314 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



habitat that seem recognizable as entities. Within each of these 

 categories are almost endless variations : for practically any change 

 of one or more of the various ecological factors dealt with in the last 

 chapter can cause marked variation in the habitat — leading in turn 

 to differences in the vegetation and, incidentally, often adding to 

 the vast assemblage of habitats on the face of the earth or in salt or 

 fresh waters. 



Perhaps the usual identity of vegetation with habitat, and the ease 

 with which the former is classified at least into such broad categories 

 as forest, scrub, grassland, etc., is in itself a reason why no satis- 

 factory over-all classification or even inventory of habitats is available. 

 For our present purposes, however, we must attempt some general 

 enumeration of the main types of habitat. But owing to the diversity 

 and variability of the factors involved, their numerous possible 

 combinations, and the frequent overlapping, a clear and unequivocal 

 delimitation of habitats according to operative factors is scarcely 

 attainable. Except for broad outlines, such as those given next 

 below^ we have to go to the plant communities and their component 

 species to obtain satisfactory criteria, and then find ourselves dealing 

 with vegetation rather than with the habitats which support it. 



Terrestrial Habitats 



Even as these are separated from the other main category, aquatic 

 habitats, by water conditions, so are their main subdivisions often 

 dependent upon the relative availability of water locally. Leaving 

 aside as largely aquatic the sea- and lake-marginal ones whose in- 

 habitants all spend at least a substantial part of their time more than 

 half submerged, we nevertheless have the marginal habitats of the 

 sea-shore that are characterized by salt spray or occasional inundation, 

 and of lakesides and streams where the bed is so shallow and sheltered 

 that ' reed-swamp ' types can grow up in such a manner that most 

 of their bodies are aerial. In addition there are the various tropical 

 swamps and mangroves. Also abundantly supplied with water (but 

 in these cases not normally covered by it) are silty marshes (whether 

 salt or otherwise), base-rich fens, acidic bogs, and the gently sloping 

 and usually mossy ' mires ' through which water percolates. 



Of less aqueous land habitats there are many, often of very various 

 types. Beginning with the ones which are reasonably moist and flat 

 or nearly so, we may mention those of a wide range of soils in regions 

 favourable for cultivation, and which still comprise habitats even 



