560 A TEXTBOOK OF BOTANY [Ch. XIV 



refine our analysis of habitats. The essential feature of a 

 vegetation form consists in the method by which the plants 

 are fitted by form and structure to perform their principal 

 functions under the physical conditions which determine 

 the habitat, — e.g. excessive dryness in deserts, and defective 

 aeration in water. The main condition of the habitat being 

 met by a common method, a diversity in other features does 

 not matter, precisely as the members of a trades union, or 

 professional association, all in equally good standing, may 

 differ in looks, height, speech, dress, and any other features 

 unconnected with the main object of the union. 



We shall now consider some of the principal vegetation 

 forms of the earth in relation to the habitats they occupy. 

 The nomenclature of vegetation forms is still chaotic. In 

 principle, they should be so named as to express the struc- 

 tural method by which they interlock with the salient 

 condition of the habitat; but in practice they are named 

 very diversely. 



Plant habitats are necessarily limited to those places on 

 the earth which provide the conditions and substances 

 indispensable to plant life. The primal necessities are six, — 

 viz. warmth, water, light, carbon dioxide, oxygen, and certain 

 mineral salts. Where any one is wanting, plants cannot 

 exist, as in polar ice or rainless deserts. Where all are 

 present, however, it is not in uniformity, but great diversity, 

 of proportions and combinations. It is this diversity which 

 determines the differences between habitats. 



The plant habitats of the world, despite their intergrada- 

 tions, are amenable to detailed classification. Most funda- 

 mentally they divide according to whether (1) they offer 

 only the raw materials for the formation of food, in which 

 case they constitute the inorganic habitats, and are 

 occupied by independent green plants, called autophytes, 

 or (2) they offer ready-formed organic food, in which case 

 they constitute the organic habitats, and are occupied 

 by parasites and saprophytes, collectively called hystero- 



