62 CARNEGIE INSTITUTION OF WASHINGTON. 



therefore, that inward diffusion of oxygen from the atmospheric air through 

 or between the tissues of the plants referred to and into the soil can occur at 

 a slow rate. 



Internal Factors governing the Seasonal Changes in the Transpiration of Encelia 



farinosa, by Edith B. Shreve. 



A publication^ appearing within the year has shown that Encelia farinosa, 

 a desert perennial having a mesophytic form of leaf in the cool, moist months, 

 and a xerophytic form in the arid season, reduces its responses to the maxi- 

 mum evaporative power of the air in June to about one-fifth of the January 

 value. It was shown further that the external factors influencing the changes 



T 



in transpiration could be expressed thus: „ „ , where Tis the transpiration 



per unit area for 24 hours, E the evaporation from an atm'ometer for the 

 same period, and S the soil-water content based on dry weight. 



A series of experiments has now been completed that was undertaken with 

 the hope of discovering the internal factors that govern this reduction in 

 transpiring power. Work was conducted on isolated disks from leaves taken 

 at the different seasons, in order to eliminate the immediate influence of 

 variations in the water-content of soil and plant parts. The conclusions 

 from these experiments are as follows: 



1. The difference in anatomical structure of the mesophytic and xero- 

 phytic leaves does not account for the greater resistance to water-loss during 

 the arid season. 



2. When disks of equal areas are cut from the two types of leaves imme- 

 diately before sunrise and placed under identical external conditions, the 

 water-loss (Ed) from them is found to vary inversely with their total imbibi- 

 tional capacity (M) and directly with the original water-content (0), so that 



M 

 the expression EaXjr gives a value that approaches a constant so nearly as 



to be remarkable for this type of work. 



3. The total imbibitional capacity of both types of leaves varies with 

 the original water-content and with the dry weight. 



4. The less the original water-content the greater the amount of soluble 

 material that diffuses into the water which surrounds imbibing disks. The 

 evidence makes it probable that in the attached leaf the differences in water- 

 content are accompanied by similar changes in the amount of soluble material 

 that passes out of the leaf by diffusion to other parts of the plant. 



5. The combined action of the external and internal factors is such that 

 the ratio of evaporation to soil moisture affects the water-content of leaf- 

 tissue; this affects the imbibitional capacity, and it in turn affects the resist- 

 ance to water-loss exhibited by the leaf tissue. 



Ecology of the Santa Lucia Mountains, by Forrest Shreve. 



In continuation of work on the vegetation of the Santa Lucia Mountains, 



instrumentation was carried on in the summer of 1923 at a locality 15 miles 



south of the Coastal Laboratory and adjacent to the sea. In this part of 



the Santa Lucias the canons are occupied by a highly mesophytic forest of 



1 Shreve, E. B., Seasonal changes in the water relations of desert plants. Ecology, iv, 3 

 (1923). 



