12 INTRODUCTION chap, ii 



cricoid (Ericaceae) ; cupressoid (Cupressus) ; scale -like in aphyllous 

 trees (Casuarina, Halimodendron). Dwarf-forms occur, some of them 

 having succulent leaves. In certain tropical forms the leaves are aggre- 

 gated at the ends of long, feebly branched twigs : for instance, in Cecropia 

 and Carica. 



Shrubs (frutices) and dwarf-shrubs {jruticuli). Low trunkless woody 

 plants, with the variety in the construction of the leaf and shoot seen in 

 canopy trees : 



Switch-shrubs : erect, long assimilating stems and small caducous 

 leaves. 



Succulent-leaved shrubs also belong here : species of Crassulaceae, 

 Mesembryanthemum, Chenopodiaceae, and others. 



Gramineous shrubs represent a pecuhar type. In the bamboo-form 

 there arise from the subterranean stem tufts of many richly and charac- 

 teristically branched evergreen stems, which undergo no secondary 

 thickening. Leaves grass-like. Particularly a tropical growth-form 

 forming forest and bush. 



Aphyllous shrubs : which sometimes have equisetoid or salicornioid 

 shoots. 



The sub-classes of the other classes of growth-forms will be referred 

 to later, when the environmental conditions relating to them are discussed. 



CHAPTER III. PLANT-COMMUNITIES 



Oecological Botany has further to investigate the natural plant- 

 communities, which usually include many species of extremely varied 

 growth-form. 



Certain species group themselves into natural associations, that is 

 to say, into communities which we meet with more or less frequently, and 

 which exhibit the same combination of growth-forms and the same facies. 

 As examples in Northern Europe may be cited a meadow with its grasses 

 and perennial herbs, or a beech-forest with its beech-trees and all the 

 species usually accompanying these. Species that form a community 

 must either practise the same economy, making approximately the 

 same demands on its environment (as regards nourishment, light, moisture, 

 and so forth), or one species present must be dependent for its existence 

 upon another species, sometimes to such an extent that the latter provides 

 it with what is necessary or even best suited to it (Oxalis Acetosella and 

 saprophytes which profit from the shade of the beech and from its 

 humus soil) ; a kind of symbiosis seems to prevail between such species.^ 

 In fact, one often finds, as in beech-forests, that the plants growing under 

 the shade and protection of other species, and belonging to the most 

 diverse families, assume growth-forms that are very similar to one another, 

 but essentially different from those of the forest-trees, which, in their turn, 

 often agree with one another .^ 



Oecological plant-geography has also to inquire into the kinds of 

 natural communities in existence, their special methods of utihzing their 

 resources, and the frequent intimate association together of species 



^ See Chap. XXVI regarding Unlike Commensals. ^ Warming, 1901. 



