CHAPTER XXIII 



AQUATIC VEGETATION 



In Chapter V. we discussed the conditions of life under 

 which hydrophytes exist, and the structural peciiliarities 

 which they exhibit in response to their environment. 

 We are here concerned more with the distribution of 

 the plants in fresh water (for marine aquatics see 

 Chapter XXVIII.). 



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, Banunculus 

 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 aU 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 all 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 {Elodea cana- 



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