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It is probable that this effusion of turpentine into the tissues of the wood, cambium, and cortex has much to do with the drying up of the parts above the attacked portion of the stem: the tissues shrivel up and die, the turpentine in the canals slowly sinking down into the injured region.
The annual rings are formed as usual on the opposite side of the stem, where the cambium is still intact, or they are even thicker than usual, because the cambium there diverts to itself more than the usual share of food substances; where the mycelium exists, however, the cambium is destroyed, and no thickening layer is formed.
One autumn I caught one of these fellows in the act of sinking long rows of his little wells in the limb of an apple-tree. There were series of rings of them, one above another, quite around the stem, some of them the third of an inch across. They are evidently made to get at the tender, juicy bark, or cambium layer, next to the hard wood of the tree.
How young the old trees suddenly become! what suppleness and grace invest their branches! The leaves are a touch of immortal youth. As the cambium layer beneath the bark is the girdle of perennial youth, so the leaves are the facial expression of the same quality. The leaves have their day and die, but the last leaf that comes to the branch is as young as the first.
There was one department of commerce, namely, cambium, or money-changing, which, while it did not give any difficulty in theory, involved certain difficulties in practice, owing to the fact that it was liable to be used to disguise usurious transactions.
If the mycelium is confined to one side of the stem, a flat or depressed spreading wound arises; if this extends all round, the parts above must die. When fairly thick stems or branches have the mycelium on one side only, the cambium is injured locally, and the thickening is of course partial.
Though he sauntered through orchards and rested under apple trees, he never observed that the rings of small holes in the bark were usually made by the yellow-bellied woodpecker, instead of by Downy, and that the bird was not searching for grubs or insects, but was feeding upon the milky cambium layer of the inner bark.
The drying up would of course occur if the conducting portions are steeped in turpentine, preventing the conduction of water from below. The mycelium lives for years in the cortex, and may be found killing the young tissues just formed from the cambium during the early summer: of course the annual ring of wood, etc., is here impoverished.
Although cambium was, strictly speaking, a special branch of commerce, it was nevertheless usually treated in the works on usury, the reason being that many apparent contracts of cambium were in fact veiled loans, and that it was therefore a matter of importance in discussing usury to explain the tests by which genuine and usurious exchanges could be distinguished.
They were delighted to have flowers, but when they recognised their mistake, they tore off three fourths of them to strengthen the remainder. Incessantly they kept talking about "sap" and "cambium," "paling up," "breaking down," and "blinding of an eye."
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