MESA AND FOOTHILL VEGETATION 25 



July and August, 1908, is seen to be 65.8, 67.9 and 75.1 respectively, 

 as calculated from maxima and minima. These figures seem high 

 for a "dry climate" and they are probably higher than the normal 

 since the present summer has been singularly free from wind and in 

 August the precipitation was very high with nearly two weeks of rainy 

 weather. The details for the month of June given in Tables III and 

 IV are of interest in showing more exactly the state of the air with 

 reference to humidity. 



Direct Heat of the Sun. — This is great in all mountain districts because 

 of the thinness of the air. Also because of this thinness the air cools 

 rapidly or does not retain heat. There is then a great difference in 

 temperature between sun and shade. Boulder has a sufficient altitude to 

 show these phenomena although not to so great an extent as high 

 mountain districts. More marked differences may be expected between 

 sun plants and shade plants than at sea-level or low altitudes generally. 

 The papers in the present number of the Studies do not take up sun and 

 shade plants but work on these forms is planned for publication at an 

 early date. 



Light. — Sunlight at Boulder and on the mesas under discussion is 

 very brilliant and frequently continuous all day long. Very few days 

 are entirely cloudy even at the time of April and May storms. The 

 number of clear, cloudy and partly cloudy days is shown in Table V. 

 It will be noted that during the entire summer there are only five cloudy 

 days while about half of the days are partly cloudy. During the spring 

 rains and snows, clouds or fog may hang over the mesas and above 

 the town of Boulder but after the growing season there is no fog. Morn- 

 ings throughout the summer are almost invariably clear, while from 12 

 to 4 o'clock it is often partly cloudy. Because of this large amount of 

 sunlight true shade plants are very scarce except in the darkest gulches 

 or underneath trees with dense foliage. The growth of "green slime" 

 so common on north walls and fences throughout the Atlantic states is 

 not found in Colorado at all. Mosses and ferns also are scarce because 

 of bright light and dryness of air. 



Comparison of Mesas and Plains. — Since the mesas support a growth 

 of coniferous trees while the plains a few hundred feet lower down 



