CHAP. Ill PLANT-COMMUNITIES 13 



differing in growth-form and economy. The physical and other charac- 

 ters of the habitat play a fundamental part in these matters, and, 

 for this reason, form the introductory subject-matter of Section I in this 

 work. 



The oecological analysis of a plant-community leads to the recognition 

 of the growth-forms composing it as its ultimate units. From what has 

 just been said in regard to growth-forms it follows that species of very 

 diverse physiognomy can very easily occur together in the same natural 

 community. But beyond this, as already indicated, species differing 

 widely, not only in physiognomy but also in their whole economy, may 

 be associated. We may therefore expect to find both great variety of 

 form and complexity of inter-relations among the species composing a 

 natural community ; as an example we may cite the richest of all types 

 of communities the tropical rain-forest. It may also be noted that 

 the physiognomy of a community is not necessarily the same at all 

 times of the year, the distinction sometimes being caused by a rotation of 

 species. 



In countries far apart there are to be found communities identical in 

 type, but entirely different in floristic composition. Meadows in North 

 America and in Europe, or the tropical forest in Africa and in the East 

 Indies, may show the same general physiognomy, the same kinds of 

 constituent growth-forms, and the same type of natural community, 

 though of course their species are entirely different and thus introduce 

 slight physiognomic differences. 



The different communities, it need hardly be stated, are scarcely 

 ever sharply marked off from one another. Just as soil, moisture, and 

 other external conditions are connected by the most gradual transitions, 

 so likewise are the plant-communities, especially in cultivated lands. 

 In addition, the same species often occur in several widely different com- 

 munities ; for example, Linnaea borealis grows not only in coniferous 

 forests, but also in birch-woods, and even high above the tree-limit on 

 the mountains of Norway and on the fell-fields of Greenland. It appears 

 that different combinations of external factors can replace one another 

 and bring into existence approximately the same community, or at 

 least can satisfy equally well one and the same species, and that, for 

 instance, a moist climate often completely replaces the forest-shade of dry 

 climates. 



It is evident that all these circumstances render very difficult the 

 correct scientific interpretation, delimitation, diagnosis, and systematic 

 classification of plant communities, especially when we consider the 

 condition of our present knowledge for we have only just commenced 

 to investigate growth-forms and communities, and what we do not know 

 seems infinite. Another difficulty, to which allusion has already been 

 made, is to assign suitable names to the more or less comprehensive, 

 principal or subordinate, plant-communities occurring on the Earth and 

 imparting to the landscapes entirely different physiognomies. Nor is it 

 easy to estimate the true significance of floristic distinctions. 



