Aphidesand may be easily penetrated by certain Fungi such as Peziza, Nectria; and when thus attacked, the repeated conflicts between the cambium and callus, on the one hand, trying to heal over the wound, and the insect or Fungus, on the other, destroying the new tissues as they are formed, results in irregular growths; the still uninjured cambium area goes on thickening the branch, the dead parts, of course, remain unthickened, and the portion in which the Fungus is at work may for the time being grow more rapidly.
Another disease which is sometimes confused with that caused by the Peziza is " heart-rot "; it occasionally attacks larches only ten years old or less, but is more common when the trees have acquired a considerable size, sometimes spreading in a short time through a whole plantation.
Such aggregations of hyphae frequently become knotted up into dense masses of interwoven and closely packed hyphae, varying in size from that of a pin's head or a pea (Peziza, Coprinus) to that of a man's fist or head, and weighing io to 25 lb or more (Polyporus Mylittae, P. tumulosus, Lentinus Woermanni, P. Sapurema, &c.).
Many of the coloured pigments of fungi are fixed in the cell-walls or excreted to the outside (Peziza aeruginosa).
The genus Peziza (in its widest sense) may be taken as the type of the group. Most of them grow on living plants or on dead vegetable remains, very often on fallen wood; a number, however, are found growing on earth which is rich in humus.