postclsia 27 



commonly grow in dry soil are certain 

 prostrate species as previously mention- 

 ed, also some species of Agropyron, 

 Sitanion, Grindelia, Artemisia and prick- 

 ly lettuce. In the slightly moist places 

 may be the Mexican poppy, alfalfa, 

 Cleome, Euphorbia marginata and rag- 

 weed. In the ditches and pools occur 

 the common green and blue-green algae, 

 the arrowheads, water plantains and 

 watercress. On the ditch banks there 

 are sedges and grasses with species of 

 Roripa, Rumex, Polygonum and Bidens. 

 Some species are able to establish them- 

 selves in all the three zones mentioned; 

 perhaps the best example is found in 

 the dandelion, which is almost univer- 

 sally distributed. Sometimes the plants 

 mentioned above as occurring in the 

 drier places may succeed in getting a 

 foothold in the more favored soil, but 

 it is seldom that the other plants are 



