234 BRITISH PLANTS 



densis), for example, was introduced into this country 

 from America about 1842, spread rapidly over the whole 

 of Britain, and extended into western Europe ; and the 

 rapidity with which it spread was probably due chiefly 

 to its being carried on barges into canals, and thence by 

 birds, which find the shoots tasty eating, into other 

 pieces of water. 



Associations of aquatic plants are usually of an open 

 type. Competition between the various plants is not 

 keen, and almost any plant suited to the particular 

 situation may obtain a footing. The distribution of the 

 different associations depends on a number of factors, of 

 which the following are the most important: the rate of 

 the current, the depth of the water, the amount of mineral 

 salts in solution, and to a slighter extent on altitude. 

 The rate of the current may be taken as a convenient 

 basis on which to classify the associations, and in this 

 way the following groups may be distinguished : 



1 . Associations in swiftly- flowing water — e.g. , mountain- 

 torrents, cascades, and waterfalls. 



2. Associations in slowly-flowing water — e.g., ordinary 

 rivers and streams. 



3. Associations in standing water — e.g., lakes, ponds, 

 and ditches. 



1. Swiftly-F:owing Water. 



Associations belonging to this group are not common 

 in this country, and no flowering plants are included^ 

 Mountain-torrents and waterfalls can only exist where 

 the underlying rock is extremely hard ; all loose material 

 will be washed away, and the only substratum on which 

 plants can grow is the solid rock. This makes it impossible 

 for a plant possessing ordinary roots to obtain a hold. 

 If a seed were to germinate in a sheltered crevice in the 

 rock, as soon as the plant produced leaves it would be 

 torn away by the current. The only plants which can 

 exist under these conditions are those which possess 

 special disc-shaped holdfasts which cement the plant to 

 the rock. These plants may form encrustations close to 

 the rock, as in some green algae and diatoms, or they are 

 smooth, slippery plants which bend before the current 

 and offer little resistance to it — e.g., some liverworts 

 (Scapania undtdata) and mosses [Fontinalis antifyretica).^ 



