times seen in
cultivation. When ripe, the fruit is hard, carrying two stout beaks
with recurved tips. Experiments show it to be admirably adapted to
catch on to the feet of sheep, goats, and cattle, or hold to the fleeces
of the two former.

[Illustration: FIG. 54.--Dry fruit of the unicorn plant adapted to
catching on to the feet of large animals or the wool of sheep.]

44. Water-fowl and muskrats carry seeds in mud.--Seeds and fruits
of aquatic and bog plants that are floating, or in the mud of shallow
water, are often carried by ducks, herons, swallows, muskrats, and
other frequenters of such places, on their feet, beaks, or feathers,
as they hastily leave one place for another. In this way seeds of
water plantain, sedges, grasses, rushes, docks, arrowhead, pondweeds,
duckweed, cat-tail flag, bur reed, bladderwort, water crowfoot, and
many others are transported from one pond, lake, or stream, to another.
In some cases enough of a living plant may be detached and carried
away to keep on growing. Darwin found on the feet of some birds six
and three-quarter ounces of mud, in which were five hundred and
thirty-seven seeds that germinated. Mud may be carried on the feet
of land animals as well as on aquatic animals, not only from ponds
and bogs, but from the fields where seeds may have accumulated in
the earth or washed down the slopes.

45. Why some seeds are sticky.--Some seeds and fruits are sticky;
in some instances the mucilaginous substance is normally moist enough
to adhere to anything that touches it, while in other cases it requires
to be wetted before it will adhere. The seeds of flax, plantain,
peppergrass, basil, sage, dracocephalum, groundsel, drop-seed grass,
and many others less familiar, possess this peculiarity. The berries
of some plants, when fully ripe, burst very easily when touched, and
some of the seeds are then likely to adhere to animals and be carried
away. Some berries of several plants belonging to the nightshade
family have this peculiarity, as well as some of the cucurbits. When
the outer covering of seeds of water lilies, arums, and others are
broken, the gummy secretion is very likely to adhere to the feathers,
or fur, or feet of animals. A number of fruits, and even the upper
fruit-bearing branches, have sticky glands with which to catch on
to any passing object. Among these are some kinds of sedges,
chickweeds, and catchflies.

The sticky substance on seeds and fruits not unfrequent