4. The most important result was the finding of three well defined colonies of living mature trees; all of which, by virtue of characters to be presently described, are offering more or less resistance to the disease.
Seedling Trees
It is well known that seedlings and young saplings are naturally immune for a certain period, which varies in extent from 8 to 15 years beyond germination of the seed beginning, of course with the first formation of the seedling. Such immunity depends, however, not on any inherent characteristic, but on the fact that at this period the bark is usually smooth, sound, and free from wounds of any sort where Endothia spores and mycelium might enter. Of course, when wounded from any cause whatever during this period of youth, this immunity ends, so that the condition might perhaps be termed physical, in contrast to physiological immunity.
As I have already said, large numbers of seedlings, for the most part still unattacked, were found in many places in the area surveyed. There are of course no grounds for believing that such seedlings, descended as they are from non-resistant trees, are physiologically immune. Where they are free from disease, this exemption is due merely to the physical immunity I have just mentioned. Since they therefore represent non-resistant stock, they were used for comparative inoculation work, which will be referred to later.
I may as well say here as anywhere, that by resistance, I do not mean total resistance, for that would be immunity. There are, of course, degrees of resistance, in the plant world just as in the animal world. One person may resist a cold germ or the influenza bacillus better than another, that is, it will cause him only a little discomfort. Another person may not be affected at all, that is, he is totally resistant or immune. I say this because I have misunderstood when I have used the term resistance. The trees in the New York 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 white inner bark is quite healthy, generally for a thickness of 5-7 mm. That the mycelium in the small lesions was unquestionably the Endothia mycelium, was shown by the appearance of the mycelium, and the presence of the Endothia pustules in many of the spots. That these were not late infections, but only slowly growing small lesions, was shown by inoculations in such bark, which revealed scarcely any growth after two months.