Most fruits of the forest tree, whether the seeds are comparatively heavy like those of the ash, or very light like those of the elm, are furnished with wings of various forms so as to be wafted away in whichever direction the wind may blow each in its own time, that all may have an equal chance of propagating whence they land. A small percentage of fruits or seeds that are adapted to wind dispersal are encircled with soft hairs like the poplar seed, or tufted with fine hairs at one end like those of the willows, or with coarser ones like the sycamore fruits, to make them fly fast and far.

The hop hornbeam produces its seeds within inflated, bag-like structures, a number of which grow together in a drooping cluster, to sail away light as balloons upon the September winds. Wherever we find spines on the outer covering of a tree fruit, as in the beechnut, it is for protection, as a rule, rather than for dispersal. The wind is the chief agent in the scattering of these fruits, and the winged seed or fruit characterizes the great forest tree. Those that are too heavy to be buoyed up and carried out from the tree by the wind are so constructed or so shaped that when they fall they will either bounce or roll into new homes.

Some of these are so attractive to the animals of the woods that in sheltered nooks on flat-topped rocks we find the remnants of many a feast. Nature did not intend that animals should disperse nuts and acorns, but many of these, carefully tucked away beneath the soft leaf mold by small animals for future use, are forgotten, and thus, by accident, have reached favorable homes. Most of the fruits of the common hardwoods ripen in late summer or autumn, but it is quite important to know approximately when the early fruits mature. Those of the elms are often ripe and swept away by the wind before the trees are fully leaved out in April. The silver and red maples scatter their seeds in May and June, six weeks after the flowers open, that their seeds may find warm, moist places in which to germinate quickly and develop sufficiently during the warm season to withstand the rigors of winter.

For the same reason other trees that love to grow in the swamps, by the river's edge, or by the brook that winds through the meadow, such as the river birch, the poplars, and the willows, hasten to ripen and send forth their offspring to seek new homes. The fruits of the willows develop from their drooping pistillate catkins into little, green pods or capsules along the catkin axis in about four weeks. In eight weeks the poplars go through the same process. The poplar capsules are larger than those of the willow and often curve outward from the central axis.