region show all grades of
resistance, from individuals where the fungus makes very little headway
in the bark, to cases where it grows almost as fast as in the average
non-resistant tree.

CHARACTERISTICS OF THE RESISTANT TREES

What now are the characteristics of these resistant trees? How are we
going to know one when we see it? I have outlined the leading features
as follows:

1. BARK. In the case of this particular disease, it is obvious
that the character of the bark is the most important feature since this
fungus is primarily parasitic in the living cortex. In other words, the
character of resistance must necessarily depend on the living cells of
the cortex. Now, careful observation of the resistant trees reveals a
most striking feature of the bark, namely its tendency to heal, by means
of a callus growth around the margins of the lesions, whether large or
small; and it is very apparent that this callus growth wards off the
advance of the fungus for a time at least. When the callus growth is
once formed, the fungus of the original canker encroaches on it very
slowly, or often not at all. Inoculations in the callused margins of
cankers showed usually only slight growth of the fungus after two
months' time in the summer, or in some cases no growth at all. Several
layers of wood could be counted underneath these callused margins--often
6 or 7--before reaching the annual ring exposed at the surface of the
canker. This of course, shows unquestionably that the callus had
remained healthy at that location for that period of time.

2. EXTENSION OF THE CALLUS TISSUE.--In many cases the callus
tissue is of considerably greater extent than the normal area one would
expect around a wound. It may even occur that the whole inner bark
around the trunk is of a callused nature, without any open cankers
showing at all. For example in a tree of which I have a photograph here
(Figs. 2 and 4), the outer bark is sloughing off, revealing callused
bark underneath of entirely different appearance, which no one would
recognize as chestnut bark. This particular tree photographed represents
an extreme in this respect. It seemed as if the whole tree was getting a
new kind of bark, and yet this same character appears in all of the
highly resistant trees. On cutting into this new callused inner bark it
was found plentifully dotted with tiny _Endothia_ lesions, which
however, never penetrated deeply. (Fig 4). Close to the cambium the
whi