Seed Dispersal

CHAPTER V
SEEDS TRANSPORTED BY WIND

17. How pigweeds get about. —In winter we often see dead tops of lamb's-quarters and amaranths – the smooth and the prickly pigweeds – still standing where they grew in the summer. These are favorite feeding grounds for several kinds of small birds, especially when snow covers the ground.

Many of the seeds, while still enclosed in the thin, dry calyx, and these clustered on short branches, drop to the snow and are carried off by the wind. Notwithstanding the provision made for spreading the seeds by the aid of birds and the wind, the calyx around each shiny seed enables it to float also; when freed from the calyx, it drops at once to the bottom. Many kinds of dry fruits and seeds in one way or another find their way during winter to the surface of the ice-covered rivers. When the rivers break up, the seeds are carried down stream, and perhaps left to grow on dry land after the water has retired. Most of the commonest plants, the seeds of which are usually transported by water, are insignificant in appearance and without common names, or with names that are not well understood. This is one reason for omitting the description of others which are ingeniously fitted in a great variety of different ways for traveling by water.

18. Tumbleweeds. —Incidentally, the foregoing pages contain some account of seeds and fruits that are carried by the aid of wind, in connection with their distribution by other methods; but there are good reasons for giving other examples of seeds carried by the wind. There is a very common weed found on waste ground and also in fields and gardens, which on good soil, with plenty of room and light, grows much in the shape of a globe with a diameter of two to three feet. It is called Amaranthus albus in the books, and is one of the most prominent of our tumbleweeds. It does not start in the spring from seed till the weather becomes pretty warm. The leaves are small and slender, the flowers very small, with no display, and surrounded by little rigid, sharp-pointed bracts. When ripe in autumn, the dry, incurved branches are quite stiff; the main stem near the ground easily snaps off and leaves the light ball at the mercy of the winds. Such a plant is especially at home on prairies or cleared fields, where there are few large obstructions and where the wind has free access.

The mother plant, now dead, toiled busily during the heat of summer and produced thousands of little seeds. The best portion of her substance went to produce these seeds, giving each a portion of rich food for a start in life and wrapping each in a glossy black coat. Now she is ready to sacrifice the rest of her body to be tumbled about, broken in pieces, and scattered in every direction for the good of her precious progeny, most of whom will find new places, where they will stand a chance the next summer to grow into plants. Sometimes the winds are not severe enough or long enough continued, and these old skeletons are rolled into ditches, piled so high in great rows or masses against fences that some are rolled over the rest and pass on beyond. Occasionally some lodge in the tops of low trees, and many are entangled by straggling bushes. In a day or two, or in a week, or a month, the shifting wind may once more start these wrecks in other directions, to be broken up and scatter seeds along their pathway.

During the Middle Ages in southern Egypt and Arabia, and eastward, a small plant, with most of the peculiarities of our tumbleweed just described, was often seen, and was thought to be a great wonder. It was called the "rose of Jericho," though it is not a rose at all, but a first cousin to the mustard, and only a small affair at that, scarcely as large as a cabbage head. A number of other plants of this habit are well known on dry plains in various parts of the world; one of the most prominent in the northern United States is called the Russian thistle, which was introduced from Russia with flaxseed. In Dakota, often two, three, or more grow into a community, making when dry and mature a stiff ball two to three feet or more in diameter.

One of our peppergrasses, Lepidium intermedium, sometimes attains the size and shape of a bushel basket; when ripe, it is blown about, sowing seeds wherever it goes. The plants of the evening primrose sometimes do likewise, also a spurge, Euphorbia [Preslii] nutans, a weed a foot to a foot and a half high.

Low hop clover, an annual with yellow flowers, which has been naturalized from Europe, has developed recently on strong clay land into a tumbleweed six inches in diameter. The tops of old witch grass, Panicum capillare, and hair grass, Agrostis hyemalis, become very brittle when ripe, and snap from the parent stem and tumble about singly or in masses, scattering seeds by the millions. I have seen piles of these thin tops larger than a load of hay where they had blown against a grove of trees, and in some cases many were caught in the tops of low trees.

Bug seed and buffalo bur are tumbleweeds. In autumn the careful observer with an eye to this subject will be rewarded by finding many other plants that behave more or less as tumbleweeds. Especially is this the case on prairies. These are annuals, and perish at the close of the growing season. There are numerous other devices by which seeds and fruit secure transportation by the wind.

19. Thin, dry pods, twisted and bent, drift on the snow. —The common locust tree, Robinia Pseudacacia, blossoms and produces large numbers of thin, flat pods, which remain of a dull color even when the seeds are ripe. The pods of the locust may wait and wait, holding fast for a long time, but nothing comes to eat them. They become dry and slowly split apart, each half of the pod usually carrying every other seed. Some of the pods with the seeds still attached are torn off by the wind and fall to the ground sooner or later, according to the force of the wind. Each half-pod as it comes off is slightly bent and twisted, and might be considered a "want-advertisement" given to the wind: "Here I am, thin, dry, light and elastic, twisted and bent already; give me a lift to bear these precious seeds up the hill, into the valley, or over the plain."

And the wind is sure to come along, a slight breeze to-day tossing the half-pod a few feet, leaving it perhaps to be again and again moved farther forward. The writer has seen these half-pods transported by this means more than a block. But many of the pods stick to the limbs till winter comes. Then a breeze tears off a few pods and they fall on the snow, which has filled up all the crevices in the grass and between the dead leaves and rubbish. Each half-pod, freighted with every other seed, is admirably constructed; like an ice boat, it has a sail always spread to the breeze. In this way there is often nothing to hinder some of the seeds from going a mile or two in a few minutes, now and then striking some object which jars off a seed or two. The seeds are very hard, and no doubt purposely so, that they may not be eaten by insects or birds; but once in moist soil, the covering slowly swells and decays, allowing the young plant to escape. Thus the locust seeds are provided with neither legs, wings, fins, nor do they advertise by brilliant hue and sweet pulp; but they travel in a way of their own, which is literally on the wings of the wind.

20. Seeds found in melting snowdrifts. —It will interest the student of nature to collect a variety of seeds and dry fruits, such as can be found still on the trees and other plants in winter, and try some of them when there is snow on the ground and the wind blows, to see how they behave. Again, when the first snow banks of the early winter are nearly gone, let him collect and melt a quantity of snow and search for seeds. By this means he can see, as he never saw before, how one neighbor suffers from the carelessness of another.

21. Nuts of the basswood carried on the snow. —Here are some notes concerning the distribution of the spherical nuts of basswood. The small clusters of fruit project from a queer bract which remains attached before and after falling from the tree.

This bract, when dead, is bent near the middle and more or less twisted, with the edges curving toward the cluster of nuts. From two to five nuts about the size of peas usually remain attached till winter, or even a few till spring. This bract has attracted a good deal of attention, and for a long time everybody wondered what could be its use. We shall see. The cluster of nuts and the bract hang down, dangling about with the least breath of wind, and rattling on the trees because the enlarged base of the stem has all broken loose excepting two slender, woody threads, which still hold fast. These threads are of different degrees of strength; some break loose after a few hard gales, while others are strong enough to endure many gales, and thus they break off a few at a time. The distance to which the fruit can be carried depends on the form of the bract, the velocity of the wind, and the smoothness of the surface on which the fruit falls. When torn from the tree the twist in the bract enables the wind to keep the cluster rapidly whirling around, and by whirling it is enabled to remain longer suspended in the air and thus increase the chances for a long journey. In throwing some of these from a third-story window, it was found that a bract with no fruit attached would reach the ground sooner than a bract that bore from two to four solid nuts. The empty or unloaded bracts tumble and slide through the air endwise, with nothing to balance them or steady their descent, while the fruit on other bracts holds them with one side to the air, which prolongs their descent. The less a loaded bract whirls, the faster its descent, and the more a bract whirls when the wind blows, the farther it is carried. The bract that is weighted with a load of fruit acts as a kite held back by a string, and when in this position the wind lifts the whole as well as carries it along. Before snow had fallen in 1896, by repeated moves on a well-mowed lawn, fruit and bracts were carried about two hundred feet, while with snow on the ground the distance was almost unlimited, excepting where there were obstructions, such as bushes and fences. When there is a crust on the snow and a good wind, the conditions are almost perfect. Over the snow the wind drives the bracts, which drag along the branch of fruit much as a sail propels a boat. The curving of the edges of the bract toward the fruit enables the wind to catch it all the better, and to lift it more or less from the snow. With changes in the direction of the wind, there is an opportunity for the fruit of a single tree, if not too much crowded by others, to spread in all directions. After watching these maneuvers, no one could doubt the object of the bent bracts of the basswood, and as these vary much in length and width and shape on different trees, it would seem that perhaps nature is still experimenting with a view to finding the most perfect structure for the purpose.

About one hundred and thirty paces west of the house in which I live stand two birch trees. One windy winter day I made some fresh tracks in the snow near my house, and within a few minutes the cavities looked as though some one had sprinkled wheat bran in them, on account of the many birch seeds there accumulated.

Other fruits in winter can be experimented with, such as that of box elder, black ash, birches, tulip tree, buttonwood, ironwood, blue beech, and occasionally a maple.

22. Buttonwood balls. —Nature seems to have no end of devices for sowing seeds to advantage. Here is one which always interests me. The fruit of the buttonwood, or sycamore, which grows along streams, is in the form of balls an inch and a half in diameter. These balls grow on the tops of the highest branches, and hold on into winter or longer. The stems are about two inches long, and soon after drying, through the action of the winds, they become very flexible, each resembling a cluster of tough strings. The slightest breeze moves them, and they bob around against each other and the small branches in an odd sort of way. After so much threshing that they can hold no longer, the little nuts become loosened and begin to drop off a few at a time. Certain birds eat a few and loosen others, which escape. The illustration shows some of these nuts, each supplied with a ring of bristles about the base, which acts as a parachute to permit the wind the easier to carry them for some distance before falling, or to drift them on the surface of the snow or ice.

23. Seeds that tempt the wind by spreading their sails. —On low lands in the cool, temperate climate of Europe, Asia, and North America, is a common plant here known as great willow-herb, a kind of fireweed (Epilobium angustifolium). There are several kinds of fireweeds. This one grows from three to five feet high, and bears pretty pink flowers. In mellow soil the slender rootstocks spread extensively, and each year new sprouts spring up all around, six to eight feet distant. Below each flower ripens a long, slender pod, which splits open from the top into four parts, that slowly curve away from a central column. The apex of each seed is provided with a cluster of white silky hairs nearly half an inch long.

The tips of the hairs stick slightly to the inside of the recurved valves, some hairs to one valve, and often others to the adjacent valve, thus spreading them apart with the seed suspended between. Four rows of the seeds are thus held out at one time. Often not over half, or even a tenth part, of the seeds are well developed, yet the silky hairs are present and float away in clusters, thus helping to buoy those that are heavy. This is a capital scheme, for when the pods are dry and unfurled, they silently indicate to the slightest breath of air that they are ready for a flight, and it doesn't take much to carry them for a long distance. As an active boy delights to venture again and again over thin ice on a shallow pond in the pasture, half fearing, yet half hoping, that he may become a hero by breaking through and escaping, so likewise many of these seeds and seed-like fruits spread themselves out, as if to tempt the wind to come along and attack them.

The twin fruits of the parsnip and some of its near relatives are light and thin and split apart, each holding on lightly to the top of a slender stem. In this position they are sure to be torn off sooner or later. Somewhat after the manner of the willow-herb behave the pods and seeds of willows, poplars, milkweeds, Indian hemp, and cotton.

24. Why are some seeds so small? —Do you know why so many kinds of plants produce very small and light seeds? Would it not be better if they produced fewer and larger seeds, which would then be stronger and better able to grow under adverse conditions? But a large number of small seeds cost the plant no more effort than a small number of large ones, and the lighter and smaller the seeds and the more there are of them, the better their chances for distribution, especially for long distances. The minute size of spores of most of the fungi are given as reasons why so many of them are so widely distributed.

Why is a boy or man of light weight chosen to ride the horse on the race track? That the animal may have less weight to carry and thereby use his surplus strength in making better time. The less weight the parachute of the seed of the willow-herb has to carry, the greater the chances for success in making a long journey. Of the willow-herb it takes one hundred seeds to weigh a milligram, including the hairs attached to them, and it would take thirty thousand to weigh as much as an ordinary white bean.

25. Seeds with parachutes. —Many years ago large portions of Huron and Sanilac counties of eastern Michigan were swept by a fire so severe that the timber was all killed. Fifteen years later the woody growth consisted mostly of willows, poplars, and birches. The seeds of all kinds of willows and poplars are very light, and are produced in immense quantities. Like those of the great willow-herb, they are beautifully constructed for making long journeys through the air – a fact that explains the frequency of these trees in burned districts. A considerable number of seeds and fruits grow with a parachute attached at one end, not to prevent injury by falling from the tree top, but to enable the wind to sustain and transport them for a longer distance.

26. A study of the dandelion. —In spring the dandelion is almost everywhere to be found; every one knows it – the child to admire, the gardener to despise. From each cluster of leaves spreading flat in the grass come forth several hollow stems, short or tall, depending on the amount of sunshine and shade. Each stem bears, not one flower, but a hundred or more small ones. Around and beneath each yellow cluster are two rows of thin, green, smooth scales (involucre).

The short outer row soon curls back, as though for rest or ornament, or for watching the progress of the colony above; but the inner row has a very important duty yet to perform in guarding the large family within. At night, or in daytime, if the day be wet, the long scales press like a blanket closely about the flowers, and do not permit them to come out; but when the sun is bright, it shrinks the outer side of these scales, which then curl apart, leaving the yellow flowers ready for bees to visit or boys to admire and study. For several days the flowers of a head blossom in succession, each night to be snugly wrapped by the scales, and the next day to be again left open, if the weather be fine. After each flower in turn has been allowed to see the light, and after all have been crawled over by bee and wasp to distribute the yellow pollen that seeds may be produced, there is nothing else to do but patiently wait for a week or two while receiving food from the mother plant to perfect each little fruit and seed. During all this period of maturing, day and night, rain or shine, the scales hold the cluster closely; the stem bends over to one side, and the rain and dew is kept from entering. After a while, on some bright morning, the dandelion stalk is seen standing erect again, and is probably surrounded by many others in a similar position. The dry air shrinks the outside of the scales, and they turn downward; the circle of feathers at the top of the slender support attached to the seed-like fruit below spreads out, and the community, which now looks like a white ball of down, is ready for a breeze. The feathery top is now ready to act as a parachute, and invites the wind to catch up the whole and float it away. If there is no breeze, the moist air of night closes the outer scales; each of the feathery tips closes, and all are secure till the next bright day.