12 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



of water requirement on the basis of the plant as a Avhole may afford 

 a more reliable index of successful groAvth in relation to drought than 

 taking into account only the aerial portion of the plant. Most of 

 the work on water requirement has been done in connection with 

 cultivated plants, the root systems of which are small as compared 

 with certain native species Avhich may be classed as conservative users 

 of water. The more dissimilar the root systems of species compared 

 the less reliable the water requirement data will be unless the roots 

 as well as the tops are taken into account. 



The appreciably greater amount of Avater used by the plants grown 

 in the fertile soil over those grown in the infertile soil is accounted 

 for by the fact that the plants greAV much more luxuriantly in the 

 richer soil ; hence the transpiration Avas much greater, and at the end 

 of the season much more dry matter had been produced on the fertile 

 than on the infertile soil. Exact data as to the vegetative develop- 

 ment and the total water requirements of the species grown in the two 

 soils are shown in Table 3. 



TABLE 3. Summary of vegetative growth and water requirements of peas, brome 



grass, and ivheat. 



The graphical representation (fig. 4) of Table 3 shows remarkable 

 contrast in the vegetative growth and total water requirement of the 

 plants developed in the two soils. The number of leaves produced 

 by field peas, for example, in the infertile soil as compared with that 

 of the fertile soil is as 1 to 2.7 ; the leaf length, 1 to 3.3 ; the total dry 

 weight produced, 1 to 8.3 ; and the water used per plant, 1 to 4.6. 

 Similar contrasts are shown in the case of the other two species. 

 The ratio in the water requirement per unit of dry matter, on the 

 other hand, is reversed in the case of each species, as has previously 

 been shown. 



The above data show clearly the importance of exercising the 

 greatest care in the selection and subsequent treatment of soils for 

 the study of comparative growth of standard plants as a means of 

 integrating climate. While soils obtained within a limited space 

 and at the same depth, and having uniform appearance in color, 

 texture, and other essentials, may be similar in many respects, they 



