NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 5 



dividuals or members of windbreaks. In the latter case, however, 

 competition could conceivably have accentuated somewhat, but not 

 altered the nature of, the anatomical features in the xylem. This 

 proved to be correct. 



All trees used from the Lubbock area, with the exception of three, 

 came from one of two places : the campus of Texas Technological 

 College or the grounds of the Texas Agricultural Experiment Sub- 

 station. Both places are very similar in being portions of nearly level 

 areas. One of the other three trees grew in a shallow valley in Mac- 

 kenzie State Park at the northeast edge of Lubbock ; the second at a 

 private residence in the west part of Lubbock; and the third in 

 Coopers Canyon, a sharp valley heading into the Plains near Post, 40 

 miles southeast of Lubbock. 



MOUNTAINS OF NORTHERN NEW MEXICO 



This region was chosen partly because of accessibility and partly 

 because of its native forest. Compared with the Lubbock region, the 

 average annual rainfall is higher, the temperature lower, evaporation 

 less, the soil moisture greater in amount and less in fluctuation, and 

 the season of growth much shorter. These facts, coupled with the 

 much greater elevation, make growing conditions less hazardous and 

 more continuously favorable than in the Lubbock region. 



A single ponderosa pine was sampled from the juniper-pinon- 

 ponderosa pine association in the foothills near Las Vegas, N. Mex., 

 at an elevation of 6,150 feet. Several trees and shrubs were sampled 

 between Mora on the east slope and Tres Ritos on the west slope of 

 the Sangre de Cristo Range at elevations ranging from 8,750 to 9,350 

 feet, elevations which put them well within the spruce-fir forest. The 

 remaining New Mexico specimens came from stunted trees and 

 shrubs at timberline near Serpent Lake above Tres Ritos at elevations 

 ranging from 12,300 to 12,400 feet. Here the forest consists chiefly 

 of Engelmann spruce, foxtail pine, and dwarf willow. 



The timberline trees in New Mexico were under severe competi- 

 tion, but the others were not. Area rainfall is concentrated in the 

 summer months, especially July and August, and averages 30 inches 

 or more. 



CHISOS MOUNTAINS OF SOUTHWEST TEXAS 



This locality, in the Big Bend country, possesses environmental 

 characteristics for all purposes strikingly intermediate between those 

 of the New Mexico and the Lubbock regions. The trees came from 



