DEPARTMENT OF BOTANICAL RESEARCH. 73 



cases the personal interests of the observers have led them to secure 

 data from depths other than 3 inches or 12 inches, to which it had been 

 hoped to confine all of the records. However, the data which are 

 accumulated indicate that the results of the survey will be of such a 

 nature as to have some value in general plant and animal ecology, as 

 well as in agriculture, horticulture, and forestry. 



The work of the committee has been confined to the recording and 

 collating of the data from the various stations, and a preliminary pub- 

 lication will be prepared on the basis of the data secured in the first 

 four years, in the hope that the observations may ultimately be ex- 

 tended over a longer period. 



Causes of the Seasonal Changes in the Transpiration of Encelia farinosa, hy 



Edith B. Shreve. 



As was mentioned in the annual report of 1919, the desert perennial 

 Encelia farinosa possesses some means of cutting down its ratio of trans- 

 piration to evaporation T/E during the months in which the aridity is 

 increasing. The evaporative power of the air is twice as great during 

 May and June as during the cool and more hmnid months of January 

 and February, while Encelia loses only 1.4 times as much water per unit 

 area in the latter months as it does in the former. The plant has two 

 distinct types of leaves, a mesophji^ic form which is present during the 

 cool months and dies when arid conditions begin, to be followed by a 

 much smaller xerophytic form. The mesophytic form is glabrous, 

 while the xerophytic is covered with a thick mat of long hairs. The 

 latter type is from two to three times thicker in cross-section than the 

 former. 



Disks of uniform diameter were cut from corresponding positions in 

 the two types of leaves and the water-loss measured under the same 

 external conditions. 



WTien disks of the two types of leaves are placed under the same 

 atmospheric conditions the xerophytic type loses 1.44 tunes as much per 

 unit area as does the mesophytic — a result contrary to that which might 

 have been expected from an inspection of the leaf structures. 



\Vhen the water-loss is calculated on the basis of dry weight, how- 

 ever, an entirely different result is obtained, for then the xerophytic 

 type loses only 0.78 as much as the mesophytic type. This is not due 

 to decreased water-content per se, for when the water-content was sud- 

 denly increased by placing branches, with leaves intact, in a moist 

 chamber, with the cut ends in water, the disks cut from these leaves 

 showed the same loss per gram of dry weight as did the disks from 

 untreated leaves. Thus it is evident that the decrease in T/E with 

 increased aridity is due to some internal change in the leaf. 



As previously noted, during drought seasons, a browm liquid is 

 present in the stems and leaves which seems to be absent during cooler 



