TYPES OF PLANT-COMMUNITIES. 887 



tributes materially to its preservation; and the same may be said of the cases where 

 a contrast between the colours of the flowers of adjacent species promotes visits from 

 msects, or where any other mutual help is afibrded by plants growing side by side 

 m a community. The general aspect of a community is scarcely influenced at all 

 by diversities in the nature of the subordinate species, but depends solely on the 

 dominant species which enter into its composition. These stamp their characteristic 

 aspect upon the entire community, and determine the general impression conveyed 

 to the observer. 



^ This fact is of great moment when we come to the task of identifying, classi- 

 fying, and naming the various communities formed by plants. Not only must the 

 gregarious dominant species afibrd the basis of description in the case of each 

 separate community, but their external appearance is the most important means of 

 classifying in groups, according to similarity of aspect, the numerous communities 

 which have been formed in the present period of the earth's history. Observations 

 made under natural conditions, and extending over many years, have led to a 

 division of plant-communities into the following nine groups: 



I. Forests. — The dominant species are plants with standard stems (see vol. i. 

 p. 712). In accordance with the common notion of a forest, the stems which con- 

 stitute its substructure are destitute of branches or leaves up to a certain height. 

 Where this height is not much above that of a man, we speak of a copse; but if 

 the standard stems remain branchless and leafless to a greater height, the assem- 

 blage of plants is called a forest proper. We might call these two kinds of forest 

 (for the purposes of this chapter) high forest and low forest, though the terms 

 are not in all ways free from objection; further, the circumstance that high forest 

 has been low forest in the younger stages of its development is an additional reason 

 against their adoption. If the trees of which a wood is composed are so close 

 together that their top leaves and branches are in contact and form a sort of roof, 

 the wood is said to be crowded or dense; whilst, if the trees are so formed and 

 situated relatively to one another as to allow the rays of sunlight to penetrate 

 between them and reach the ground, the wood is said to be thin. 



II. Scrub. — The dominant species are shrubs, semi-shrubs, and cactiform plants 

 growing in thickets, and never developing standard-stems, but branching from the 

 very base, even when full-grown. The transition is quite gradual from erect scrub, 

 reaching to a height of 2 or 3 metres, to those in which the stems lie upon the 

 ground, and only lift their woody branches a few decimetres above it. It is the 

 nature of shrubs and semi-shrubs to form thickets. Most of the bigger shrubs are 

 impenetrable if not modified by human agency. In special situations, and under 

 certain annually recurring conditions, woody plants of a kind, which usually develop 

 into trees and exhibit standard-stems, may be dwarfed and assume the form of tall 

 shrubs. For example, in the Alps, where trees growing near the boundary-line, 

 beyond which their existence is impossible, are liable to be loaded with heavy masses 

 of snow, and again, in the valleys annually exposed to avalanches, the Beech grows 

 in regular thickets. They are, nevertheless, to be looked upon as forests which have 



