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 alga3 and mosses are by no means unusual. In 

 autumn the stones of a turnip-field are sometimes 

 green with alga; 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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