as developed recently on strong clay land
into a tumbleweed six inches in diameter. The tops of old witch grass,
_Panicum capillare_, and hair grass, _Agrostis hyemalis_, become
very brittle when ripe, and snap from the parent stem and tumble about
singly or in masses, scattering seeds by the millions. I have seen
piles of these thin tops larger than a load of hay where they had
blown against a grove of trees, and in some cases many were caught
in the tops of low trees.

[Illustration: FIG. 26.--The top of old witch grass as a tumbleweed.
(Reduced two thirds.)]

Bug seed and buffalo bur are tumbleweeds. In autumn the careful
observer with an eye to this subject will be rewarded by finding many
other plants that behave more or less as tumbleweeds. Especially is
this the case on prairies. These are annuals, and perish at the close
of the growing season. There are numerous other devices by which seeds
and fruit secure transportation by the wind.

19. Thin, dry pods, twisted and bent, drift on the snow.--The common
locust tree, _Robinia Pseudacacia_, blossoms and produces large
numbers of thin, flat pods, which remain of a dull color even when
the seeds are ripe. The pods of the locust may wait and wait, holding
fast for a long time, but nothing comes to eat them. They become dry
and slowly split apart, each half of the pod usually carrying every
other seed. Some of the pods with the seeds still attached are torn
off by the wind and fall to the ground sooner or later, according
to the force of the wind. Each half-pod as it comes off is slightly
bent and twisted, and might be considered a "want-advertisement"
given to the wind: "Here I am, thin, dry, light and elastic, twisted
and bent already; give me a lift to bear these precious seeds up the
hill, into the valley, or over the plain."

[Illustration: FIG. 27.--Two views of a half-pod of common locust,
dry, twisted, and bent, ready for a breeze.]

And the wind is sure to come along, a slight breeze to-day tossing
the half-pod a few feet, leaving it perhaps to be again and again
moved farther forward. The writer has seen these half-pods
transported by this means more than a block. But many of the pods
stick to the limbs till winter comes. Then a breeze tears off a few
pods and they fall on the snow, which has filled up all the crevices
in the grass and between the dead leaves and rubbish. Each half-pod,
freighted with every other seed, is admirably constructed; like an