THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 43 



assumed from field observation, the only noticeable difference being 

 that some are a little more coherent than others. The soil of station 

 2 is strikingly different, having much less of the coarser grades and 

 a correspondingly larger proportion of silt and clay, especially the 

 latter; consequently it is sticky and heavy when wet, is unfavorable 

 to water movement (shown by the fact that water stands in holes 

 for many hours after a rain), and is very hard and almost rock- 

 like when dry. Though still belonging to the sandy-loam class, it 

 tends strongly toward clay. This physical character results in 

 quite a different set of soil-moisture values, as will appear later, 

 and it is therefore not surprising to find the difference in the vege- 

 tation which has been noted— a difference, however, entirely in 

 relative luxuriance. The importance of the mechanical analysis to 

 the present study is that it demonstrates that the soils of most of 

 the localities studied do not differ in physical nature (excluding the 

 humus portion), and that therefore this factor as a determinant of 

 water-content and of vegetation has been excluded, since it is not 

 a variable. The stations in which no analyses were made, from 

 field observation should be grouped as follows: station 1 like sta- 

 tion 2, 5 like 4, 8 like 7, and 9 like 10. 



The same samples were submitted to Dr. Charles B. Lipman, of 

 the University of California, for determination of humus. The 

 depths are those noted in the last section. T indicates less than 

 0.1 per cent. 



Table 4. — Average humus content. 



The importance of this reaction-factor in the surface layer is 

 seen, and its penetration to a considerable depth in the forest. 



The Soil Moistuke. 



The determinations of water-content were made in two series. 

 In the 'first and more important, two stations were utilized, Nos. 

 7 and 10, the object being to contrast the conditions in chaparral 

 and forest. Determinations were made weekly for a period of one 

 year, and the series was later resumed for a few weeks for a special 

 purpose. In the second the stations selected were the six used in 

 the evaporation experiments and the same from which the samples 

 for mechanical analysis were taken. This series ran parallel to the 

 evaporation work, determinations being made at intervals of four 

 weeks from June to April. 



