Chapter IX 

 VITAL IMPORTANCE TO MANKIND 



Early in this work we indicated that the green plant is the only 

 satisfactory mechanism for transforming the energy of the sun into 

 organic compounds on which, as an animal, Man is dependent for 

 food and other requisites of life. In the last chapter we mentioned 

 some instances of such dependence, chiefly in connection with the 

 distribution of leading crops. It is the object of the present chapter 

 to give a systematic account, with chosen examples, of the multi- 

 farious and often vital ways in which plants and plant products are 

 important to mankind. For Man is unable to synthesize, at all 

 events economically and in useful bulk, most of the materials which 

 he needs in such great quantities — often for his very existence. 

 Even though he can, for example, convert starch into alcohol and 

 the latter in turn into all manner of useful products, he needs the 

 Wheat or some other plant to make the starch for him. In such 

 food materials is locked radiant energy from sunlight, which can 

 then be liberated by the process of respiration that goes on in all 

 living bodies and is rapid in warm-blooded animals. This process 

 usually requires oxygen in large quantities and consequently is again 

 dependent upon green plants as, during photosynthesis, they liberate 

 this vital gas and return it to the air, so purifying and maintaining 

 the atmosphere. 



Not only, as we shall see, do plants afford for mankind, either 

 directly or indirectly, his food and many other requisites of life, 

 but they largely condition his environment. Thus, for example, 

 forests are very different to live in from grassy plains, and deserts 

 and areas of arctic tundra are again widely different. Many present- 

 day grasslands and treeless cultivated areas are, however, due to 

 Man's clearance of forests, and although he shows a natural tendency 

 to avoid desert and tundra areas, the correlation of forest or grass- 

 land {see Fig. 65) with dense human population (Fig. 78) is not 

 always close. Rather has Man wandered and settled where he most 

 conveniently could, having in mind his need for subsistence, which 

 meant in large measure the finding or growing of plants — or of 



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