On the other hand when the temperature rapidly rises, the outside layers of the tree expand so much more rapidly than the inside, that they separate with a dull muffled chug, the check extending in a circular direction following the annual rings. Such checks are often called "wind shakes" and "cup shakes," Fig. 41._c_, p. 47. These injuries are found in regions where sudden changes of temperature occur, rather than in the tropics or in very cold climates. [Illustration: Fig. 79. Contraction Frost Check. _U. S. Forest Service._] VEGETABLE ENEMIES. Under this head may be classed, in addition to fungi, a number of unrelated plants, including such as: moosewood and dogwood, Fig. 80, which crowd out young trees; vines, like bitter-sweet, which wind about trees and often choke them by pressure, cutting thru the bark and cambium; saprophytes, which smother the foliage of trees, of which Spanish moss, Fig. 47, p. 201, is an example; and finally such parasites as the mistletoes, which weaken and deform the trees. [Illustration: 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.