EVAPORATION AS A CLIMATIC FACTOR. 51 



what is known of the low rates of water absorption by stems 

 and leaves it seems very improbable that the latter supposition is 

 true. If the other is correct, it would appear that the Fou- 

 quicria plant in question was absorbing water and transmitting 

 it up the stemi at a rate which was inadequate for the develop- 

 ment of foliage, as long as the evaporation (and transpiration) 

 remained excessive, but that this rate became adequate for leaf 

 formation with the local decrease in evaporation rate. 



Another apparent instance of a response to evaporation 

 conditions is afforded by Sphacralcea pedata, a small red-flow- 

 ered mallow attaining a height of about two feet, and bearing 

 flowers and leaves the year round. In the winter and early 

 spring (the season under the influence of the winter rains) the 

 leaves have an area of from twenty to thirty times as great 

 as in the spring dry season. While the leaves of the earlier sea- 

 son are bright green and nearly smooth, those formed in the 

 season of drought are densely covered with white tomentose 

 hairs. Growth is less i»apid in the dry season and the longer 

 branches often die back to the ground at this time. The leaves 

 of the more mesophilous type mostly succumb before the ar- 

 rival of the summer rains, leaving a plant of an appearance 

 entirely different from that of the early spring. In the summer 

 rainy season this plant resumes its more rapid growth and the 

 leaves then produced are of the mesophilous type in the shade 

 and partake of this character to a large extent even in the sun- 

 shine. It was observed that in the dry season potted plants of 

 Spliaeralcca growing in a soil kept at the optimum water con- 

 tent refused to develop the mesophilous type of leaf, and that 

 the same specimens produced that type during the season of low 

 evaporation rate. The main difference noted between the potted 

 plants and those in the open ground lay in the fact that the 

 former retained the mesophilous leaves produced in the early 

 spring somewhat longer than did the latter. 



This evidence is of course very inconclusive, but it seems 

 indicated at least that even with the soil kept at its optimum 

 water content, Sphaeralcea responds to high evaporating power 

 of the air by the assumption of an entirely different form and 



