Seed Dispersal Part 3

[Ill.u.s.tration: FIG. 33.--At the left a dry fruit of a lily opening to permit seeds to dry and the wind to enter; to the right, a fruit later in the season. Two views of a flat seed.]

Observe that the three opening cells are lashed together loosely with a latticework. No slight breeze can dislodge the seeds, but just see how they behave in a good gale! The elastic stems are swayed back and forth against each other, and some of the upper seeds are tossed out by the wind that pa.s.ses through the lattice, and at such times are often carried to some distance. The seeds at the top having escaped, the dry pods split down farther and still farther and open still wider, till the bottom is reached. As the seeds are not all carried away the first or even the second time, and as succeeding breezes may come from different directions, it is thus possible for the lily to scatter its seeds in all directions.

The seeds of the lily are flat, very thin, and rather light, not designed to be shot out like bullets, but to be carried a little way by the wind; the pods are erect, and open at the top, that the seeds need not escape when there is no wind or unless some animal gives the stem a strong shake. The latticework was made for a purpose, and the gradual opening of the pods prevents the supply from all going in one direction or in one day, for a better day may arrive. The student will look for and compare the following: Iris, figwort, wild yam, catalpa, trumpet-creeper, centauria, mulleins, foxglove, beardtongue, and many other fruits.

28. Large pods with small seeds to escape from small holes.--The large ripe pod of the poppy stands erect on a stiff stem, with a number of small openings near the top. The seeds are nearly spherical, and escape, a few at a time, when the stem is shaken by the wind or some animal, thus holding a reserve for a change of conditions. Here is an ill.u.s.tration of ripe pods of a bellflower, _Campanula turbinata_, nodding instead of erect.

[Ill.u.s.tration: FIG. 34.--Ripe pod of poppy on an erect, stiff stem, ready for wind or animal to shake out a few seeds at the top.]

[Ill.u.s.tration: FIG. 35.--Ripe pods of bellflower bent over; the holes opening when dry to allow seeds to be shaken out.]

The small holes are still uppermost, but to be uppermost in this case it is necessary for them to be at the base of the pod.

29. Seeds kept dry by an umbrella growing over them.--When mature, the apple of Peru, _Nicandra_, keeps every dry bursting fruit covered with a hood, umbrella, or shed, so that seeds may be kept continually dry and may be spread with every shake by the wind, or by an animal, in rainy weather as well as in dry.

In the words of Dr. Gray, "The fruit is a globular dry berry, enclosed by a five-parted, bladdery inflated calyx." The margins of the lobes of the calyx curl upwards and outwards as the berry hangs with the apex downward.

[Ill.u.s.tration: FIG. 36.--Mature fruit of apple of Peru covered by an enlarged calyx.]

[Ill.u.s.tration: FIG. 37.--The same with the calyx removed.]

[Ill.u.s.tration: FIG. 38.--The same as Fig. 37, except that it is older and somewhat changed.]

The berry is as large as one's thumb, and when ripe, bursts open irregularly on the upper side as it hangs up under the calyx. As the covering of the pod opens more and more, a few seeds at a time may be rattled out by wind or animal. The numerous large and light fruits, with calyx surrounding them, are each supported on a nodding stem, stiff and elastic, which gives the wind a good chance to sway them about. Water does not seem to get into the berries even when they are torn open, for when it is poured over the branches it rolls off the calyx roof as freely as from a duck's back. The fruits of _Physalis_ are apparently kept dry in a manner similar to the apple of Peru, although when first mature they are soft and juicy, considerably like a ripe tomato.

30. Shot off by wind or animal.--The calyx of sage, bergamot, and most other mints, remains dry and stiff, as a cup to hold one to four little round nutlets as they ripen. The figure shows two of these in section, as they are attached to the main stem of the plant, or one of its branches. Observe the direction taken by the upper and by the lower points of the calyx. When dry, the plant behaves somewhat as follows: when the wind jostles the branches against each other, or when an animal of some kind hits the plant, this movement causes many of these cups to get caught; but the elastic stem comes suddenly back to its place, and in so doing flips a nutlet or more from its mouth one to six feet, somewhat as a boy would flip a pea with a pea-shooter. In our garden, July 2, when plants of sage, _Salvia interrupta_, were ripening their fruit, we found it difficult to collect any seeds, but seedlings were observed in abundance on every side of the plant, some to the distance of six feet. Plants dispersing seeds in this manner have been called catapult fruits. Examine ripening fruits of blue curls, pennyroyal, germander, balm, h.o.r.ehound, dittany, hyssop, basil, marjoram, thyme, savory, catmint, skullcap, self-heal, dragon's head, motherwort, and various dry fruits of several chickweeds.

[Ill.u.s.tration: FIG. 39.--Ripe calyx of sage, first as pushed down; second as let loose throwing nutlets.]

31. Seed-like fruits moved about by twisting awns.--Most of the grains of gra.s.ses are invested with glumes, or chaff, and a considerable per cent of the chaff has awns, some of which are well developed and some poorly developed. The distribution of such gra.s.ses depends on several agents--wind, water, and animals. The chaff and awns of all are hygroscopic; that is, are changed by differences caused by variation of moisture in the air. Sweet vernal gra.s.s, tall oat gra.s.s, holy gra.s.s, redtop, animated or wild oats, blue-joint, and porcupine gra.s.s are among them. When mature, the grain and glumes drop off, or are pushed off, and go to the ground. When moist, these awns untwist and straighten out, but when dry they coil up again; with each change they seem to crawl about on the ground and work down to low places or get into all sorts of cracks and crevices, where the first rain is likely to cover them more or less with earth, after which they are ready for growth.

32. Grains that bore into sheep or dogs or the sand.--Porcupine gra.s.s, _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 gra.s.s, 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 gra.s.ses 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.

[Ill.u.s.tration: FIG. 40.--Ripe fruit of pin clover, or Alfilerilla, ready to twist into fleeces of sheep or into loose soil.]

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.

[Ill.u.s.tration: FIG. 41.--Single fruit of silver maple.]

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, fis.h.i.+ng, or coasting on the hillside. Try experiments with seeds of catalpa, trumpet-creeper, wild yam, pine, spruce, arbor vitae, 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.

[Ill.u.s.tration: FIG. 42. Winged seed of pine. Want of symmetry causes it to whirl about while falling.]

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 ill.u.s.trated 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 pa.s.ses 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 pa.s.sed 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 gla.s.s 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.

[Ill.u.s.tration: FIG. 43.--Cone of jack-pine closely covering its seeds, often for several years.]

[Ill.u.s.tration: FIG. 44.--Cone of jack-pine as opened by heat, sowing seeds.]

CHAPTER VI.

PLANTS THAT SHOOT OFF THEIR SPORES OR SEEDS.

By numerous devices a large number of the lower plants send off their ripe spores with considerable force. Some call them sling fruits.

One in particular, _Pilobolus cristallinus_, found about damp stables, I have observed to shoot black ma.s.ses of spores to a spot on a wall six feet above the ground, with enough force to have carried them not less than twelve feet. When ripe and dry, the spores of most ferns are shot from the parent plant by a motion forcible enough not only to burst the _sporangium_, the vessel that contains the spores, but also to turn it inside out.

[Ill.u.s.tration: FIG. 45.--Spores of _Pilobolus_ before and while shooting its spores.]

35. Dry pods twist as they split open and throw the seeds.--In December, while absent from home, I collected for future study some pods of the Chinese wistaria, and left them on my desk in the library for the night. The house was heated by a hot-air furnace. In the morning the pods were in great confusion; most of them had split and curled up, and the seeds were scattered all about the room. As usual the little daughter, an only child, was accused of spoiling my specimens, but she showed her innocence. A little investigation and a few experiments with some pods not yet opened explained the whole matter satisfactorily. The stout pods grow and ripen in a highly strained condition, with a strong tendency to burst spirally, the two half-pods being ready to coil and spring in opposite directions; when the valves can no longer hold together, they snap with a sharp noise and sling the heavy seeds, giving them a good send-off into the world.

As a pair of birds build a nest, hatch eggs, rear their young, and then send them forth to seek their fortunes, so for months the mother plant had labored, had produced and matured seeds, which at last it scattered broadcast. Goethe, Kerner von Marilaun, each independently, and very likely others, had an experience with ripe pods brought to a warm room very similar to my own. In many cases the ripe and drying fruits are "touched off" by wind jostling the branches or by animals pa.s.sing among them; in the latter case there is a chance that a portion of the discharges will be lodged somewhere on the animal and be carried along with it.

36. A seed case that tears itself from its moorings.--The perennial phlox in cultivation distributes its seeds in the following manner: when ripe, the calyx becomes dry and paper-like, and spreads out in the form of a saucer. The thick-walled dry pistil opens from the top into three pieces with a snap, spreading open so far against the calyx that it is torn from the brittle attachment; away go the seeds, mingled with the fragments of the pistil, no longer of any use.

[Ill.u.s.tration: FIG. 46.--A dry pod of wild bean bursting spirally to throw the seeds.]

[Ill.u.s.tration: FIG. 47.--Fruit of violet partially dried and split into three pieces, each piece pinching the seeds so closely that sooner or later all are thrown out.]

Fruits that sling their seeds are to be found in every neighborhood, and are first-cla.s.s objects for the curious person to see and handle.

Very fortunate is the girl or boy who is never fully satisfied with what he reads and sees pictured, but has a strong desire to learn how plants are made and how they behave. A considerable number of seed pods have been ill.u.s.trated with notes in recent schoolbooks.

Here are some of them: peas and vetches, and some kinds of beans, violets, balsams, wood sorrel, geranium, castor bean, some of the mustards and cresses and their cousins, Alfilerilla, richweed, _Pilea_, witch-hazel, and others. Each of those will well repay study, especially the fruit and seeds of oxalis. The witch-hazel bears a hard, woody, nut-like fruit, as large as a hazelnut; when ripe, the apex gaps open more and more, the sides pressing harder against each smooth seed, till finally it is shot, sometimes for a distance of thirty feet. The girl who has shot an apple seed or lemon seed with pressure of thumb and finger across a small room, can understand the force needed to shoot a seed but little heavier than that of the apple two or three times that distance.

[Ill.u.s.tration: FIG. 48.--Dry fruit of witch-hazel shooting seeds.]

CHAPTER VII.

PLANTS THAT ARE CARRIED BY ANIMALS.

With the frosts of autumn ripe acorns, beechnuts, bitternuts, b.u.t.ternuts, chestnuts, hickory nuts, hazelnuts, and walnuts are severed from the parent bush or tree and fall to the ground among the leaves.

37. Squirrels leave nuts in queer places and plant some of them.--Even before the arrival of frosts many of these are dropped by the aid of squirrels, gray and red, which cut the stems with their teeth.

The leaves, with the help of the s.h.i.+fting winds, gently cover the fruit, or some portions of it, and make the best kind of protection from dry air and severe cold; and they come just in the nick of time.

Dame Nature is generous. She produces an abundance; enough to seed the earth and enough to feed the squirrels, birds, and some other animals. The squirrels eat many nuts, but I have seen them carry a portion for some distance in several directions, and plant one or two or three in a place, covering them well with soil. It may be the thought of the squirrel--I cannot read his thoughts--to return at some future time of need, as he often does. But in some cases he forgets the locality, or does not return because he has stored up more than he needs; or in some cases the squirrels leave that locality or are killed; in any such case the planted nuts are not disturbed. At all events, some of the nuts--one now and then is all that is needed--are allowed to remain where planted. In this way the squirrel is a benefit to the trees and pays for the nuts he eats. He has not lived in vain, for he is a tree planter and believes in arboriculture. His arbor day comes in autumn, and he needs no message from the governor to stimulate him to work.

After some red squirrels had been given black walnuts, a member of my family saw them hide the nuts in all conceivable places, and in some instances place them above a cl.u.s.ter of small branches of a tree for support where three or more twigs spread from nearly the same place. Here the nuts, one in a place, were left till perhaps shaken to the ground by a severe wind or by some other cause. In one winter, without hunting for them, six to ten places were found in one neighborhood of Michigan, where something had placed a single walnut or acorn in the forks of small branches. In some cases a severe wind could have dislodged the nut.

[Ill.u.s.tration: FIG. 49.--A black walnut as left by a red squirrel on a small oak tree.]