DISTRIBUTION OF PLANTS 307 



328. Economic Plants. The economic plants, or 

 plants useful to man, are very numerous and very im- 

 portant. Some have already been named. Out of trees 

 man gets lumber, and by grinding up certain woods he 

 gets the pulp for paper. Certain grasses, developed long 

 before the dawn of history, became wheat, oats, rye, 

 barley, etc. Indian corn was native to America, and was 

 developed by its primitive inhabitants. The potato is a 

 native of South America, but was developed, so that it 

 became edible, in Europe. The flax plant has given man 

 linen since the distant past. Cotton was known in ancient 

 Egypt, but its use on a large scale was impossible until the 

 invention of the cotton gin, which made cotton-raising 

 profitable in America. 



Two of the great problems of agriculture in America 

 to-day are: (1) how the soil may be improved, so that it 

 shall give the greatest possible yields, and yet retain its 

 fertility; and (2) how our grain plants may be improved, 

 so that they shall bear most abundantly, shall be free from 

 pests and diseases, and shall be adapted to our varied 

 climate. To these problems we may add the problem of 

 the forests how they may be used without being 

 destroyed, and the problem of the fruit supply, a supply 

 that is making constantly increasing demands upon 

 gardeners, both here and in the tropics. 



329. Distribution of Plants. Plants have distributed 

 themselves all over the earth; but there are certain great 

 factors, or conditions, that fix the regions where any 

 particular kind of plant can grow. The different kinds of 

 soil, and the amount of rainfall are two of these conditions 



