sheep or dogs or the sand.--Porcupine grass,
_Stipa spartea_, grows in dry soil in the northern states, but more
particularly on the dry prairies of the central portion of the United
States. This grass, when ripe, has a very bad reputation among
ranchmen for the annoyance the bearded grain causes them. The grains
are blown into the stubble among grasses with the bearded point down,
sticking into the soil. The first rain or heavy dew straightens out
the awns, which are twisted again as they dry. The bearded point works
a little farther with each change, and after twisting and untwisting
a number of times it gets down three or four inches into the sand,
often to moisture, where the awns decay and the grain germinates.
Here is an admirable scheme for moving about and for boring into the
ground. But this is not all. The grains are quick to catch fast to
clothing, as people move among the plants, and they are admirably
fitted for attaching themselves to dogs and sheep, which they annoy
very much. These animals transport the grains for long distances.
The twisting and untwisting of the awns enable the grain to bore
through the fleeces, and even to penetrate the skins and make wounds
which sometimes cause the death of the animal. Examine also seeds
of pin clover, Alfilerilla, which is becoming abundant in many parts
of the world.

[Illustration: FIG. 40.--Ripe fruit of pin clover, or Alfilerilla,
ready to twist into fleeces of sheep or into loose soil.]

33. Winged fruits and seeds fall with a whirl.--The large fruit of
the silver maple falls in summer. As these trees are most abundant
along the margins of streams, the fruit often drops into the water
and is carried down stream to some sand drift or into the mud, where
more sand is likely to cover them. Thus sown and planted and watered,
they soon grow and new trees spring up. But in many instances a strong
breeze, sometimes a whirlwind, has been seen to carry these mature
fruits from the tree to a distance of thirty rods.

[Illustration: FIG. 41.--Single fruit of silver maple.]

A thin sheet of paper descends more slowly than the same material
put in the form of a ball. On the same principle, many seeds and fruits
are flattened, apparently for a purpose; not that they may be easily
shot through the air by some elastic force, not to increase their
chances for attachment to animals, but to enable the wind to sustain
them the longer and carry them farther. Some seeds and dr