vStudies in Soil Physics. 115 



It must be remembered, of course, that this concentration 

 factor has nothing to do with the total amount of water trans- 

 pired or absorbed, or with the draft of transpiration and the plant 

 upon the water of the soil. So long as water be freely supplied 

 to the plant roots and freely absorbed by them the sap con- 

 centration will be the same whether the yearly transpiration 

 rate be one ton or ten tons. It is only when the soil water 

 begins to run low or the rate of transpiration grows high enough 

 to tax the absorbing and transmitting mechanism, that the in- 

 crease of sap concentration begins to operate and restrict the 

 outgo. 



In summary, we may say that the rate of transpiration is 

 controlled by the mechanism of exposure, the concentration of 

 the cell solution and the evaporating power of the air, and that 

 the rate of absorption is controlled by the mechanism of contact, 

 the concentration of the cell solution and the supply of capillary 

 water by the soil. The upper limits of transpiration and ab- 

 sorption are fixed by the mechanisms concerned, — -by the 

 mechanical construction of the plant; the equality of transpira- 

 tion and absorption is maintained, under normal conditions, 

 by the concentration of the cell sap; and the actual amounts 

 of transpiration and absorption are determined primarily by 

 the evaporating power of the air and secondarily by the rate of 

 capillary supply from the soil. These latter factors also deter- 

 mine whether or not the plant can be kept within that range of 

 conditions, which is called normal, and between the limits of which 

 alone is change of sap concentration able to maintain the proper 

 balance of outgo and income. The danger of the fatal disturb- 

 ance of this balance is almost entirely in one direction only. 

 Plants frequently surfer from a deficiency of water but very 

 seldom from an excess. Too much transpiration or too little 

 absorption are the ever present dangers; there need be no fear 

 of too little transpiration or too much absorption. To sum- 

 marize again, a plant may die of drouth either (1) because the 

 soil water has given out; (z) because the transpiration rate is 

 greater than the maximum rate of absor, tion which the absorb- 

 ing mechanism will permit ;*or (3) because the evaporating power 

 of the air is persistently high enough to require a transpiration 



*For an instance of this see Livingston. Plant World 10: 269, 1907. 



