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-Flowing 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 alge and diatoms, or they are 
smooth, slippery plants which bend before the current 
and offer little resistance to it—e.g., some liverworts 
(Scapania undulata) and mosses (Fontinalis antipyretica). 
