72 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



build up simple raw materials to complex carbohydrates. But 

 besides carbon dioxide and water they also require, like the lower 

 plants, certain mineral elements which are usually obtained from the 

 soil or water in which they grow ; alternatively, some other materials 

 may be objectionable or even poisonous to them. The likes and 

 dislikes, as well as requirements and inabilities, of different plants 

 in the matter of water or soil constituents, often complicate their 

 distribution patterns, and constitute, as we shall see later, one of 

 the many sets of factors determining their actual or potential areas 

 on earth. Not a few of the Angiosperms which are primarily 

 photosynthetic appear to be aided in their nutrition by mycorrhizal 

 associations with Fungi in their roots — examples being the Heaths, 

 Orchids, and many forest trees. It has even been suggested that 

 the majority of vascular plants may be so aided. In addition there 

 are entire groups of Angiosperms that are either wholly or partially 

 parasitic on other plants, or saprophytic on a variety of decaying 

 substrata — again largely with the aid of mycorrhizas. 



As for their habitats, Angiosperms tend to be plentiful almost 

 everywhere anything can grow on land or in shallow fresh water, 

 though they may be scarce in, or even absent from, some of the most 

 inhospitable situations such as rock faces, deserts, or mountain 

 summits where nevertheless some lower plants may exist. Except 

 in warm regions where free-floating types flourish, they do not 

 normally occur in deep open water ; nor do they grow directly on 

 snow or ice, as some lower organisms can. Relatively few live in 

 the sea, and these appear to be limited to rather shallow water and 

 in no instance to extend northwards beyond the low-arctic zone. 

 But, in general, Angiosperms are practically ubiquitous in anything 

 approaching orthodox situations for plant growth, and a recital of 

 their habitats would be practically that of plants in general as outlined 

 in Chapter XI. Within the limits stated they are also virtually 

 cosmopolitan, extending from the tropics to the farthest north land, 

 which many attain, and also to the Antarctic Continent, which does 

 not appear to be reached by any other vascular plants nowadays. 

 Moreover, it seems likely that in the matter of number of species 

 they may be the largest of all plant groups, the order of 250,000 

 being currently suggested, though of course a great deal depends 

 on what precisely is understood by a species. 



Finally, in the matter of vegetational and economic significance, 

 Angiosperms are of paramount importance in the world today, 

 aflrording the main dominants of most plant communities on land 



