CHAP, xxxin SOME STRUCTURAL CHARACTERS 131 



vegetation ; the grasslands of northern and central Europe and of the 

 Alps, and similar types of vegetation, are very rich in low, perennial 

 rosette-herbs such as Plantago major, Taraxacum officinale, Achillea 

 Millefolium, and Pimpinella Saxifraga, species of Primula, Draba, Saxi- 

 fraga. Bonnier * showed that certain species which in the plains have 

 shoots with long internodes, when grown at alpine heights produce 

 rosettes. 



PROSTRATE SHOOTS 



Many species growing on dry, warm, sandy soil have prostrate shoots, 

 at least so far as these are vegetative. As was shown on p. 26 this is 

 to be attributed to the thermal relations prevailing in the soil. 



CHAPTER XXXIV. OECOLOGICAL CLASSIFICATION 2 



THE foregoing chapters have made it clear that the distinctions 

 between water-plants and land-plants are deep-seated, and concern 

 the external form as well as the internal structure. Plant-communities 

 must therefore be grouped in the first place into aquatic and terrestrial ; 

 but between these there is no sharp boundary, for there is a group of 

 plants, marsh-plants (helophytes), which, like water-plants, develop 

 their lower parts (roots, rhizomes, and, to some extent, leaves) in water 

 or at least in soaking soil, but have their assimilatory organs mainly 

 adapted to existence in air, as is the case with land-plants to which they 

 are closely allied. Helophytes give rise to special forms of communities. 

 Yet we must include among water-plants all those plants that, like 

 Nymphaeaceae, approximate to land-plants in so far as they have floating- 

 leaves, which are more or less adapted to existence in air, but are never- 

 theless mainly designed for existence upon water. 



It has already been shown that land-plants exhibit many grades of 

 adaptation to their mode of life in contact with air, and that those which 

 encounter the greatest difficulties in regard to securing water are termed 

 xerophytes ; while others are described as mesophytes because in some 

 respects they stand midway between the two extremes, hydrophytes 

 and xerophytes. The differentiation of the land-plant in one or the 

 other direction is decided by the oecological factors, edaphic and climatic, 

 that prevail in the station or habitat. But edaphic and climatic factors 

 cannot be regarded separately : the plant-community is always the pro- 

 duct of both together. The nature of a soil is also influenced by climate, 

 and it is incontestible that climate (rainfall) calls forth the wide differences 

 between, say, desert and tropical rain-forest. But it is far from being 

 true that climate alone calls into existence the different communities 

 of plants which will hereafter be defined as formations. Characters of 



1 Bonnier, 1890, 1894. 



* In the classification of plant-communities these are grouped into successively 

 smaller subdivisions that are, only to some extent, analogous with systematic 

 families, genera, species, and varieties. The most comprehensive group is termed 

 in German a ' Vereinsklasse ' or ' Formationsklasse ', which we propose to translate 

 as ' oecological class ', or when the context permits, as ' class '. 



K 2 



