellular alteration.
There is a considerable
histological variation in reaction to the crown-gall organism reported in the
literature.
In general, the tissues are more distorted than normal, and giant
cells with many nuclei may be present (9).
It has been reported by Braun (4)
that cells which have undergone the transformation induced by the crown-gall
organism can change back into normal cells and give rise to organse
The host renges of other galli-inducing bacteria are quite limited and are
not applicable to this study (27).
Insects are probably the most common cause of galls in plants.
case Of a great many insects
In the
there is no mechanical injury, but in all cases
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