PLANT INDUSTRIES 309 



about the growth of plants. This requires a knowledge of the 

 facts that are known about the structure of plants, the con- 

 ditions of soil, temperature, moisture, etc. under which they 

 thrive best, the nature of plant and animal diseases and the 

 means of preventing them, and the best methods of utilizing 

 plant products after the plant has made them. Second, the life 

 of a plant, like that of an animal, is a very intricate matter, and 

 there are many highly important questions upon which science 

 as yet has little positive knowledge. Each plant industry, there- 

 fore, involves many unsolved problems, about which we need 

 to secure additional knowledge. Thus, science has discovered 

 what kind of cultivation will best enable the corn plant to 

 thrive ; but, although botanists know the life cycle of the 

 smut which attacks corn, a practical and thoroughly effective 

 way of preventing the attack has not yet been found. 



294. The sugar industry. Nearly all plants that do the work 

 of photosynthesis produce sugar of some kind. In some plants 

 the amount produced may be extremely small, and it may re- 

 main as sugar only for a brief period before it is made into 

 some other compound and assimilated into living substance or 

 stored within the plant. In other plants the amount may be 

 large, and may be retained as sugar for long periods. It may 

 be stored as sugar or, more often, as starch, which is usually 

 changed to sugar again for transference through the plant. 

 Plants of the latter class may prove useful in the sugar in- 

 dustries. It is possible that wild plants may yet be found 

 that are valuable for this sugar content, but at present a few 

 species produce the sugar of the world. The sugar maple 

 (Acer saccharum) (fig. 233) has long been used as a source 

 of sugar. Other species of the same genus produce sweet 

 sap, but not so abundantly as the sugar maple. The tree 

 belongs to the type of primeval forest that prevailed in the 

 north-central and northeastern parts of North America. In 

 early spring the previous season's surplus of sugar is trans- 

 ported to the parts of the tree where, a little later, there is 

 active growth of new leaves. This stored sugar is used in 



