128 



WILLIAM H. BROWN 



In two cases plants which had wilted in the open, under a 

 high rate of evaporation, were transferred to the house where 

 the rate was much lower. Here they revived quickly without 

 any addition of water, and wilted again in the new situation; 

 but, like Martynia, with a considerably lower soil moisture con- 

 tent than was present in the first case. These results are given 

 in table 5. 



TABLE 5 



Percentage of water left in the soil by the same plants of Physalis wilting under 



different rates of evaporation 



Tropacolum majus Linn. This plant makes a good growth 

 in the region of Tucson if started early in the spring, before the 

 rate of evaporation becomes excessive. For these experiments 

 seeds were planted in the early summer, which is the dryest 

 season of the year, and the seedlings appeared to grow normally 

 until they had developed eight or ten leaves. After this, growth 

 was very slow and the plants did not appear healthy. The appar- 

 ently unhealthy condition of the plants was probably due to 

 their inability to obtain water from the soil fast enough to replace 

 that lost through transpiration and at the same time to maintain 

 the usual rate of growth Livingston 6 has shown experimen- 

 tally that with a high evaporating power of the air, a number 

 of plant forms undergo a cessation of growth after the forma- 

 tion of only a few leaves, even though the soil in which they are 

 rooted be maintained at or near what would be the optimum 

 moisture content under less strenuous conditions of evaporation. 

 With a lowering of the evaporation rate, and without any alter- 

 ation in the soil condition, many of these same plants resumed 

 their growth. 



8 Livingston, B. E., Operation of the porous cup atmometer. 

 13: 111-18. 1910. 



Plant World, 



