CULTIVATION OF WASTE LAND 385 



open fields prevail in the east and south. Yet the enormous value of a 

 wind-screen to vegetation can be readily observed, and the market- 

 gardeners both in England and the still dryer districts of the south of 

 France make great use of them. Lastly, we must have more knowledge 

 about the relation between transpiration-water and growth: we do not 

 know if the high ratios we have spoken of hold for all plants. Xero- 

 phytic plants are supposed to be possessed of protective devices to reduce 

 loss of water: Are they merely effective in preserving the plant from 

 destruction during the fierce insolation and drying it receives? and do 

 they enable a plant to make more growth on a given amount of water? 

 Wheat, for example, puts on its glaucous waxy bloom under dry condi- 

 tions : Is this really accompanied by a lower rate of transpiration per 

 unit surface of leaf? and is it more than defensive, connoting a better 

 utilization of the water the plant evaporates ? 



The cultivation of these soils with a minimum rainfall necessitates 

 varieties of plants making a large ratio of dry matter to water transpired 

 and also with a high ratio between the useful and non-useful parts of 

 the plant. Mr. Beaven has shown that the difference in the yields of 

 various barleys under similar conditions in England is due to differ- 

 ences in their migration factors : the same amount of dry matter is pro- 

 duced by all, but some will convert 50 per cent, and others only 45 per 

 cent, into grain. This migration ratio, as may be seen by the relation 

 between corn and straw on the plots at Eothamsted, is greatly affected 

 by season ; nevertheless Mr. Beaven's work indicates that under parallel 

 conditions it is a congenital characteristic of the variety and therefore 

 one that can be raised by the efforts of the plant-breeder. The needs of 

 dry-land-farming call for special attention on the part of the breeder to 

 these two ratios of transpiration and migration. 



Closely linked up with the problems of dry-land-farming are those 

 which arise in arid climates from the use of irrigation-water on land 

 which is either impregnated with alkaline salts to begin with or develops 

 such a condition after irrigation has been practised for some time. The 

 history of irrigation-farming is full of disappointments due to the rise 

 of salts from the subsoil and the subsequent sterility of the land, but 

 the conditions are fully understood and there is no longer any excuse 

 for the disasters which have overtaken the pioneers of irrigation in 

 almost every country. Sterility may arise from two causes — overmuch 

 water which brings the water-table so close to the surfac£ that the 

 plants' roots may be asphyxiated, or the accumulation by evaporation 

 of the soluble salts in the surface layer until plants refuse to grow. The 

 annual ratting off of the cotton crop in Egypt as the water-table rises 

 with the advance of the Nile flood affords a good example of asphyxia- 

 tion, but in the neighborhood of irrigation canals we also find many 

 examples of sterility due both to the high water-table and an accom- 



