General 
of vegetation pressing upon and replacing the lower 
association. 
This way of considering the plants even in a country 
so well known as Great Britain makes many small 
points very much more interesting. 
When a stone wall or a new bridge is built, one sees 
first lichens, then mosses, and occasionally ferns begin- 
ning to occupy the new territory. 
It is new untrodden ground, and requires the old 
primeval form of attack, 
But when ploughed land or the pared edges of a 
roadside are overgrown by weeds during the summer, 
the conditions are not exactly the same. The earth is 
rich soil full of dead vegetable matter, and probably 
full of bacteria and worms. 
So one often finds grasses and various weeds 
springing up only a very short time after the earth 
is exposed. Even on rich soil of this kind, however, 
both algze and mosses are by no means unusual. In 
autumn the stones of a turnip-field are sometimes 
green with alge and occasional patches of moss may 
be found. 
In the more advanced kinds of plant associations, 
such as, eg. a well-grown tropical jungle, the amount of 
vegetation formed per square yard is extraordinary, 
indeed almost incredible. Almost every kind of plant 
life may be represented. 
There are first the tallest trees whose leaves forms 
the uppermost “canopy” or foliage surface ; from their 
upper branches there usually hang down coiled, en- 
twined and rope-like lianas, sometimes twining round 
the tree stems, more often hanging like festoons or 
wreaths from above. The foliage of these creepers 
forms a second leaf-surface or floor. Below these are 
the younger trees which have been arrested in their 
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