188 3] Means of Plant Dispersion. 819 
flora to that of the mainland near by is a proof of this, since many 
of their plants could have been received in no other way, unless 
the islands have been at some former era physically joined to the 
continents, and afterward cut off by water through subsidence. 
Hard fruits like walnuts, butternuts and acorns readily bear this 
transportation, and their thickened coats serve not only to keep 
the water from injuring the germ, but also help to float them by 
rendering them lighter in proportion to the water displaced. So 
too with hard seeds that may be dropped into the water by plants 
which grow along its margin. In this way streams, and currents 
on ponds, lakes and arms of the sea, aided by winds that drift 
bodies along the surface of the water, bear their freight of seeds 
and fruits from land-grown plants. 
But aquatic plants, whose fruits and seeds ultimately sink in 
water, must be principally dispersed in this manner. They gen- 
erally grow in the stiller parts of streams and sheltered places of 
pords and lakes, and their seeds are borne away by currents or 
drifted by winds till lodged or dropped in quiet spots. Some- 
times there are special equipments to facilitate this, like wings or 
other floats, such as are also suitable for wind transportation. A 
good example of this is the fruit of the common arrow-head 
(Sagittaria), which, surrounded by a wing, easily floats away on 
the water. The flattened spikelets of aquatic grasses, like Leer- 
‘Sia, and the inflated perigynea of many sedges of the genus 
Carex, as well as the bristies that surround the fruit of some, like 
Eleocharis, contribute to this result. It is a common thing to see 
plants floated off wholly or in part by water. The current of any 
stream shows this. The shores of ponds and lakes, and the banks 
of rivers are often strewn with species of the plants that grow 
in them. By their examination the collector learns what may be 
found in them even before seeking for their particular location. 
If torn from their place of growth at the time of ripened fruit, 
this becomes an effectual means of propagation. Stems of Pot- 
amogeton and other Naidacex, of Myriophyllum and Ceratophyl- 
lum, carry their seed in this way. This is especially true of some 
plants that are rootless and float naturally on the surface, like the 
Utricularie, or bladder-worts, some of which are provided with. 
bladders to sustain them on the water. The duckweed (Lemna) 
and Wolffiia are in this manner carried everywhere in the waters 
they frequent, till they find some quiet place where they can mul- 
