EVAPORATION AND PLANT DEVELOPMENT. 275 



(minimum). During the summer rainy season, which lasted 

 from July 6 to about September i, the temperature and hu- 

 midity averages were about lOO degrees and 80 degrees, and 

 95 and 59 per cent. The average evaporation rate for the 

 second period of the experiment was only 57 per cent of that 

 for the first period. 



With the exception of castor bean and muskmelon, 

 which developed quite normally from the first, all of the 

 plants practically ceased to grow after the development of a 

 few leaves, and remained quiescent till the coming of the sea- 

 son of lower evaporation. They did not wilt, and appeared 

 healthy except for lack of growth. 



With the advent of the rainy season all the plants re- 

 sponded in a marked manner, with the exception of Tropaeo- 

 linii and Dipnicus, which finall succumbed in spite of the 

 change in seasons. Castor bean and muskmelon increased 

 their rate of growth, which was rapid before, and the other 

 'forms suddenly began to develop rapidly and soon came into 

 flower and fruit. 



The meaning of this experiment may be stated briefly 

 in this way, that the castor bean and melon were able to 

 absorb and transmit water to their foliage faster than it was 

 lost by transpiration, and hence were able to carry on a vig- 

 orous growth even during the intense drought. Tropaeolmn 

 and Dipsaciis faileci to provide the excess of water needed for 

 growth even in the rainy season. The other forms of the ex- 

 periment were unable to provide the needed excess in the 

 season of high evaporation rate, but provided it in a normal 

 way when that rate decreased. Only in castor bean and 

 muskmelon was the power to absorb and transmit water ade- 

 quate for growth during the dry season. 



That the responses just described were due to the 

 change in the evaporation power of the air is hardly to be 

 doubted. There was not a sufficiently great alteration in 

 temperature conditions to explain them, and, as has been 

 stated, the conditions of soil moisture were kept practically 



