4 THE YOSEMITE FLORA 



by cool summers and much snow and ice in the winter time, for 

 it extends up to timber-line, where Alpine conditions prevail. 



4. Belt Above Timber-line (Arctic-Alpine Life Zone). As 

 in the last preceding belt, the plant species here are entirely of 

 boreal origin. For those who are not particular as to technical 

 requirements, this is the most easy of all belts to distinguish, 

 since its lower limits are fixed by the upper line of the forests, 

 from which it extends to our highest summits. Here grow only 

 those plants which, through the peculiar constitution of their 

 protoplasm, are enabled not only to endure the rigors of winter 

 but to make their growth and form flowers and seed with a rela- 

 tively small amount of heat. In these regions spring comes on 

 with a rush after the melting of the snow, for each plant must 

 hasten to mature its crop of seed before it is caught by the cold 

 storms of early autumn. All have deep, perennial roots, while 

 low and tough stems, often much gnarled, are the fashion. The 

 leaves are likewise tough in most cases, having a thick epider- 

 mis, and they are mostly huddled near the base or along the short 

 stems. These characteristics are due not only to the short grow- 

 ing period but also to the need of conserving moisture, since, as 

 a result of low temperature, steep slopes, porous soil, strong 

 winds, and reduced atmospheric pressure, a lack of sufficient 

 water is one of the plant's chief contentions. Among the more 

 interesting of these Alpine plants may be mentioned the Arctic 

 Willow, which creeps along the ground, rising only to a height of 

 three or four inches ; the Cassiope, with thick, overlapping leaves 

 and dainty pendent flowers; and the Alpine Sorrel, which ex- 

 tends around the world in Arctic regions, ranging southward to 

 high peaks in the Rocky Mountains and in the Alps. The Pole- 

 monium shown in the illustration facing page 190 is a typical 

 Alpine species, having a strong, perennial root, numerous short 

 stems with compact leaves, and showy flower-clusters. 



As has been intimated in the preceding paragraphs, tempera- 

 ture is not the only factor to be considered in a study of dis- 

 tribution. Among the other environmental influences we should 

 note the effect of soil, light, air, animals, and especially water. 

 Let us now briefly look into a few of these factors, observing both 

 their influence on distribution and their effect on the appearance 

 of the vegetation. 



The moisture relation often determines the kind of plant that 

 can grow in a given place. We therefore find, within each of the 

 great belts as already outlined, markedly different types of vege- 

 tation, known technically as plant formations. These formations 

 often extend from one belt into another. Where the subsoil is 

 moist and the surface soil only moderately so, as over most of 



