CHIEF ENVIRONMENTAL CONDITIONS. 117 



which the transpiring portions of a non-storage plant may receive water 

 depends upon several conditions : the rate at which water may move 

 in through to the absorbing surfaces, the nature and condition of the 

 roots, and the nature and condition of the water-conducting tissues. 



In most cases where growth ceases or wilting occurs, the inadequacy 

 in the water-supply seems to arise, not from the attainment of the 

 physiological maximum of absorption and transmission, but from a 

 greater or less drying-out of the soil, whereby it fails to transfer water 

 to the absorbing roots at a rate adequate to supply the demand of 

 absorption. Quantitative information bearing upon the relations 

 between soil-moisture and plant absorption and transpiration is not 

 yet available, and our consideration of this exceedingly important 

 subject must be very brief and very tentative.^ 



It is certain that, with diminished supply of soil-moisture and with 

 other conditions remaining unchanged, transpiration in any plant 

 must be decreased in amount, also that this diminution in the trans- 

 piration-rate does not progress parallel to the continuous drying of 

 the soil, so that it ultimately comes about that the supply fails to equal 

 the demand, transpiration becomes greater than absorption, the non- 

 storage plant ceases to grow, and wilting or even partial or total death 

 ensues. 



Such a decrease in the rate of movement of water to the roots does 

 not necessarily mean any considerable fall in the average percentage 

 of moisture present in the soil, but gives evidence merely of the fact 

 that the movement of water through the soil-films and into the roots 

 has become less rapid. Under such conditions the soil immediately 

 surrounding the absorbing portions of a root-system becomes drier 

 than that at a greater distance, and the movement of water into the 

 drier layer is too slow to keep the surface of the root adequately 

 moist. This matter of the possible rate of water transfer from soil 

 to root, fundamental as it is, is greatly in need of thorough investiga- 

 tion. The subject has been opened by Livingston and Hawkins and 

 by Livingston and Pulling in the papers cited above. This should 

 prove a wonderfully productive field, both for scientific ecology and 

 for agriculture when serious attention is at length turned to it. 



Great differences in the water-relations of plants in different habitats 

 are secondarily occasioned by the nature and exposure of the soil in 

 which they are rooted. Surface drainage often conducts the water 

 of precipitation away before it can penetrate the soil to an adequate 

 degree, and underground drainage frequently depletes the moisture- 

 supply of porous soils almost as fast as water enters from a shower. 

 Evaporation removes water rapidly from some soils and but slowly from 



^ In this connection see Livingston, B. E., and Lon A. Hawkins, The water-relation between 

 plant and soil, Carnegie Inst. Wash. Pub. No. 204: 3-4S, 1915. — Pulling, H. E.. and B. E. Liv- 

 ingston, The water-supplying power of the soil as indicated by atmometers. Ibid. 204: 49-84, 1915. 

 Also see: Livingston, B. E., and Riichiro Koketsu, The water-supplying power of the soil as re- 

 lated to the wilting of plants. Soil Science, 9: 469-485, 1920. 



