the plow sole and thus did not affect either
cultivation nor the growth, of grasses and cereals beneath them.

The pros and cons of the problem have been reviewed several times in the
recent proceedings of the Northern Nut Growers Association. (Greene,
1930; MacDaniels and Muenscher, 1942; Brown, 1943.) That the roots of
walnut trees are toxic to the roots of certain crop plants in direct
contact with them is widely accepted. In nature this toxicity seems to
be limited to plants with tap root systems such as tomato and alfalfa
(Davis, 1923) and those with other types of deep root systems such as
apple trees (Schneiderhan, 1927), rhododendrons (Pirone, 1938), and
privet. This toxicity is exhibited only when there is a direct contact
between the roots of the two plants involved. (Jones, 1903; Massey,
1925). That the wilting observed under walnuts is due to a toxic product
from the bark of the walnut, and does not result from a lack of water,
is substantiated by the fact that the vascular or water conducting
system is discolored for several inches above the point of contact with
the walnut root. This symptom is very similar to that produced by
vascular disease fungi. No such discoloration results from wilting due
to competition for water. This symptom of toxicity has been overlooked
by many workers in the field.

Massey (1925) suggested that the toxic component of walnuts might be
juglone. This idea was further supported by Davis (1928). Today this
concept is widely held. Chemically this substance is known as 5,
hydroxy-1, 4, naphtho-quinone and belongs to a group of strong oxidizing
agents with commercial uses, including tanning agents, medicinals,
poisons, etc.

A knowledge of the physiology of juglone in the walnut is essential to
an understanding of the divergent results obtained by various
experimenters. Juglone, as such, occurs probably only in minute
quantities in the inner root bark, and in the green husks of the nuts.
These regions are, however, rich in a substance known as hydrojuglone.
This compound, the colorless, non-toxic, reduced form of juglone is
immediately oxidized to its toxic form upon exposure to the air or some
oxidizing substance from the roots of other plants. Upon standing in the
air juglone again disappears, being either changed back to hydrojuglone
or broken down into other non-toxic substances.

This sequence of events may be noted in a fresh green husk of a black
walnut. When the fresh hus