10 



BULLETIN 100, U. S. DEPARTMENT OF AGRICULTURE. 



ment led to an examination of the character and extent of the root 

 systems of the species in question. The examinations showed that the 

 extent of root system varied widely, as is shown in Table 2, and fig- 

 ure 3. 



TABLE 2. Relation of aerial and subterranean parts of peas, wheat, and l)rome 

 grass, and comparative water requirements of aerial growth and of aerial 

 and root development combined. 



Table 2 show r s that the dry weight of the roots of peas was approxi- 

 mately 3 per cent of the dry weight of the tops, and the roots of wheat 

 about 3.3 per cent ; in the case of the brome grass the roots weighed 

 21.4 per cent as much as the tops. Hence the ratio of roots to tops 

 in the case of brome grass was about 1 : 5, while in wheat and peas 

 the roots showed a ratio of root to top by weight of about 1 : 30. In 

 other words, brome grass had about six times as much root in com- 

 parison to the top as the other two species. 



From these figures it would seem that the determination of the 

 water requirement of the plant, on the basis of the dry weight of the 

 aerial growth, is not necessarily an index to the ability of the plant 

 to grow successfully in dry situations. To determine the moisture- 

 absorbing power of a species account must also be taken of the depth 

 and spread of the root system, as the volume of soil through which 

 the roots penetrate is of the utmost importance in determining not 

 only the amount of water available to the plant but the amount re- 

 quired by the tops. A plant may- have a high water requirement 

 when it is calculated on the basis of the weight of the tops, but at the 

 same time it may be possessed of a root system great enough to supply 

 the water necessary to the tops through its increased power to absorb. 

 When the total water transpired by the plant is charged to the dry 

 weight of the plant as a whole that is, both aerial and subterranean 

 parts the water requirement data per unit of dry matter are quite 

 different from those calculated on the aerial basis, as is shown in 

 figure 3. 



Since two factors, (1) water requirement, or expenditure, and (2) 

 water gathering, or accumulation, are involved in the development of 

 vegetation, further investigations may prove that the determination 



