15. An air-tight sack buoys up seeds.—Here are several dry fruits of sedges—plants looking considerably like grasses. There are a good many kinds, and most of them grow in wet places. The seed-like fruit of those we examine are surrounded each by a sack which is considerably too large for it, as one would be likely to say, but in reality it serves to buoy the denser portion within, much after the plan of the bladder nut. In some instances the sack is rather small, but a corky growth below the grain helps to buoy it on water.

FIG. 19.—Fruit of Carex communis, an upland sedge, that readily sinks when placed in water; the sack fits closely.

Sedges that grow on dry land usually have the sack fitted closely, instead of inflated, and the whole mass sinks readily in water. Now we see the probable reason why the sack is inflated in some species of sedges and not in others.

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.

FIG. 20.—Seed-like fruit of arrowhead with corky margins to float on water. FIG. 21.—Seed-like fruit of bur reed with corky lining ready to float on water, and a naked seed, such as sinks promptly.
FIG. 22.—Grains of lyme grass with two corky empty glumes attached, which serve as a raft.

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.

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.