CHIEF ENVIRONMENTAL CONDITIONS. 121 



thus falls during bright days and rises again at night. In the arid 

 regions it appears that this night period of recovery is of very great 

 importance. Many plants that are normally very resistant to 

 drought conditions as they occur would probably succumb com- 

 pletely on the second day if the night period of recovery of the water 

 ratio were omitted. A shower of rain affects both terms of the water 

 ratio; it increases soil-moisture and decreases evaporation, while 

 incipient drying or partial plasmolysis may sometimes be almost 

 immediately corrected through actual absorption of moisture through 

 leaf surfaces wetted by rain. 



In the majority of ordinary plants the water of transpiration passes 

 with comparative directness from the absorbing surfaces of the root- 

 system to the transpiring surfaces of the foliage. Stored water is 

 here of little general importance. Outside of the arid regions such 

 plants appear to absorb and transmit moisture from a moist or wet 

 soil with sufficiently great rapidity to prevent any serious wilting, 

 even with the highest transpiration-rates; that is, the maximum 

 possible rate of absorption is seldom inadequate. But, with a soil 

 that is becoming dry, there comes a time when the actual rate of ab- 

 sorption fails to keep the water ratio above unity, and in such cases 

 wilting soon occurs. If a plant wilts for this cause it may be made to 

 revive by mere addition of water to the soil about its roots. How- 

 ever, if the maximum possible rate of conduction is at fault (which 

 depends, as has been seen, upon the structure and condition of the 

 roots, vessels, etc.), such treatment will fail to produce a complete 

 return to the usual condition. (Caldwell 1913.) 



Before the problem of the quantitative aspects of wilting and of 

 general plant behavior with regard to moisture may be seriously ap- 

 proached, the study of soil physics and of water absorption, conduc- 

 tion, and transpiration, must furnish us with means of determining 

 with fair accuracy the terms of the water ratio. The study of soil 

 transmission and plant absorption have been strangely neglected by 

 students of plant physiology. That of transpiration and the condi- 

 tions controlling it has progressed somewhat further, but much remains 

 to be determined. No field of plant physiology promises greater 

 conquests than this one of the water-relations, either from the stand- 

 point of pure science or from that of a rational plant-culture. (Living- 

 ston and Hawkins 1915; Pulling and Livingston 1915, also Pulling 1917. 



When a plant wilts from lack of soil-moisture it is well known that 

 the soil about its roots is not dry, but always contains a considerable 

 amount of water. This residual water, left after the roots have ceased 

 to absorb, has been called "non-available." Under a given set of con- 

 ditions this moisture-content appears to be constant for any plant and 

 for any soil, but the conditions upon which the magnitude of the resid- 

 ual soil-moisture content depend are much more complex than has 

 usually been thought (Shive and Livingston 1914). 



