238 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



and the common thistle (Cirsium lanceolatum) . None of these, 

 with the possible exception of the thistle on the west slope, is 

 sufficiently plentiful to constitute a prominent element of the flora. 



LIFE ZONES. 



In the study of the vegetation of any region it soon becomes 

 apparent that many of the species are restricted in their distribu- 

 tion. Some plants, of course, gi*ow only in water or in wet soil, 

 and others only in dry or well-drained situations, but most species 

 are not generally distributed even when such habitats are disre- 

 garded. In the mountains, as one ascends the slopes, it is noted 

 that the character of the vegetation changes, either gradually or 

 abruptly, certain plants disappearing and new ones taking their 

 places. Very few plants that grow on mountain tops grow also in 

 the foothills or valleys or on the plains. In general, it is found 

 that the vegetation is divided roughly into belts or zones, which 

 are sometimes well marked, but often of indefinite limitation. As 

 a general rule the trees and shiaibs are more definitely distributed 

 in belts than are the herbaceous plants. This zonal distribution of 

 plants is the result of varying conditions with regard to temperature 

 and moisture, as influenced by exposure and elevation. Moisture 

 and especially temperature vary greatly at different elevations, and 

 each plant reaches its best development at the elevation at which 

 conditions are most fully suited to its growth. 



After studying the broader aspects of the distribution of plants 

 and animals, botanists and zoologists have evolved a general classi- 

 fication of regions into life zones, each of which occupies a wide 

 geographic area. For North America these zones have been worked 

 out with great care, and a knowledge of the characteristics of each 

 is desirable in the study of the flora of any region, especially a 

 mountainous one. In regions of little variation in elevation a single 

 zone often continues without interruption for hundreds of miles, 

 but in the Rocky Mountains one may often pass through three or 

 four different zones in a half day's walk. 



The life zones, of course, are not always sharply marked; indeed, 

 more often they are not, but their general features can usually be 

 recognized, and the more familiar one becomes with a certain region 

 the more apparent are the broader features of the zonal division. 

 If temperature and moisture were uniform at a given altitude, 

 probably the so-called zones would be sharply marked, but this is 

 obviously not the case. On northward slopes there is less evapora- 

 tion and consequently more moisture, and also the temperature is 

 somewhat lower at a given altitude than on a sunny slope; on a 

 southward slope the conditions vary in the opposite direction. As a 

 consequence, plants of high altitudes are often found at compara- 



