4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



Plains are so flat that runoff in the area where the trees grew is at a 

 minimum. The soil is a sandy chernosem of rather high porosity ; it 

 absorbs water and yields it to evaporation rapidly. 



The climate of the area is typified by extremes of varying intensity. 

 Annual rainfall averaged 19.09 inches over a period of 40 years. 

 Table i gives certain rainfall data at the Texas Agricultural Experi- 

 ment Substation, 5 miles east of the College campus. On the whole, 

 rainfall is concentrated in the summer months, and therefore the trees 

 may start growth in March or early in April with very little soil 

 moisture available. The rains of spring and summer are commonly 

 intense, of short duration, and separated by relatively long periods of 

 drought. Because of the nature of the rainfall, soil moisture fluctuates 

 widely. 



Table 2 sets forth temperature data. Latitude causes a rather high 

 average annual temperature, and altitude gives rather sharp fluctua- 

 tions in daily and seasonal temperatures. Because of the rather high 

 average annual temperature, growth begins rather early in the spring, 

 sometimes as early as the first week of March, and temperatures 

 which would permit growth continue late into autumn. However, this 

 lateness is of little significance to tree growth except in certain in- 

 stances where so-called postseasonal growth occurs. 



The impact of a cold front during spring may delay initiation of 

 growth or may force the temperature so low, after growth has started, 

 as to kill or seriously injure the cambium and newly formed tissues. 

 Temperature records permit us easily to identify the years of killing 

 late frosts. The average date of the last killing frost in spring is April 

 14, and that of the first in autumn is November i. Because of the 

 highly unique character of the anatomical effects of a few of the late 

 frosts, coupled with the knowledge of their exact dates, absolute 

 dating of growth layers has been possible. 



Table 3 sets forth data on evaporation and wind movement. It 

 shows the relatively high rate of evaporation for all times of the year. 

 There can be no doubt that a combination of high rate of evaporation, 

 high temperatures in the summer, and long drought intervals, has 

 detectable effects on tree growth. 



The natural vegetation of the Lubbock area consists principally of 

 short grass and widely scattered shrubby growth of catsclaw, yucca, 

 and mesquite. Junipers are common at the Break of the Plains and 

 in the transitional area immediately below. Locally, broad-leaved trees 

 and shrubs are common in the canyons which fray the edge of the 

 Plains. 



The trees used for experimental purposes were either isolated in- 



