n: FIG. 10.--Portion of a branch of the cottonwood as
it fell from the tree.]

In a greenhouse a potted plant of _Selaginella emiliana_(?) was
placed on the bench near the aisle, where it was often brushed by
people in passing. Small branches, not being firmly attached, were
frequently broken from the main plant and fell upon the moist sand,
where they rooted in abundance.

CHAPTER IV.
WATER TRANSPORTATION OF PLANTS.

11. Some green buds and leaves float on water.--Loosely floating on
slow streams of the northern states, in water not the purest, may
often be found the common bladderwort, _Utricularia vulgaris_,
producing in summer a few yellow flowers on each stem, rising from
six to twelve inches above the water. The lax, leafy branches in the
water are from six inches to a foot long. The leaves, or thread-like
branches, are about half an inch long, more or less, and several times
divided.

[Illustration: FIG. 11.--A free branch and two buds of bladderwort.]

Scattered about are large numbers of flattened scales, or bladders,
sometimes one-sixth of an inch long, which give the plant one of its
names. For a long time the bladders were thought to serve merely as
life-preservers; it was supposed that they were constructed to keep
the plant from sinking to the bottom. In reality these bladders help
preserve the plant in another sense, by catching and killing large
numbers of minute animals, on which the plant lives in part. The tips
of the stems at all times of the year are rather compact, made up
of young leaves and stems, and in the middle of the summer, as well
as at other times, many may be seen severed from the parent plant,
floating in the water, ready to accept the assistance of any favorable
current or breeze and start out for homes of their own to found new
colonies. These olive-green tips, or buds, vary much in size, but
the largest are the size of the end of one's little finger. Late in
autumn or early winter, when cold threatens, the tender buds contract
a little, and, having thus become heavier than water, slowly go to the
bottom to spend the winter safely protected in the soft mud. All the
plant perishes except these buds. With the lengthening days of spring
the melting ice disappears, and genial sunshine gives notice to the
dormant buds that it is safe to come out again. The buds begin to
expand, become lighter than water, and are soon seen spreading out at
the surface and producing branches a