178 Drying and Wilting of Plants [376 



cannot recover without special treatment), and finally by 

 death and actual desiccation. 



Incipient drying of leaves, whether they show any signs 

 of wilting or not, may be said to be due to inadequate water 

 supply to these organs; no matter how great might be the 

 rate of foliar water loss, the transpiring cells should not suf- 

 fer any diminution in their water content if the rate of 

 entrance of water into these cells were only sufficiently great. 

 The question therefore arises, as was mentioned by Living- 

 ston and Hawkins, to what extent is this inadequacy in the 

 rate of foliar water supply to be considered as due to inade- 

 quate water supplying power, of the soil, and to what extent 

 may it be due to inadequate absorbing power of the roots and 

 inadequate conducting power of the stems, petioles, etc. ? In 

 wilting leaves, for example, is the insufficient rate of water 

 supply due to an external condition in the soil or to an inter- 

 nal condition, within the plant body? 



This is a very important question, both with regard to the 

 general problem of plant water relations and with respect to 

 the practical problem of drought resistance in plants. A 

 quantitative answer for plants growing in the open is of 

 course impossible at present, but some light has been thrown 

 upon the consideration of this question by some experiments 

 recently carried out in the Laboratory of Plant Physiology. 8 

 The matter in hand was approached by making the water- 

 supplying power of the root surroundings very great ; the test 

 plants were grown in water-culture instead of in soil, so that 

 the external resistance offered to water absorption by the root 

 surfaces may be considered as practically nil and therefore 

 constant. Under such conditions the actual rate of water 

 absorption must be very nearly proportional to the absorbing 

 power^of the root system. 



Two methods were employed, for both of which the tran- 

 spirational rates were determined by weighing, in the ordi- 



8 Mr. E. S. Johnston carried out the manipulations in these 

 experiments. 



