EVAPORATION AS A CLIMATIC FACTOR. 49 



the same concentration as well-water. The responses of the 

 plants were immediate and no time elapsed for the leaching of 

 soluble salts from the soil by the first rains ; in fact, a change in 

 the behavior of the plants was observed before any precipitation 

 occurred, for the evaporating power of the air began to decrease 

 several days before rain fell. 



With the coming of the summer season the plants quite 

 generally responded in a very marked degree. The castor bean 

 and muskmelon increased their growth, which had been rapid 

 before. Morning-glory, sunflower, marigold, mustard and jim- 

 son-weed, all came rapidly into flower and fruit in a perfectly 

 normal manner, and this within a surprisingly short time. The 

 small rosettes of teasel which had been developed showed almost 

 no response. They are still alive at the time of writing, but 

 are only about three-quarters of an inch across. Nasturtiums 

 were also scarcely affected by the change in conditions ; they 

 finally succumbed during the rainy season. A single plant, 

 which was situated so as to be in the shade of a creosote bush 

 for a few hours of the day, lived several days longer than the 

 others, suggesting that light intensity may possibly have had a 

 part to play in the failure of this form to succeed. But it seems 

 altogether more probable that the plants of this form had been 

 so injured by the untoward conditions of the dry season that 

 they were unable to recover. The latter idea is supported by an 

 observation made several times previous to the present experi- 

 ment, that nasturtiums failed to succeed at the Laboratory, when 

 grown in pots which were kept well watered. I have never 

 started the plants from the seed in the rainy season. 



The meaning of this entire experiment may be expressed in 

 this way : that castor bean and muskmelon were able to absorb 

 and transmit water from the soil to their foliage considerably 

 faster than it was lost by transpiration, and hence were able to 

 carry on an active growth even during the intense drought. The 

 garden nasturtium and teasel failed to provide the excess of 

 water needed for good growth even in the rainy season (al- 

 though the latter form did not fail as completely as did the 

 former). The other forms of the experiment were unable to 

 supply the needed excess of water in the dry season, but pro- 



