Trees with winged seeds, however, Fig. 68, as bass, maple and pine, or with light seeds, as poplar, often have their seeds carried by the wind to great distances. [Illustration: Fig. 68. Winged Seeds. 1, Basswood; 2, Box-elder; 3, Elm; 4, Fir; 5, 6, 7, 8, Pines. _U. S. Forest Service._] Again some trees, as spruce, are very fertile, while others, like beech, have only occasional seed-bearing seasons, once in three or four years. Willow seeds lose their power of germination in a few days, and hence, unless they soon reach ground where there is plenty of moisture, they die. This is why they grow mostly along water courses. On the other hand, black locust pods and the cones of some pines keep their seeds perfect for many years, often until a fire bursts them open, and so they live at the expense of their competitors. It is such facts as these that help to account for some of the acts of forest composition,--why in one place at one time there is a growth of aspens, at another time pines, at still another oaks; and why beeches spring up one year and not another. That red cedars grow in avenues along fences, is explained by the fact that the seeds are dropped there by birds, Fig. 69. [Illustration: Fig. 69. Red Cedar Avenue. Seeds dropped by birds which perched on the fences. Indiana. _U. S. Forest Service._] The fact that conifers, as the longleaf pine, Fig. 46, p. 200, and spruce, Fig. 55, p. 212, are more apt to grow in pure stands than broad-leaved trees, is largely accounted for by their winged seeds; whereas the broad-leaved trees grow mostly in mixed stands because their heavy seeds are not plentifully and widely scattered. This is a rule not without exceptions, for beech sometimes covers a whole mountain side, as Slide Mountain in the Catskills, and aspens come in over a wide area after a fire; but later other trees creep in until at length it becomes a mixed forest. The essential facts of the relation of trees to each other in the forest has been clearly stated by Gifford Pinchot thus:[1] The history of the life of a forest is a story of the help and harm which trees receive from one another. On one side every tree is engaged in a relentless struggle against its neighbors for light, water and food, the three things trees need most. On the other side each tree is constantly working with all its neighbors, even those which stand at some distance, to bring about the best condition of the soil and air for the growth and fighting power of every other tree.