116 The Plant World. 



rate greater than the maximum rate of capillary movement of 

 soil water to the roots. 



All this concerns extreme conditions, — conditions which 

 have become "abnormal ' and under which life itself is not long 

 possible. From the practical viewpoint the more normal, less 

 extreme, conditions are of far greater importance. We are less 

 interested in why plants die than in how (or how efficiently) 

 they live. Under these normal conditions, which we may 

 define as the conditions adapted to the particular plant under 

 consideration, the total absorption will be equal to, and deter- 

 mined by, the amount of transpiration, the latter quantity being 

 fixed by the variable factor of the evaporating powxr of the air 

 and the relatively constant factor of the mechanism of transpira- 

 tion. Changes in the evaporating power of the air will be fol- 

 lowed by corresponding changes both in transpiration and in 

 absorption and the necessary balance of the two will be main- 

 tained without change in the internal conditions of the plant. 

 Suppose, however, that the exaporating power of the air in- 

 creases to a point at which the transpiration it necessitates 

 can just be balanced and no more by the supply of soil water. 

 Suppose, then, the evaporating j ower of the air to increase a 

 little more. Transpiration will increase but absorption can not 

 because water can not be supplied any faster. Obviously the 

 plant will lose water. But this loss of water causes an increase 

 of sap concentration, this causes a reduction of transpiration, 

 and equilibrium is restored, the only difference being that the 

 cell solution is a little more concentrated than it was. Un- 

 fortunately, however, this increase of sap concentration is (for 

 some reason not fully known) unfavorable to the plant. Life 

 may go on but rapid growth does not and this is obviously a 

 matter of extreme importance to practical agriculture. Maxi- 

 mum yields can be obtained only when the possible rate of sup- 

 ply of soil moisture is sufficient to balance any transpiration 

 which the climatic situation is likely to require. 



The Wilfing Point. The ideas elaborated in the preceding 

 section can be a])plied with profit to a few of the conceptions 

 current in the borderland of plant physiology and soil physics. 

 Let us take first the so-called "wilting point," usually defined 

 as the water content at which a given plant will wilt permanently 



