he sack is inflated in some species
of sedges and not in others.

[Illustration: FIG. 19.--Fruit of _Carex communis_, an upland sedge,
that readily sinks when placed in water; the sack fits closely.]

Here are some small, seed-like fruits, _achenes_, not likely to be
recognized by every one. They belong to the arrowhead, _Sagittaria_,
found in shallow ponds or slow streams. They are flattened, and on
one edge, or both, and at the apex is a spongy ridge. Very likely,
by this time, the reader has surmised that this serves the purpose
of a raft to float the small seed within, which would sink at once
if separated from the boat that grew on its margins. In this connection
may be studied achenes of water plantain, _Alisma_, bur reed,
cat-tail flag, arrow grass, burgrass, numerous pondweeds, several
buttercups, the hop, nettles, wood nettle, false nettle, cinquefoil,
avens, ninebark, buttonbush, and in fact a large number and variety
of plants usually found on river bottoms.

[Illustration: FIG. 20.--Seed-like fruit of arrowhead with corky
margins to float on water.]

[Illustration: FIG. 21.--Seed-like fruit of bur reed with corky
lining ready to float on water, and a naked seed, such as sinks
promptly.]

One of the lyme grasses, _Elymus Virqinicus_, is a stiff, short grass,
growing along streams. Each spikelet with its chaff adheres to two
empty glumes, stout, thick, and spongy, which make a safe double boat
for transportation down stream whenever the water is high enough.
The grains of rice-cut grass, grown in ditches and spring brooks,
sink if separated, but in the chaff, as they fall when ripe, they
are good floaters.

[Illustration: FIG. 22.--Grains of lyme grass with two corky empty
glumes attached, which serve as a raft.]

In the driftwood, which we still have under consideration, are some
fruits of maple, beech, oak, tulip tree, locust, and basswood. Maples
are well scattered by the wind, but these seed-like fruits have taken
to the water, and a few still retain vitality. An acorn, while yet
alive, sinks readily, and is not suited for water navigation, unless
by accident it rides on some driftwood. The fruits of the tulip tree,
locust, and basswood behave well on the water, as though designed
for the purpose, though we naturally, and with good reason, class
them with plants usually distributed by wind.

16. Fruit of basswood as a sailboat, and a few others as adapted to
the water.--In spring, when the bract