32 THE BIOLOGY OF FLOWERING PLANTS 



depleted. By June, on exposed slopes, there is no water 

 available for plant growth in the top 6 in., by July there 

 is none in the top 2 ft., but even in August there is still 

 a reserve at 4 to 5 ft. Comparing the water relation of 

 this soil with that of the eastern prairies, it will be seen 

 how the shallower root systems are related to the altered 

 conditions. 



On the eastern borders of the eastern prairies there is a 

 chaparral or scrub community with such shrubs as Rhus 

 glabra, Symphoricarpus vulgaris, Vitis vulpina, and Rosa 

 arkansana. The community also exists in the prairie itself, 

 where moister patches of soil allow it to supplant the prairie 

 herbs and grasses. The growth of the shrubs increases the 

 soil moisture by lowering evaporation, and the formation of a 

 humus mulch. The roots of some of these shrubs extend 

 to great depths, to 21 ft. in the case of Rosa arkansana. 

 Generally, however, a large absorbing system is developed 

 near the surface and the lateral spread is sometimes great — 

 over 20 ft. in the case of Rhus glabra. 



Contrasting more strongly with the root systems of the 

 prairie plants are those of plants inhabiting the Colorado 

 sandhills. Depths of 8 or 9 ft. are attained by some plants 

 {Psoralea lanceolata, Artemisia fili folia, Eriogomim micro- 

 thecum), but of nineteen species examined eight had roots 

 entirely confined to the surface 2 ft. Of the deep-rooted 

 species Psoralea lanceolata alone has a W'Orking depth of 

 7 ft. ; the others all show a predominant development of 

 absorbing roots within 3 ft. of the surface. The rainfall is 

 about 23 in., but it is all absorbed rapidly, and the first 

 few inches of the soil act as an efficient mulch, so thoroughly 

 preventing evaporation that below 6 in. the soil remains 

 more or less uniformly moist (cf. Fig. 3). 



Three communities on the east slope of the Rockies at 

 an elevation of 8000 ft. may be considered. On the gravel 

 slides, with a moving surface of small stones overlying a 

 shallow (4 to 6 in.) soil, and coarse subsoil that passes into 

 rock at 2 to 4 ft., a sparse vegetation of dwarf shrubs {Apo- 

 cynum atidrosamifolium, Eriogonumflavmn, etc.) is developed. 



