Trees, like people, often entertain a number of troubles at once and have misfortunes in series. A seedling injured by one insect is more likely to be attacked again, and by some other insect, than is the sound seedling by its side. Let a seedling be injured, and relays of insects--often several species at a time and each species with a way of its own--will attack it through the seedling, sapling, pole, tree, and veteran stages of its growth until it succumbs. Or let a vigorous tree meet with an accident, and like an injured deer it becomes food for an enemy. If lightning, wind, or sleet split the bark or break a limb, through these wounds some spore or borer will speedily reach the tree's vitals. In many cases the deadly work of parasitic plants and fungi is interrelated with, and almost inseparable from, the destructive operations of predacious insects. Many so-called tree diseases are but the spread of rot and fungi through the wood by means of an entrance bored by a borer, weevil, or beetle. The bark of a tree, like the skin on one's body, is an impervious, elastic armor that protects blood and tissues from the poisonous or corrupting touch or seizure of thousands of deadly and incessantly clamoring germs. Tear the skin on one's body or the bark upon a tree, and eternally vigilant microbes at once sow the wound with the seeds of destruction or decay. A single thoughtless stroke of an axe in the bark of a tree may admit germs that will produce a kind of blood-poisoning and cause slow death. The false-tinder fungus apparently can spread and do damage only as it is admitted into the tree through insect-holes or the wounds of accidents. Yet its annual damage is almost beyond computation. This rot is widely distributed and affects a large number of species. As with insects, its outbreaks often occur and extend over wide areas upon which its depredations are almost complete. As almost all trees are susceptible to this punk-producer, it will not be easy to suppress. [Illustration: A TREE KILLED BY MISTLETOE AND BEETLES] The study of forest insects has not progressed far enough to enable one to make more than a rough approximation of the number of the important species that attack our common trees. However, more than five hundred species are known to afflict the sturdy oak, while four hundred prey upon the bending willow. The birches supply food to about three hundred of these predacious fellows, while poplars feed and shelter almost as many. The pines and spruces are compelled permanently to pension or provide for about three hundred families of sucking, chewing parasites.