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
Agricu