ROOT-SYSTEMS OF PERE.NN'IAI.S. 8 1 



water supply from the rains directly, and also, even where the roots may 

 penetrate deep and actually do so, it appears from the abundance of 

 superficial roots that the species growing imder such conditions also derive 

 their water supply mostly from surface waters. Thus, although it is char- 

 acteristic of the young plant that a strong tap root is developed (plate 21), 

 ni the mature form many of the laterals remain near the surface of the 

 ground and, with little change of level, may run 15 meters from the central 

 axis under especially favorable conditions, as by an irrigating ditch. This 

 observation was verbally communicated by Prof. R. H. Forbes, of the Ari- 

 zona Experiment Station. 



CONDALIA SpATHULATA AND ZlZYPHUS PaRRYI. 



Condalia and Zizyphus are both dwellers on the flood-plain of the Santa 

 Cruz and of West Wash, but the characteristic habitat of both forms is on 

 the river bottom. In this vicinity Zizyphus reaches its largest size in the 

 forest of Prosopis vduiina near the old San Xavier mission, 9 miles south 

 of Tucson. Both species, however, were studied near the western edge of 

 the Santa Cruz flood -plain about 6 miles west of the domain of the Desert 

 Laboratory. 



In the neighborhood of the plants examined the leading perennials, in 

 addition to Condalia and Zizyphus which were fairly abundant, were Acacia 

 constricta, Bigelowia hartwegii, and occasionally Kacrhcrlinia spinosa, with 

 many arborescent specimens of Prosopis velutina. 



Soil conditions where Condalia and Zizyphus were studied are character- 

 istic of the edge of the flood-plain, that is, adobe clay with particles of rock 

 fragments to a considerable depth. Condalia is evergreen, but Zizyphus is 

 deciduous, dropping its leaves soon after the beginning of the arid autumn. 

 The specimen of Condalia studied was 74 cm. in height. The root-system 

 of the plant consisted mainly of a tap root with numerous small and a few 

 large laterals. The tap root was traced directly downward 1.3 meters and 

 where left it was 8 mm. in diameter, so that it would not be possible to state 

 the depth attained, but without question it was much more than actually 

 demonstrated. At itscrown the tap root was 2.5 cm. in cross-section. The 

 leading laterals arose as follows: three, 5 mm. in diameter, originated just 

 below the surface of the ground, three more were given off at a depth of 

 36 cm., and one arose 90 cm. beneath the surface. Throughout the first 

 meter of the tap root there were, between the larger laterals, very many 

 smaller ones about i mm. in diameter, but these were relatively long. The 

 length of the larger laterals was not learned. Filamentous roots were not 

 seen on the laterals. 



The shoot of the specimen of Zizyphus examined was 1.4 meters high and 

 bore numerous branches. At the time its roots were examined (April 22) 

 it was not in leaf. The root-system closely resembled that of Condalia; 

 it was characterized by a stout tap root from which arose several laterals; 

 the main root was 3.5 cm. in diameter at the crown and was traced straight 



