3S 



The Plant World. 



its presence is taken to indicate a good water supply. Its roots 

 extend widely, to a distance of 50 or 60 feet according to credible 

 observers, and possibly to a corresponding depth; and when cut 

 green its wood tissue, which is hard and heavy, carries a large 

 percentage of water, precisely as do the hardwoods of the eastern 

 United States, and strikingly different from the creosote bush, 

 its near neighbor on the slopes beyond the flood-plai._ At the 

 same time, the general structural peculiarities of the mesqc.ite are 

 xerophytic. It is commonly armed with spines, and its coria- 

 ceous leaves are well protected against excessive transpiration. 

 It is a plant requiring a better supply of water than many of its 



i ici. 6. Edge of mesquite forest near Tucson. 



associates, yet well adapted to the low relative humidity of the 

 desert air, and its occurrence beyond its own special area, ranging 

 as it does to the top of Tumamoc Hill, in spots where a soil re- 

 tentive of moisture affords the conditions it needs, corresponds 

 with this peculiarity. Thus it is, in a sense, a desert plant, yet 

 one of high water requirement — characteristics which it shares 

 with various other species. 



The capacity of this plant for taking possession of wide areas 

 beyond its earlier limits of distribution is of special interest. 



