VITAL IMPORTANCE TO MANKIND 257 



animals which are dependent upon them. Thus the migrations of 

 Man have been dependent in considerable degree on plant distribu- 

 tion, even as his present population-density is conditioned by the 

 crop-growing potentialities of different areas. 



From the point of view of what they are used (or, occasionally, 

 to be avoided) for, plants and plant products may be classified into 

 seventeen main (or often multiple) categories whose consideration 

 will occupy the remainder of this chapter. As each is apt to be a 

 large subject, the accounts will be brief or in mere outline ; it should 

 also be noted that the categories are neither hard and fast nor 

 mutually exclusive. Indeed a good deal of repetition is inevitable. 

 Many of the more important plants are dealt with elsewhere in this 

 work, though usually in other connections, and some are mentioned 

 in more than one category — though without cross-referencing, as this 

 can be done through the index. Nor, in this primarily ' economic ' 

 chapter, will technical botanical usages and names be maintained 

 in the manner in which they usually are elsewhere in this book, 

 and practically have to be for the use of scientists. 



Foods 



Though whole volumes can be — and have been — written about 

 food plants and Man's dependence upon them, this theme is too 

 obvious to require detailed treatment here. It has latterly been 

 extended to include vitamins, of which plants are the main primary 

 source. Suffice it to say that practically every item of food of all 

 animals comes from plants. For even if one animal eats another, 

 and it in turn consumes yet another, when we follow back the food- 

 chain we come, sooner or later, to a point of dependence upon green 

 plants, as these alone are able economically to build up complex 

 food substances from simple inorganic materials. This is true not 

 only on land but also in fresh and salt waters, where the ' producer ' 

 plants are often microscopic, larger and larger ' consumer ' animals 

 succeeding one another to constitute the later stages in the food- 

 chain. Any exceptions are insufficient in scale to have serious effect. 



The most important plants used directly for food by Man (as 

 opposed to those used indirectly through pasturage of his domestic 

 animals) are those affording abundant carbohydrates and other 

 energy-producing materials. Outstanding are the cereal and other 

 ' grains ' (such as Wheat, Rice, Maize, Barley, Rye, Oats, Sorghums, 

 Millets, Quinoa, and Buckwheat) and ' roots ' (such as Potatoes, 



