MAN DISPERSES SEEDS AND PLANTS.
In describing the various means by which plants are dispersed, people
are very likely not to mention the aid supplied by man, or to speak
of his efforts as artificial or unnatural, forgetting for the time
that man so far appears to be the crown of earthly existence, and
that his works are a necessary part of a complete world.
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.
[Illustration: FIG. 63.--Seed of cockle (enlarged).]
[Illustration: 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.
[Illustration: 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.