DEPARTMENT OF BOTANICAL RESEARCH. 



77 



soil is relatively cold, but at Tucson it is in midsummer as well. In the 

 latter region, in fact, the season of most active root-growth is in sum- 

 mer, when the soil is relatively warm as well as moist. Whether any 

 species characteristic of either of these regions will grow in the other 

 regions depends largely, therefore, on the temperature of the soil at 

 the time it is suitably moist, since an appropriate soil-temperature is 

 a condition indispensable to an effective rate of growth of roots. 



The following summary gives the maximum and minimum tempera- 

 tures of the soil at the Coastal Laboratory for the summer in 1916 and 

 for the depths indicated: 



It should be said that at the depth of 30 and 60 cm. the soil in mid- 

 summer at Carmel is moist, but at the least depth it is air-dry at that 

 time; at all depths, owing to the sandy nature of the soil, it is well 

 aerated. 



The types of root-systems represented by the species referred to are 

 the specialized superficial type of the cacti and the deeply penetrating 

 type of Prosopis and Pistacia. Under natural conditions the roots of 

 the cacti, for the most part, lie within 5 to 10 cm. of the surface of the 

 soil and extend away from the central axis as far as 1 to 3 meters 

 or more. 



Growing in the soil of the garden at the Coastal Laboratory the 

 Opuntias, from whatever region, tend to form generah zed root-systems. 

 Specimens which had been over two years at Carmel had formed root- 

 systems which may be described as constituting a tuft of approxi- 

 mately equal length, none of which exceeded 25 cm. These took a 

 downward as well as an outward course in their growth. However, 

 most of the roots were confined to the uppermost 15 cm. of soil, or that 

 stratum where, as the accompanying table indicates, the soil tempera- 

 ture is the highest. 



The specimens of Pistacia, five years from the seed, had shoots about 

 25 cm. in length. The root-system was dominated by a tap-root which 

 was traced down over 105 cm. There were deeply penetrating laterals 

 also, and within 15 cm. of the surface of the soil many short, fibrous 

 rootlets had been formed. 



The Prosopis observed were seedUngs one year old. The seed was 

 sown in September. In the following May the roots were seen to have 



