Seed Dispersal

51. Burs stick to clothing. —Late in summer or in autumn, who is there who has not returned from a walk along the river or from a tramp through thickets or the open woods, to find large numbers of half a dozen kinds of seed-like fruits sticking to his clothes? When ripe, these fruits usually separate from the parent plant very easily, by a joint or brittle place well provided for in the early part of the season. In pursuing your way you rub off a portion of these fruits, and at the end of the journey, or before, you sit down in some comfortable spot and deliberately pick off the unwelcome stick-tights. At such times you have been the means of transporting seeds, and you have left them scattered about ready to grow. If you ever were so fortunate as to live on a farm, you must have seen your father or his hired help carefully look about the field or the wood lot and remove all the bur-bearing plants that could be found before turning in his flock of sheep or the colts and cattle; for if this were not done, he knows that hair and mane will surely be disfigured, and that the wool will be rendered unsalable. In removing the weeds he defeated the plans of Nature in her devices for sowing seeds.

The agency of man in the distribution of plants exceeds in importance that of all other means combined. He buys and sells seeds and plants, and sends them to all parts of the habitable globe. He exterminates many plants in large areas, and substitutes in large measure those of his choice. Mixed with seeds of grasses, clovers, or grains, he introduces many weeds and sows them to grow with his crops.

L. H. Dewey, in the Yearbook of the Department of Agriculture for the year 1896, p. 276, says: "Cockle seeds are normally somewhat smaller than wheat grains. In some parts of the northwest, where wheat for sowing has been cleaned year after year by steam threshers, all the cockle seeds except the largest ones have been removed, and these have been sown until a large-seeded strain has been bred which is very difficult to separate from the wheat." For illustration, some years ago I purchased of a dealer in Michigan a small quantity of what was being sold on the market as seed of red clover; this specimen contained 40 per cent of seeds of rib-grass or narrow-leaved plantain.

Man introduces some seeds of weeds with unground feed stuff. He introduces some with barnyard manure drawn from town. He gets some in the packing of nursery stock, crockery, baled hay and straw. For example, in 1895, baled hay from Kansas or that vicinity examined at the Missouri Agricultural College was found to contain fifteen species of weeds. Others from the west were examined in Michigan and found to contain much foul stuff. Some are carried from farm to farm by wagons, sleighs, or threshing machines; or they are spread by plows, cultivators, and harrows. A few are introduced to grow for ornament or food, and afterwards spread as weeds. A number have been shipped to distant lands in the earth of ballast, which is often unloaded and reloaded at wharves where freight is changed. They are carried along the highway, strung along the towpath of canals, or are carried in the trucks or in the cars of railroads. They are imported and exported around the world in fleeces of wool. They float down irrigating ditches from farm to farm, and with the water are well distributed.

52. Man takes plants westward, though a few migrate eastward. —So far as man's agency is concerned, the direction for plant migration is generally westward, in the course taken by himself. In case of two hundred kinds of weeds named by the United States Department of Agriculture, one hundred and eight species are of foreign origin. Three notable samples of weeds in the United States have gone from the west to the east, carried in seeds of grasses or clovers. These are Rudbeckia hirta, Artemisia biennis, Plantago aristata. To these Mr. Dewey adds buffalo bur, Solanum rostratum, squirreltail, Hordeum jubatum, false ragweed or marsh elder, Iva xanthifolia, Franseria hookeriana, alfalfa dodder, Cuscuta epithymum.

Above I have barely mentioned a few of the methods by which man is an unwilling agent in distributing plants. Large volumes could be filled with statements of man's more or less carefully planned attempts to transport seeds and living plants from one part of the world to another.

CHAPTER IX

SOME REASONS FOR PLANT MIGRATION

53. Plants are not charitable beings. —Man uses to his advantage a large number of plants, but there appears to be no evidence that the schemes for their dispersion were designed for anything except to benefit the plants themselves. The elegant foliage and beautiful flowers, the great diversity of attractive seeds and fruits, all point to plants as strictly selfish beings, if I may so use the term; and not to plants as works of charity, to be devoured by animals without any compensation. By fertilizing flowers, by distributing plants, and by other helpful acts, animals pay for at least a portion of the damage they do.

By an almost infinite number of devices, we have seen that seeds and fruits flee from the parental spot on the wings of the wind, float on currents of ocean, lake, and river. They are shot by bursting pods and capsules in every direction. With hooks, barbs, and glands they cling to the covering of animals. Allured by brilliant colors, birds and other animals seek and devour the fruits of many plants, the seeds of which are preserved from harm by a solid armor; these seeds are then sown broadcast over the land, ready to start new colonies. Nuts are often carried by squirrels for long distances, and there securely buried, a few in a place. By a slow process, which, however, covers a considerable space, in a few years many plants send forth roots, rootstalks, stolons, and runners, and thus increase their possessions or find new homes.

54. Plants migrate to improve their condition. —The various devices by which plants are shifted from place to place are not merely to extend and multiply the species, and reach a fertile soil, but to enable them to flee from the great number of their own kind, and from their enemies among animals and parasitic plants. The adventurers among plants often meet with the best success, not because the seeds are larger, or stronger, or better, but because they find, for a time, more congenial surroundings. We must not overlook the fact, so well established, that one of the greatest points to be gained by plant migration is to enable different stocks of a species to be cross fertilized, and thereby improved in vigor and productiveness.

55. Fruit grown in a new country is often fair. —Every horticulturist knows that apples grown in a new country, that is suited to them, are healthy and fair; but, sooner or later, the scab, and codling moth, and bitter rot, and bark louse arrive, each to begin its particular mode of attack. Peach trees in new places, remote from others, are often easily grown and free from dangers; but soon will arrive the yellows, borers, leaf curl, rot, and other enemies. For a few years plums may be grown, in certain new localities, without danger from curculio, or rot, or shot-hole fungus. It has long been known that the nicest way to grow a few cabbages, radishes, squashes, cucumbers, or potatoes is to plant a few here and there in good soil, at considerable distances from where any have heretofore been grown. For a time enemies are not likely to find them. I have often noticed that, while pear-blight decimated or swept large portions of a pear orchard, a few isolated trees, scattered about the neighborhood, usually remain healthy. The virgin soil of the Dakotas produced, at a trifling cost, healthy, clean wheat, but it was not long before the Russian thistle, false flax, and other pests followed, to contest their rights to the soil.

As animals starve out, in certain seasons when food is scarce, or more likely migrate to regions which can afford food, so plants desert worn-out land and seek fresh fields. As animals retreat to secluded and isolated spots to escape their enemies, so, likewise, many plants accomplish the same thing by sending out scouts in all directions to find the best places; these scouts, it is needless to say, are seeds, and when they have found a good place, they occupy it, without waiting for further instructions.

56. Much remains to be discovered. —"In this, as in other branches of science, we have made a beginning. We have learned just enough to perceive how little we know. Our great masters in natural history have immortalized themselves by their discoveries, but they have not exhausted the field; and if seeds and fruits cannot vie with flowers in the brilliance and color with which they decorate our gardens and our fields, still they surely rival them – it would be impossible to excel them – in the almost infinite variety of the problems they present to us, the ingenuity, the interest, and the charm of the beautiful contrivances which they offer for our study and our admiration."[5 - Flowers, Fruits, and Leaves, by Sir John Lubbock, p. 96.]

Frequent rotations seem to be the rule for many plants, when left to themselves in a state of nature. Confining to a permanent spot invites parasites and other enemies, and a depleted soil, while health and vigor are secured by frequent migrations. The more we study in detail the methods of plant dispersion, the more we shall come to agree with a statement made by Darwin concerning the devices for securing cross-fertilization of flowers, that they "transcend, in an incomparable degree, the contrivances and adaptations which the most fertile imagination of the most imaginative man could suggest with unlimited time at his disposal."[6 - Fertilization of Orchids, p. 351.]

Let no reader think that the topics here taken up are treated exhaustively, for if he will go over any part of this work and verify any observation or experiment, he will be sure to find something new, and very likely something different from what is here stated.

BIBLIOGRAPHY

Means of Plant Dispersion. By E. J. HILL. Am. Nat. Vol. xvii, pp. 811, 1028. 1883.

Why Certain Kinds of Timber Prevail in Certain Localities. By JOHN T. CAMPBELL. Am. Nat. Vol. xix, p. 337. 1885.

Report of the U. S. Commissioner of Agriculture for 1888. Article on the "Food of Crows." By W. B. BARROWS. p. 498.

Report of the U. S. Secretary of Agriculture for 1890. Article on "Seed Planting by Birds." By W. B. BARROWS. p. 280.

Report of the U. S. Secretary of Agriculture for 1893. Article on "Food Habits of the Kingbird, or Bee Martin." By W. B. BARROWS. p. 233.

Bulletin No. 6, U. S. Department of Agriculture. Division of Ornithology and Mammology. "The Common Crow of the United States." By W. B. BARROWS and E. A. SCHWARZ. 1895.

Bulletin No. 7, U. S. Department of Agriculture. Division of Ornithology and Mammology. "Food of Woodpeckers." By F. E. L. BEAL.

Causes of Forest Rotation. By JOHN T. CAMPBELL. Am. Nat. Vol. xx, p. 521. 1886.

Seeds of the Violet and Other Plants as Projectiles. By MOSES N. ELROD, M.D. Am. Nat. Vol. xiii, p. 93. 1879.

The Natural History of Plants. By KERNER and OLIVER. Henry Holt & Co., New York. 1895.

Flowers, Fruits, and Leaves. By SIR JOHN LUBBOCK. Macmillan & Co., New York.

Origin of Cultivated Plants. By ALPHONSE DE CANDOLLE. D. Appleton & Co., New York.

Distribution of Weed Seeds by Winter Winds. By H. L. BOLLEY. Bulletin No. 17. Fargo, North Dakota.

Weeds of California. By E. W. HILGARD. Report of the Experiment Station, pp. 238-266. 1890.

Migration of Weeds. By L. H. DEWEY. Yearbook of the U. S. Department of Agriculture, pp. 263-286. 1896. Washington, D. C.

Squirrels Carrying Nuts. Nature. Vol. xv, p. 117. Macmillan & Co., New York.

Natural History of Plants. By KERNER and OLIVER. Distribution of Species. Vol. ii, pp. 790-885. Henry Holt & Co., New York. 1895.

notes

1

C. D. Lippincott believes that this is a provision of nature to dispose of the now unnecessary branchlets without leaving a knot. Plant World, Vol. I, p. 96.

2

The brittle branches of salix were noticed by the author in Bull. Torr. Bot. Club, Vol. IX (1883), p. 89.

3

In the fall of 1897, Prof. C. F. Wheeler saw a blue jay fly from a white oak tree with an acorn in its mouth. The bird went to the ground four or five rods distant and crowded the acorn into the soil as far as it could, covering the spot with a few leaves. A member of my family saw a blue jay leave half of a black walnut in the forks of several small branches.

4

It should be noted that the blackbird here mentioned is not the same as either of our blackbirds, but a thrush much like our robin; that the robin mentioned is a ground warbler nearly related to our bluebird. It should also be noted that jackdaws, ravens, thrushes, and probably many others eject thousands of seeds by the mouth for one which passes through the intestines.

5

Flowers, Fruits, and Leaves, by Sir John Lubbock, p. 96.

6

Fertilization of Orchids, p. 351.