| FIG. 40.—Ripe fruit of pin clover, or Alfilerilla, ready to twist into fleeces of sheep or into loose soil. |
32. Grains that bore into sheep or dogs or the sand.—Porcupine grass, Stipa spartea, grows in dry soil in the northern states, but more particularly on the dry prairies of the central portion of the United States. This grass, when ripe, has a very bad reputation among ranchmen for the annoyance the bearded grain causes them. The grains are blown into the stubble among grasses with the bearded point down, sticking into the soil. The first rain or heavy dew straightens out the awns, which are twisted again as they dry. The bearded point works a little farther with each change, and after twisting and untwisting a number of times it gets down three or four inches into the sand, often to moisture, where the awns decay and the grain germinates. Here is an admirable scheme for moving about and for boring into the ground. But this is not all. The grains are quick to catch fast to clothing, as people move among the plants, and they are admirably fitted for attaching themselves to dogs and sheep, which they annoy very much. These animals transport the grains for long distances. The twisting and untwisting of the awns enable the grain to bore through the fleeces, and even to penetrate the skins and make wounds which sometimes cause the death of the animal. Examine also seeds of pin clover, Alfilerilla, which is becoming abundant in many parts of the world.
| FIG. 41.—Single fruit of silver maple. |
33. Winged fruits and seeds fall with a whirl.—The large fruit of the silver maple falls in summer. As these trees are most abundant along the margins of streams, the fruit often drops into the water and is carried down stream to some sand drift or into the mud, where more sand is likely to cover them. Thus sown and planted and watered, they soon grow and new trees spring up. But in many instances a strong breeze, sometimes a whirlwind, has been seen to carry these mature fruits from the tree to a distance of thirty rods.
| FIG. 42.—Winged seed of pine. Want of symmetry causes it to whirl about while falling. |
A thin sheet of paper descends more slowly than the same material put in the form of a ball. On the same principle, many seeds and fruits are flattened, apparently for a purpose; not that they may be easily shot through the air by some elastic force, not to increase their chances for attachment to animals, but to enable the wind to sustain them the longer and carry them farther. Some seeds and dry fruits are said to have wings, with the general understanding that they are by this means better fitted to be sustained in air. We shall find that all or nearly all flattened seeds and dry fruits, also winged seeds and fruits, are one-sided, unbalanced, and more or less twisted; consequently, in falling to the ground they whirl about, and are thus kept much longer in the air than they would be if shaped more like a winged arrow. Even the wings on the fruit of some of the ashes are twisted, though many of them are flat. Experiments with these things are sure to interest inquisitive children, or even older persons, when once started right; they are likely to prove as interesting as flying kites, skating, fishing, or coasting on the hillside. Try experiments with seeds of catalpa, trumpet-creeper, wild yam, pine, spruce, arbor vitæ, and fruits of maple, box elder, birch, hop tree, blue beech, ailanthus, ash, tulip tree,—in fact, anything of this nature you can find, whether the name is familiar or not. No two of them will behave in all respects alike.
34. Plants which preserve a portion of their seeds for an emergency.—Many a great general or business man has learned by experience and observation that it is usually unwise to exhaust all resources in one effort. If possible, he always plans to have something in reserve for an emergency—a loophole for escape from difficulty. We have seen in many instances that plants are endowed with the same trait. This is well illustrated by the way in which the jack-pine, Pinus [Banksiana] divaricata, holds in reserve a portion of its seeds, to be used in case the parent trees are killed by fire. In 1888 I made a study of this tree as it lives on the sandy plains of Michigan. The tree is often killed by fire, and never sprouts from the stump, as do oaks, willows, cherries, and most other trees. The jack-pine grows readily and rapidly from seed dropped on the sand, and begins to bear cones and seeds in abundance while it is yet only a few years old, perhaps as young as five years in some instances. The cones open slowly to liberate their seeds, some of them only after months or even years, and in some cases they never open at all. I have seen cones containing good seeds that had been nearly grown over by the tree. Dry weather, the dryer and hotter the better, causes many of these stubborn old cones to open their scales and allow the seeds to escape. What can be the advantage in cones of this nature? Let us see. A brisk fire passes over the ground at irregular intervals, usually of from one to ten years; it licks up all dry leaves and sticks, and kills the pine trees and all else above ground. The soil and the trunks of trees are blackened, and by lack of reflection the heat of the sun is rendered more intense; besides, the heat of the fire acts slowly on the unburned cones as they are left on the dead trees. By the time the quick hot fire has passed over, the cones have slowly opened and begun scattering seeds on the vacant and newly burned ground, at a time when there is the best possible chance for them to grow. I picked a few unopened cones which, according to my judgment, were from two to four years old. They were placed under glass in a dark sheet-iron dish and exposed to the sun. The extra heat caused the cones to open; many seeds were obtained and sown, and in five days they began to come up, 95 per cent germinating. From the same tree I selected at the same time older cones, which I believe to be from four to six years old at least. From these, 225 seeds were sown, 191 of which germinated—about 85 per cent.