Fig. 80. A "Forest Weed," Flowering Dogwood. North Carolina. U.S. Forest Service.
The most important of the vegetable enemies of trees are fungi. It should be remembered, however, that, without the decay produced by them, the fallen trees would soon cover the ground, and prevent any new growth, thus destroying the natural forest.
Every tree, as has been noted ([p. 17]), is composed of two parts, one part, including leaves, young branches, roots and sap-wood, living, and the other part, namely, the heart-wood, practically dead.
Fungi that attack the live parts of a tree are called parasites, while those that live on dead trunks and branches are designated as saprophytes. The line, however, between these two classes of fungi is not well defined, since some parasites live on both living and dead wood. The parasites are of first importance, for, since they kill many trees, they control to a large extent the supply of living timber.
Nearly all parasitic fungi have two portions, an external fruiting portion which bears the spores—which correspond to the seeds of flowering plants—and an internal portion consisting of a tangle of threads or filaments, which ramify the tissues of the tree and whose function is to absorb nutriment for the fungus. Fungi are classified botanically according to the spore-bearing bodies, their form, color, etc.
The parasitic fungi which are especially destructive to wood are those that have naked spores growing on exposed fruiting surfaces (the Hymenomycetes). In toadstools (the agarics) these exposed surfaces are thin, flat plates called gills. In the polypores, which include the shelf fungi, the spore surfaces are tubes whose openings constitute the pores. In the dry-rot, or tear fungus (Merulius lacrymans), the spore surfaces are shallow cavities.
Some varieties, called true parasites, develop in uninjured trees, while others, called wound parasites, can penetrate the tissues of trees, only where a cut or injury makes a suitable lodgment for the spores. Some fungi attack only a single species of trees, others whole genera; some attack only conifers, others deciduous trees, while a few attack trees of nearly all kinds alike.
Fungal spores when brought in contact with a wound on a tree or other suitable place, and provided with suitable conditions of growth, germinate, penetrate the tissues and grow very rapidly. These spores send out long threads or filaments which run thru the cells lengthwise and also pierce them in all directions, soon forming a network in the wood called the mycelium.
Rotting, in a large number of cases, is due to the ravages of fungi. This sometimes shows in the color, as the "red rot" of pine or the "bluing" of ash. Sometimes as in "pecky" or "peggy" cypress, the decayed tracts are tubular. More commonly the decayed parts are of irregular shape.