THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 55 



(like that of 1913), which recur every few years with tolerable 

 regularity, is fundamental. 



In station 2, the constantly high water-content is rendered in- 

 effective by the high wilting coefficient. At 10 cm. depth, during 

 the period from June 15, 1913, to April 17, 1914, the soil furnished 

 available water only from November 1 to April 1. It seems un- 

 necessary to seek further for the explanation of the poverty of the 

 vegetation in that station and its vicinity. 



The Soil Temperature. 



In making determinations of soil temperature, the simplest 

 method seemed the best — the plunging of a chemical thermometer 

 into the freshly cut surface at the time of taking water-content 

 samples. During the dry season it was found necessary at the 

 lowest level to bore a hole with a small auger. The observations 

 were made in the two stations of water-content Series I, at all 

 depths used in that series. They were begun March 25, 1913, and 

 with a few minor interruptions continued to January 26, 1914, the 

 closing date for the soil-moisture work. During much of the period 

 the temperature of the surface was also observed, in shade in 

 station 7 and in sun and shade in station 10. In these cases the 

 bulb of the thermometer was covered with a thin layer of soil or 

 litter. The results are plotted in figure 13. 



The first point to note is that during the dry season the soil 

 temperatures in the two stations are widely different, the three 

 lines representing station 10 being all of them regularly below those 

 of station 7. With the arrival of the wet season they all descend 

 and at the same time converge, the differences between them almost 

 disappearing. This is exactly the reverse of the soil-moisture's 

 behavior, where convergence takes place in the dry season. It is 

 due of course to the greatly increased water-content of both soils. 

 The surface temperatures, naturally the highest in each station 

 through most of the period, show the same tendency, and in the 

 case of station 7 the line actually drops below those representing 

 the subterranean temperatures. 



Comparing the lines of a single habitat, we find that in station 7, 

 during the dry season, the temperatures vary inversely with the 

 depth, the order being, from highest to lowest, surface, 10 cm., 

 50 cm., 100 cm. During the wet season the order is roughly the 

 reverse, the order most of the time being 100 cm., 50 cm., 10 cm., 

 surface. In station 10 the case is the same, except that the shaded 

 surface and 10 cm. lines do not greatly differ, because of the lack of 

 cover, and in the wet-season portion the lines do not hold constant 

 relative positions. In both stations the greatest fluctuations occur 

 at the surface and at 10 cm. depth, and the greatest uniformity at 

 100 cm. depth. 



