| FIG. 63.—Seed of cockle (enlarged). |
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.
| FIG. 64.—Grain of wheat (enlarged), scarcely larger than a seed of cockle. |
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.
| FIG. 65.—Two seeds of narrow-leaved plantain such as are becoming common in clover seed. The lower one and the one at the left are seeds of red clover. |
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.