[Illustration: FIG. 33.--Vertical section through a very young Æcidium of Peridermium Pini (var. acicola), with part of the subjacent tissue of the leaf, h, the mycelium of the parasitic fungus running between the cells of the leaf; immediately beneath the epidermis of the leaf, the ends of the hyphæ give rise to the vertical rows of spores (b), the outermost of which (p) remain barren, and form the membrane of the blister-like body. The epidermis is already ruptured at p by the pressure of the young Æcidium. (After K. Hartig: highly magnified.)] Much more disastrous results can be traced directly to the action of the mycelium in the cortex. The hyphæ grow and branch between the green cells of the true cortex, as well as in the vast tissues beneath, and even make their way into the medullary rays and resin canals in the wood, though not very deep. Short branches of the hyphæ pierce the cells, and consume their starch and other contents, causing a large outflow of resin, which soaks into the wood or exudes from the bark. 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 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. 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. 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. From this cause arise cancerous malformations which are very common in pine woods (Fig. 34).