[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 237 



Similar results were obtained in experiments with wheat: 



Acre-inches of Yield in Bushels 



Water Added per Acre 



4 6 4.5 



8.9 11.8 



17.5 15.8 



30.0 26.6 



In the eastern United States where irrigation is usually considered 

 unnecessary, market gardeners ha\'e found it quite profitable to add 

 water in addition to what enters the soil from rains. During prolonged 

 droughts this practice has often returned profitable crops when adjoin- 

 ing unirrigated gardens were a total loss. 



Geographic allocation of crops. Crops are planted and cultivated for 

 economic reasons. The amount and quality of the \ield are accordingly 

 of the greatest importance. During pioneer days a far greater variety 

 of crops was planted on individual farms and in local regions. But as 

 transportation facilities increased, the commercial production of the 

 more important crops has gradually become allocated to those regions in 

 which the different varieties grow best. 



Among the conditions which influence the yield of a particular crop 

 in various regions is the available water supply in comparison with the 

 rate of water loss. The center of broomcorn production, for example, 

 has been moved westward from the eastern seaboard to Illinois and 

 finally to Oklahoma. There broomcorn grows in company with other 

 species of sorghum. These plants ha^•e extensive root systems and smaller 

 leaf areas than corn and >'ield a larger profit than other grain crops in 

 dry regions. Corn is a more profitable crop in the corn belt states from 

 Ohio to Nebraska, where the soil is fertile, and, in addition, the tempera- 

 ture is high enough for maximum photosynthesis in the davtime and 

 not too low at night for continued growth, and the transpiration rate 

 is not so excessive. Macaroni or hard wheats attain their best quality in 

 the northern plains states, where drought is frequent but rarely exces- 

 sive for this kind of wheat. Tobacco that is culti\ ated for cigar wrappers 

 should have large thin leaves. Such leaves develop best where transpira- 

 tion is low, and the plants are set out either in moist or shaded valleys 

 or in the reduced light under cloth shades. 



Irrigation is an ancient practice, but rapid means of transportation, 

 methods of preserving and shipping perishable crops, and scientific 

 methods of obtaining varieties of plants that mav be cultiA^ated in drier 



