DEPARTMENT OF BOTANICAL RESEARCH. 73 



A partial report upon the results of this work has previously been made.^ 

 Since then further experimentation has been carried on with these plants 

 under controlled conditions. Simultaneous measurements have been made 

 of the daily march of transpiration, of water-content of plant parts, of stomatal 

 apertures, of leaf temperatures, of the evaporating power of the air, and of 

 soil water-content. The entire series of experiments has extended over 

 several years and the results have now been brought together and presented for 

 pubHcation. Following are the outstanding conclusions: 



During the season when abundant moisture is accompanied by relatively 

 low temperatures, the spring annual is living under conditions which are very 

 near the limit of its endurance to aridity. It could not live during the summer 

 rainy season, when favorable soil-moisture conditions are accompanied by 

 high temperatures and a greater evaporative power of the air, even if it 

 could be made to germinate under artificial conditions. The summer annual 

 is able to live through temporary drought periods of several weeks' duration 

 by lowering its response to the evaporative power of the air and by remaining 

 in a wilted condition from which it easily recovers. It seems probable that 

 it could flourish in the spring rainy season, except for the fact that it will not 

 germinate at the lower temperatures which exist at that time. 



Phaseolus grew well under conditions of high humidity and relatively low 

 temperatures, but died when exposed to air of low humidity and high tempera- 

 ture, even when the soil water-content was kept high. The perennial ap- 

 proaches its limit of endurance only in the arid fore-summer. It is able to 

 live through this unfavorable period by lowering its response to the evapora- 

 tive power of the air and by decreasing its total leaf-surface. It has a some- 

 what mesophytic type of leaf during the favorable late winter and spring 

 months and a much smaller, more xerophytic form during the drought season. 

 Furthermore, in the arid season the stems and leaves contain in abundance 

 a dark-colored, gummy semi-fluid, which is either entirely or almost absent 

 in the more humid season. The theory is advanced that this substance is 

 at least partially responsible for the greater resistance to water-loss in the arid 

 season. 



It appeared clearly that the best measure of the response of a plant to any 

 given aridity is indicated by the ratio Tc/Em, the denominator of which is 

 the maximum rate of evaporation from an atmometer for the day under con- 

 sideration and the numerator the corresponding rate of transpiration per 

 unit area. In all but a very few cases the maximum transpiration for the 

 day occurred earlier than the maximum evaporation. The more favorable 

 the humidity conditions the closer the two maxima approached in time. 

 Humidity indices were obtained by dividing the evaporation-rate for 24 hours 

 by the soil-moisture per unit dry weight (E/S) ; and the TJEm ratios for the 

 various plants in their several seasons were compared with the corresponding 

 humidity indices. For the three native species the Tc/Em ratio increases 

 directly with the humidity index. The beans stand out in strong contrast to 

 the successful species in showing the ratio to have the same value for the 

 humid season in which they flourished and in the dry hot season which caused 

 their death. 



^Carnegie Inst. Wash. Year Book for 1919, p. 101. 



