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, ete. 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 corm 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 saecharum) (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 
