118 ENVIRONMENTAL CONDITIONS. 



others, and the supply remaining to a plant after a period of dry 

 weather depends very largely upon this factor. Fine-grained soils 

 resist both evaporation and subdrainage, but they also resist water- 

 absorption by plants, while coarser ones give up their water more 

 readily^ so that in a region frequently visited by drought the upland 

 vegetation is, in general, most highly developed on the heavier soils. 



As to the internal conditions that influence the rate of water-supply 

 to the transpiring parts of the plant, there appears to be, for any 

 plant at any time, a maximum rate at which water can enter the roots 

 and pass through the vessels. This rate seems to depend upon the 

 extent of the root-system and upon the condition of the absorbing 

 portions of the roots as well as upon that of the conducting tissues in 

 general. It is probably never for very long periods that this maximum 

 rate is attained in moist weather; most plants at such times do not 

 appear to transpire at a rate that exceeds their maximum rate of ab- 

 sorption and conduction. Evidence has been obtained in the arid 

 regions, however, and this probably holds also for dry periods in 

 humid areas, that this maximum rate may frequently be reached when 

 the rate of transpiration is greatly increased through high evaporating 

 power of the air.^ 



In such cases growth ceases even with an ample supply of soil- 

 moisture and the plant remains quiescent, without other sign of in- 

 jury, till a lower evaporation-rate allows absorption and transmission 

 again to surpass the rate of water-loss. Thus, in the spring dry season 

 at Tucson, Arizona, morning-glory plants attained a few leaves and 

 then rested without growth until the higher humidity of the sununer 

 season arrived, although the soil in which they were rooted was 

 kept continuously at or somewhat above its optimum water-content 

 by irrigation. When the evaporation-rate had fallen markedly, with 

 the coming of the cloudy and more humid summer rainy season, these 

 plants resumed their growth in the usual manner.^ 



The ability of a plant to absorb and conduct water is, of course, 

 an internal condition, which depends upon many things. Naturally, 

 the more extensive is the absorbing surface of the roots the more 



1 Of several soils with approximately the same chemical composition, but differing in the size 

 of their particles, that with the finest particles gives ordinarily the most luxurious vegetation. 

 The physical and physiological reasons for these phenomena have apparently not been taken up 

 in detail. In this connection see Livingston, B. E., and G. H. Jensen, An experiment on the 

 relation of soil physics to plant growth, Bot. Gaz., 38: 67-71, 1904. On the capillary move- 

 ment of water in natural soils, see: Pulling H. E., The rate of water movement in aerated 

 soils. Soil Science, 4: 239-268, 1917. 



2 Li\'ingston and Brown (1912): Brown, W. H., The relation of evaporation to the water- 

 content of the eoil at the time of wilting. Plant World, 15: 121-134, 1912. — Briggs, L. J., and H. L. 

 Shantz, The wilting coefficient for different plants and its indirect determination, U. S. Dept. 

 Agric, Bur. Plant Ind. Bull. 230, 1912. Other citations of this work are given in the two fol- 

 lowing papers: Caldwell, J. S., The relation of environmental conditions to the phenomenon 

 of permanent wilting in plants, Physiol. Res., 1: 1-56. 1913. — Shive, J. W., and B. E. Livingston, 

 The relation of atmospheric evaporating power to soil-moisture content at permanent wilting in 

 plants, Plant World, 17: 81-121, 1914. 



' Livingston, 1907. a 



