ROOT HABITS OP DESRRT PLANTS. 



The course of the temperature for the year at the depth of 30 cm. is 

 somewhat different from that at the lesser depth. This is graphically- 

 shown by the figure below, which was made from an inspection of the 

 records for the year 1 908-1 909, and is accurate only in general form, not in 

 detail, but illustrates the facts. As fig. i indicates, the highest temperature 

 for the year is in July, or August, just prior to the rains of midsummer. 

 With the coming of the rains the temperature falls and continues to decline 

 until the middle of March, when it begins to rise and quickly reaches the 

 degree characteristic of summer. There is thus a grand maximum, which 

 precedes the time of the germination of the summer annuals and the active 

 growth of the summer perennials, and a grand minimum which follows the 

 most active period of vegetation of the winter season. Broadly speaking, 

 therefore,when the activities of the two large classes of plants are awakened, 

 there is seen to be a difference of approximately 30° F. in soil temperature 

 at the 30 cm. depth. 



70 



60 



50 



Fig. 1. — Temperature of the soil, at a depth of 30 cm., for the year 1908-1909. 

 Desert Laboratorj-. 



SOIL TEMPERATURES: 2.5 CM. DEPTH. 



The only available records of temperatures of the soil at less depths than 

 those above given are thermometer readings made at a station less than 

 50 meters from the location of the thermograph records given above. These 

 readings extend from March 22 to May 10, 1907, and owing to the rather 

 brief period covered by the thermometer record a discussion of it per se is 

 precluded. However, it will be of interest to compare the thermograph 

 record and that of the thermometer for the same days, as shown in table 4. 



The most interesting facts revealed by the records are the differences in 

 temperatures which the soil shows at any moment, and the relatively high 

 temperatures of the least depth. During the days of which the temper- 

 ature record is available the greatest difference of temperature between 

 the depths referred to is shown in the following summary : At the 30 cm. 

 depth the variation was 3° F. ; at the 15 cm. it was 11°; and 2.5 cm. it was 

 40°. The greatest difference in maximum temperatures at any moment 



