STUDIES IN LAKE AND STREAMSIDE VEGETATION 1 43 



(3 miles) north of Redrock Lake. Here an average difference of 10 

 degrees was noted in the temperature of gulch and ridge at night, the 

 gulch being the colder. 



Daily Range. — It is shown in Table I that the daily range of tempera- 

 ture is less at Redrock Lake than at Boulder. Most persons acquainted 

 with high altitudes believe that the range there is much greater than at 

 lower points. It is probable that this misconception is due to the fact 

 that the sun temperature at midday is high in mountain districts and 

 people think of that rather than of the shade temperature in forming 

 their opinions of the climate. One reason for the smaller daily range 

 at Redrock Lake is the frequent clouding up in the afternoon. If there 

 is no afternoon shower the range may be nearly as great as at Boulder. 

 The clouds of afternoon are probably of some importance to plants, 

 especially to those in dry soil. The direct heat of the sun is very great 

 and relative humidity is low during the day. Plants which are barely 

 able to get water to replace that transpired may be saved by the after- 

 noon clouds from death by wilting. It is certainly true that many appar- 

 ently mesophytic species are able to live in the dry forest and this 

 almost daily clouding of the sky in the afternoon may be of consequence 

 in this regard. The chart, Fig. 4, made by combining hourly records 

 for the week ending June 28, 1908, shows the mean hourly march of 

 temperature for that week at Redrock Lake and at Boulder. The de- 

 pression which occurs between 12:00 m. and 1:00 p.m. at Redrock 

 Lake keeps the maximum low and gives the curve a more flat appearance 

 than that showing the temperature at Boulder. 



Insolation. — Direct heat of the sun is very great at high altitudes and 

 the difference between sun and shade temperatures is often remarked. 1 

 No suitable instrument is known to the writers which may be used for 

 measuring this ecological factor. The black bulb thermometer is, 

 according to Hann, open to serious objections. Insolation is supposed 

 to be one of the causes for the dwarfing of alpine plants but it is to be 

 noted that such true alpine plants as grow at Redrock Lake have just 

 the same habit and appearance there as in the much higher regions 

 where insolation is greater. The great metabolic activity of high- 



1 Schimper, Plant Geography (English Translation), pp. 692, 693, 1903. 



