CHAPTER XXIII 
AQUATIC VEGETATION 
In Chapter V. we discussed the conditions of life under 
which hydrophytes exist, and the structural peculiarities 
which they exhibit in response to their environment. 
We are here concerned more with the distribution ot 
the plants in fresh water (for marine aquatics see 
Chapter XXVIIT.). 
The aquatic vegetation throughout all temperate regions 
is remarkably constant. Not only do we find the habit 
of the various plants similar in all parts of the world, 
but the same species may be found in all countries. 
Thus the commonest British aquatics, Ranunculus 
aquatilis (water-crowfoot), Lemna minor (duckweed), and 
species of Potamogeton (pondweeds), are spread over the 
entire Temperate Zone of Europe, Asia, and America, and 
even occur in the southern hemisphere in Australia. 
This widespread distribution is accounted for by the ease 
with which detached portions of the plant can be carried 
in water-currents (p. 54), and the seeds by migratory 
water-fowl. The latter are the most efficient of all birds 
for the dispersal of seeds ; they settle down only in the 
neighbourhood of water, and any seeds of aquatics 
adhering to the mud on their feet are pretty sure of 
finding a congenial home. The temperature of the water 
and other conditions of life vary little in different countries, 
and once a plant or seed has reached a new district it 
finds aJl its surroundings favourable to growth, and it 
becomes established. In civilized countries the barge is 
the most efficient of all carriers of water-plants. Many 
plants may be conveyed over whole continents, through 
rivers and their connecting canals, attached to the bottom 
of the barge. The Canadian pondweed (Hlodea cana- 
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