Plant Associations" at the^Deseirt Laboratory. 37 



tains. It approaches adobe in texture, and bricks of a rather 

 inferior quality are manufactured from it, but it is of a high degree 

 of fertihty, as is evidenced by the crops of grain, fruits, and vege- 

 tables which it produces. 



As already stated, the flood-plain in the immediate vicinity 

 of the river suffers from erosion when the stream is high. Figure 

 4 shows the condition of affairs at a point less than a mile south 

 of Tucson, where the roots of mesquite and other plants have 

 been exposed along the deeply cut channel. 



According to statements of residents, this extensive erosion 

 is of recent date. Previous to the advent of cattlemen some 20 

 years ago, and the destructive effects of over-pasturing, the 

 valley of the Santa Cruz had a luxuriant growth of saccaton and 

 other vegetation, which prevented the cutting of channels, and 

 the water spread out over the whole valley instead of flowing 

 through the deep cuts it has since made; tules grew thickly in the 

 springy places, and a fine forest of mesquite covered the ground. 

 At present the effects of such erosion are seen most plainly from 

 a point below Tucson to one about 2 miles above the city. It is 

 inevitable that such changes, where they occur, should be fol- 

 lowed by a lowering of the water-table of the flood-plain, re- 

 sulting in more or less pronounced changes of vegetation. Never- 

 theless,although the formerlyabundant saccaton has disappeared,' 

 and along with it doubtless other species, the vegetation of the 

 flood-plain shows clearly enough what were in earlier times, and 

 are still, its essential features. 



The dominant species is the mesquite (Prosopis), here in the 

 form veluhna, a highly characteristic species of the Lower Sonor- 

 an zone as defined by Merriam, but extending in its various forms 

 far beyond. It is by preference a plant of low flats, though it 

 occurs far beyond these on the uplands in situations where a 

 sufficient water-supply is obtainable. In the neighborhood of 

 Tucson the mesquite ranges in size from a mere shrub a few feet 

 height of a tree 2 feet or more in diameter and upwards of 40 feet 

 in height. Such trees grow thickly on the bottom-land near the 

 old mission of San Xavier, forming the fine forest that stretches 

 for miles up the river, in the shade of which grows a rank vege- 

 tation similar to that of eastern mesophytic forests in luxuriance. 



The habits of the mesquite are popularly well known, and 



