24 
SEED DISPERSAL. 
another mode of travel, not by wings of a bird, but by 
floating on water. 
Clean grape seeds sink at once, but covered by the 
dry skin and pulp, they float. In a similar manner the 
dry seeds of several dogwoods are eaten for the pulp by 
birds, but in case any are left they behave after the 
manner of grapes. 
14. Bits of cork around the seeds prevent them from sink- 
ing. — Narrow-leaved dock is a prominent weed, and is 
Fig. 17 . — Fruit and adherent calyx of narrow-leaved dock ; a cross section and a naked, 
seed-like fruit (enlarged). 
especially at home on river bottoms and on low land that 
is flooded once in a while. 
Did you ever wonder what could be the object of a 
round, spongy tubercle on the outside of each of these 
sepals which hold the ripened seed closely ? I did not 
know their use for a long time, but now think I have 
discovered their meaning. They are not exactly life- 
preservers, but the next thing to it. The naked, seed- 
