.26 
SEED DISPERSAL. 
Fig. 21. — Seed-like fruit of bur reed 
with corky lining ready to float on 
water, and a naked seed, such as 
sinks promptly. 
head, 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 sepa- 
rated 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, nine- 
bark, buttonbush, and in fact a large number 
and variety of plants usually found on river 
bottoms. 
One of the lyme grasses, Elymus Virginicus, 
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 trans- 
portation down stream whenever the water 
is high enough. The grains of rice-cut grass, grown 
in ditches and spring brooks, sink if separated, but 
Fig. 22. — Grains of 
lyme grass with 
two corky empty 
glumes attached, 
which serve as a 
raft. 
