SUMMARY. 75 



(5) The moisture determination should be made as soon as the 

 plant has first reached a wilted condition from which it can nob re- 

 cover when placed in a damp chamber. 



The method employed consists in growing the plants in a small 

 glass pot, evaporation from the soil surface being prevented by means 

 of a seal of wax which is melted and flowed over the soil surface. In 

 the case of monocotyledons this wax seal can be applied immediately 

 after planting the seeds and the seedlings will grow readily through 

 the wax, which forms a perfect seal around the stems. In the case of 

 dicotyledons the wax, which is usually a mixture of paraffin and petro- 

 latum having a low melting point and low heat conductivity, can be 

 melted and flowed around the stems of the seedlings without injury. 

 During growth the pots are -kept immersed in a water bath to avoid 

 condensation of the soil moisture on the pot walls. 



When these precautions are observed, the probable error of the 

 mean of the determinations from 12 pots does not usually exceed 1 

 part in 100, which is comparable with the accuracy with which the 

 moisture retentiveness of the soil used can be determined by purety 

 physical methods. 



The wax-seal method is also particularly adapted to the study of 

 transpiration, since all loss of water is avoided except that taking 

 place through the plant. 



Wilting-coefhcient determinations have been made in a series of 20 

 soils ranging from sands to clays. In this work, involving about 

 1,300 determinations, a large number of varieties of the different crop 

 plants have been tested, as well as many native plants from the Great 

 Plains. 



The results obtained show that species differ only slightly as 

 regards the soil-moisture content at which permanent wilting first 

 takes place. Taking 100 to represent the average wilting coefficient, 

 the different species tested (except Colocasia and Isoetes) give an 

 extreme range from 92 for Japan rice to 106 for a variety of corn. 

 Most of the species and varieties tested differ much less than this. 

 On the same scale the great crop plants gave the following values, 

 obtained by combining the different varieties: Corn 103, wheat 99, 

 oats 99, sorghum 98, millet 97, barley 97, rye (one variety only) 94> 

 rice 94, grasses 97, and legumes 101. 



The conclusion is thus reached that the differences exhibited by 

 crop plants in their ability to reduce the moisture content of the 

 soil before wilting occurs are so slight as to be without practical sig- 

 nificance in the selection of crops for semiarid regions. Furthermore, 

 it is believed that the slight differences which have been observed 

 are largely due not to the ability of one variety to exert a greater 



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