78 ROOT HABITS ()F DESERT PLANTS. 



THE FLOOD-PLAIN. 



The bottom lands which come into this study comprise not only the flats 

 near West Wash and those along the Santa Cruz river, but also a branch 

 of the latter, the mouth of a wash having no w^ell-defined channel, about 

 9 miles down the river, west from Tucson. The first two physiographic 

 areas both have a deep surface soil, and in both cases, in former years 

 before the Santa Cruz had cut itself a channel, flood waters at times spread 

 over the bottoms. The water table of the two areas is at different depths : 

 that of the flood-plain of the river is from 5 to 12 meters from the surface, 

 while that of the wash is much deeper and probably fluctuates with the 

 seasons or the years. While a certain amount of excavating has been done, 

 reliance has also been put on authenticated reports and use has been made 

 of natural excavations, as the caving of the banks of the river and the 

 exposing of roots by other severe washouts. On the river flood-plain have 

 been examined the rootsof Koerbeyliniaspinosa, Prosopis vclutina, and Zizy- 

 pkus parryi; by the Wash those of Pcnioccreus greggii, Ephedra trifurca, 

 and Lycium andersonii; Opuntia vivipara was studied in its habitat near 

 the Nine-mile Water-hole. The root-systems of some other trees or shrubs, 

 as Olneya tesota, Parkinsonia torreyana, Condalia spatlmlata were seen in 

 part or incidentally in connection with other plants. 



THE FLOOD-PLAIN OF THE SANTA CRUZ 

 KcERBERLINIA SpINOSA. 



Koerberlinia is one of the most striking of the desert plants of the vicinity 

 of Tucson. It is without leaves throughout its existence and the branches 

 are reduced to rather short, stout spines. The plants are usually under 

 a meter in height and occur in colonies which are frequently circular in 

 form and 4 meters more or less in diameter. Where the species has formed 

 such a thicket it is secure from injury by grazing animals and constitutes a 

 safe retreat for small rodents. The distribution of the plant in this vicinity 

 is somewhat limited. It does not occur on Tumamoc Hill or by West Wash, 

 but on the flood-plain of the Santa Cruz, by the edge of the plain as well as 

 nearer the river, and on the bajada where soil conditions are favorable. 



The habitat of the specimen oi Kmberlinia studied is on the line between 

 the bajada and the flood-plain of the river and on the lower slopes of the 

 bajada itself. The top soil, to a depth of 30 cm., is adobe clay with sand 

 or gravel admixture and under this is the caliche hardpan. On the flood- 

 plain just below, where the most perfect specimens of the plant were grow- 

 ing, the soil had a depth characteristic of the plain. The character of the 

 specimens, as wxll as the greater depth of soil, made it impracticable to 

 study the roots of the better-developed plants of the flood-plain. 



The root-systems of three individuals, or groups, were examined with 

 the purpose of learning not only the type of the roots but also the manner 

 of development of the colonial character, a prevailing habit of the species. 



