THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 59 



etation are included when the evaporating power of the air is 

 measured, as are also in part the effects of temperature, wind, and 

 light. The two series of observations shown in table 11 are of 

 interest, however, in illustrating the values of this simple factor in 

 the various stations. The readings were made with a cog psy- 

 chrometer, breast-high, except where dense chaparral made a 

 lower level necessary. 



Table 11. — Relative humidity, percentages. 



The percentages given in table 11 do not approach to what we 

 are accustomed to think of as xerophytic values. It may be that 

 they do not represent average dry-season conditions, though there 

 is ample reason in the proximity of the bay and the ocean for 

 relatively high humidity. Other chaparral localities would cer- 

 tainly show a much lower average. The sun humidities in the 

 Adenostoma habitats are higher than those in the shade; in the other 

 stations the order is reversed. 



Evaporation. — Observations were made by means of the Livingston 

 cylindrical porous-cup atmometer, standardized at the beginning and 

 end of their use by the Plant World Company. For dry-season 

 study instruments were set out in all the stations except No. 10, 

 two per station, one at the level of the ground, and one near the top 

 of the vegetation in the region of most abundant foliage. The 

 instruments at the ground-level were set in small excavations, so 

 that the tops of the porous cups were about 2 dm. above the surface. 

 Those at the upper levels were set upon posts of proper height, 

 ranging from 0.7 meter in station 1 to 2.5 meters in station 6. In 

 the forest stations, No. 7 and 8, it was necessary to construct wire 

 trolleys running to the tops of trees, so that the instruments could 

 be lowered for measurement. 



