BOG XEROPHYTES AND ACID SOILS. 139 



high lime requirements and relatively weak feeding powers, from getting the" 

 lime from the soil at a sufficiently rapid rate to meet their needs. This is 

 further substantiated by the parallel relation found between the amount of 

 growth of alfalfa on acid soils and the amount of calcium which could be 

 extracted with carbonated water from these soils. These considerations are 

 especially important in formulating a practical system of using hme, especially 

 as regards the amount to be used which, as is discussed in detail, is dependent 

 on the Ume-requirement of the crop, the degree of acidity of the soil, and the 

 fertihty of the soil." 



Truog and Meacham (1919 : 469) have found the acidity of the 

 cell-sap of various plants to vary from pH 6 to pH 4 and to be regu- 

 larly greater in the case of plants grown on unlimed acid soil than in 

 those on limed acid soil. The juice from plants cut in the morning 

 was more acid than that of plants cut later in the day, indicating an 

 accumulation of acids at night, and it was also more acid during warm, 

 dry weather than after a heavy rain. For each species there is a 

 certain acidity most favorable for the life processes, and in many 

 cases soil acidity affects the acidity of the sap by limiting the supply 

 of lime available. Lime and other bases are needed to neutralize the 

 acids formed during metabolism, some of which are mere by-products. 

 If the supply of bases is inadequate, the acidity of the sap rises to 

 a point where the accumulation of acids limits the processes that 

 produce them. While such a condition of self-regulation probably 

 prevents death from over-acidity, slow growth and a weakened con- 

 dition result in the case of plants of high lime requirement when 

 grown on acid soils. 



Truog (1918 : 177) points out that the acidity of the plant-sap is 

 of the same order as that of the water extracts of acid soils, and 

 hence it is not to be expected that the acidity of such soils often be- 

 comes high enough to be directly of serious injury to plant roots. 



Haas (1916 : 233) has demonstrated that the cell-sap of normal 

 cells is often decidedly acid (pH 3), contrary to the accepted view 

 that the cell-sap must be neutral or nearly so for normal physiologi- 

 cal functioning. Moreover, the blue coloring of living cells does not 

 indicate an alkaline reaction, but one decidedly acid to neutral or 

 barely alkaline (pH 3 to pH 8). The reaction of the cell may change 

 from pH 3 to pH 7 as it dies. 



In reviewing Hart well and Pember's work, Crocker (1919) states 

 that the hydrogen-ion concentration found in acid soils by the gas 

 method is generally but a fraction of that necessary to reduce the 

 growth of plants in water or sand cultures. 



The extent to which competition is a decisive factor in the appar- 

 ent preference of many native species for acid soils is indicated by the 

 behavior of sorrel, Rumex acetosella, which has perhaps received the 

 most attention experimentally. Tacke (1910) stated that sorrel 

 did as well on plats heavily limed as upon acid soils, while Wheeler 

 (1905) reported that lime seemed to be unfavorable to the growth of 



