344 General Botany 



for the plants. He may irrigate the land with water from the 

 mountains, or he may modify the climate by growing the crops 

 under glass, or under shades and screens. Such devices lead to 

 wide extension of the areas of crop production beyond their nat- 

 ural areas. The hundreds of tracts of irrigated lands in the 

 Western states ; the growing of tobacco under shade in Connecti- 

 cut, Florida, and the West Indies ; the growing of vegetables 

 under palms in desert oases ; and the growing of tropical plants 

 and summer vegetables in greenhouses in winter are familiar 

 examples of the extension of crop production into regions nat- 

 urally unfavorable. 



Extending areas of crop plants through plant breeding. There 

 is still another reason why crop plants are less restricted than 

 wild plants. This is the production of new plant forms through 

 the activities of plant breeders. Ever since plants were first 

 cultivated, men have tried to find better or more suitable varie- 

 ties for cultivating in particular localities. As a result we now 

 have hundreds of varieties of crop plants, some of which grow 

 better in one climatic region and others in another. By produc- 

 ing or discovering new varieties, the areas of all the familiar crop 

 plants have been greatly extended. 



The factor of transportation. Some plant products — like 

 the potato, for example — are bulky and the cost of transpor- 

 tation correspondingly great. Although potatoes grow best 

 on sandy loam in the cool Northern states, they can be produced 

 at a profit elsewhere, in spite of lower yields, when sold locally 

 and transportation charges avoided, or when they can reach the 

 market earlier. A map of potato production shows that potatoes 

 are grown in quantity near all the large cities of the country. 



Other plant products — like mahogany, oils, resins, rubber, and 

 spices of tropical forests — are so valuable that they may be 

 transported long distances before they are made into commercial 

 products. Consequently, the industries dependent upon plants 

 of this type may be far removed from the source of supply. 



