HOW PLANTS ARE SCATTERED 381 
are: aquatic grasses, rushes and sedges, polygonums, water-dock, 
bur-reed, arrowhead, water-plantain, pickerel-weed, alder, button- 
bush, water-parsnip (Sium), water-hemlock (Cicuta), water penny- 
wort (Hydrocotyle). 
451. Distances traversed by Floating Seeds. — Ocean 
currents furnish transportation for the longest journeys 
that are made by floating 
seeds. It is a well-known 
fact that cocoa-palms are 
among the first plants to 
spring up on newly formed 
coral islands. The nuts 
from which these palms 
grew may readily have 
floated a thousand miles 
or more without injury. 
On examining a cocoanut 
with the fibrous husk at- 
tached, just as it fell from 
the tree, it is easy to see -— 
how well this fruit is 
adapted for transportation 
by water. There are al- 
together about a hundred 
drifting fruits known, one 
(the Maldive nut) reach- : 
ing a weight of twenty to Fic. 270. — Panicle of Tickle-Grass, a 
a ty- rao poun ds: Common Tumbleweed. 7 
452. Burs. — A large class of fruits is characterized by 
the presence of hooks on the outer. surface. These are 
sometimes outgrowths from the ovary, sometimes from 
