860 THE AMERICAN NATURALIST.  [Vou. XXXII. 
In wet places sedges abound. Those of the genus Carex 
have each fruit enclosed by a sack (perigynium). In most 
sedges growing in wet land the sack is considerably larger 
than the enclosed achene and serves to float the denser por- 
tion. Without the perigynium the ripened achene sinks at 
once. 
Some of the lowland sedges, like Carex stzpata and C. sterile, 
have a perigynium only slightly inflated, but to buoy up the 
achene well there are small masses of corky substance inside. 
Species of Carex which grow on dry land, like C. pennsyl- 
vanica and the rest of the tribe, have the sack fitting closely 
instead of inflated, and the whole mass sinks readily in the 
water. 
In the drifted material under consideration are achenes of 
arrowhead, Sagittaria. They are flattened, and on one edge 
or both, and at the apex is a spongy ridge that serves the pur- 
pose of a raft to float the small seed within, which would sink 
readily if separated from the light substance that grew on its 
sides. In this connection may be studied achenes of Alisma, 
bur reed, cat-tail flag, arrow grass, burgrass, numerous potamog- 
etons, several buttercups, the hop, nettles, false nettle, cinque- 
foil, avens, and others. z 
There grows along streams a common grass known as 
Elymus virginicus. A pair of corky, empty glumes adhere to 
one or more of the mature florets between them and serve 
as boats to carry the ripened grain to a new spot. 
The compressed grain of rice-cut grass (Homalocenchrus ory- 
zoides) is enclosed by a pair of glumes, and they float well on 
the water, but if the glumes are removed the grain drops to 
the bottom immediately. 
Noticeable among seeds in the floodwood are some of the 
milkweeds, which every one would say at a glance were espe- 
cially fitted for sailing through the air, aided by numerous long, 
silky hairs. These hairs are no hindrance to moving by water. 
The flat seed has a hem-like margin, which must aid the wind 
in blowing it about, but this margin is thickened somewhat by 
a spongy material. With the margin attached, it floats; without 
it, the seed sinks in fresh water. 
