TEMPER.\TURE OF THE SOIL. 



was observed on April 15, when there was a variation of 2_V5° between the 

 upper two levels ; unhappily the record for the greatest depth for the time is 

 missing, or the difTerence would doubtless be much more. PVom these 

 records it appears that the roots of a plant which reach as deep as 30 cm. 

 may at one moment in springtime experience a range of temperature as 

 great as 22.50° F., and inother portionsof the vear probablva much greater 

 range. 



T.\BLE 4. — Soil Temperatures at Different Defiths, March-Mav, 1907. 



*Maximum and minimum thermometers were employed in the study. 



SUMMARY OF ENVIRONMENTAL CONDITIONS. 



The habitats, which have been studied in connection with the observa- 

 tions on the roots of plants inhabiting them, comprise mesa, or bajada, 

 and include locations on the flood-plain of the Santa Cruz river, the West 

 Wash (near Tumamoc Hill), and on Tumamoc Hill. The studies on the 

 soils, the moisture in them and their temperatures, adopted from various 

 reports, have been limited to Tumamoc Hill, or the bajada and Hood-plain 

 near by. 



The surface soils are an adobe clay with a varying admixture of rock 

 fragments, either of volcanic origin or of caliche, and the leading apparent 

 differences in the soil are dependent on the amount of this coarser material 

 present. The upper soil is usually from 15 to 30 cm. in thickness. Beneath, 

 to a considerable but varying depth, lies a hardpan caliche. Between 

 the upper stratum and the hardpan is an intermediate zone in which there 

 are large fragments of caliche, or other rock, mingled with adobe. The 

 intermediate stratum is usually not more than 20 cm. in thickness. The 

 total soil generally available for the roots is thus 50 cm., and frequently 

 (usually on the bajada) it is much less than this. 



The depth of perennial water varies with the habitats, although uu 

 Tumamoc Hill no water table exists. On the bajada it lies 25 meters or 

 more beneath the surface, and on the flood-plain of the river the water table 

 is 5 meters deep, or more. 



