30 DROUGHT RESISTANCE OF OLIVE IN SOUTHWESTERN STATES. 



these were all okl, hard roots it seems probable that they had gone 

 down to these depths during the years when the orchard was still 

 irrigated. The rootlets branching from these and all the deeper 

 lying roots were much fewer than those nearer the surface. 



A point previously referred to of special interest in this study is 

 that of a number of deep-lying roots at points several feet from the 

 tree sending off branches which rise suddenly to the level of 4 to 8 

 inches, along which levels they make a growth of several feet, sending 

 off numerous branchlets and small feeding rootlets. These ascending 

 laterals were found to show but three or four rings of annual growth 

 while the main root (though it was impossible to count the rings accu- 

 rately) was several years older. Evidently the upper growth had been 

 made since irrigation ceased in an efiort to reach the more favorable 

 conditions for moisture and air at the upper level. In both trees 

 studied, as well as in many others where small excavations were made, 

 fine-feeding rootlets were found in considerable numbers at 2 to 24 

 inches in depth, Init the zone of their greatest abundance was at 

 4 to 10 inches. Plate IV, figure 2, shows a section of a long root 

 from a depth of 6 inches, with lateral branches and feeding rootlets, 

 from a photograph of exactly natural size. 



MOISTURE ECONOMY AIDED BY THE STRUCTURE OF THE OLIVE 



LEAF AND STEM. 



In addition to the elaborate arrangement of olive roots for collecting 

 the last particle of moisture possible from a sandy desert soil, there 

 must still exist a remarkable economy in tissues and functions to 

 enable a tree to survive, not to say to make growth, under such con- 

 ditions. 



It is to the leaves that we must look chiefly for this work. Their 

 narrow form, reflexcd margins, thickness, and tough leathery texture, 

 as well as the densely hairy, almost felted under surface, all indicate 

 that they are prepared to resist to the extreme the drying influence 

 of the desert air. The minute anatomy of the leaf and stem of the 

 olive has received considerable attention from botanists, but ap- 

 parently no attempt has hitherto been made to ascertain to what 

 extent difl'erent environments aft'ect modifications of structure. That 

 some light might be had on this most interesting point, material was 

 procured by the writer of this bulletin from olive groves of California 

 and Arizona, the samples being obtained from such widely diverse 

 environments as the moist, fog-laden air and ample irrigation of 

 Niles, near the San Francisco Bay, and the extreme of desert dryness 

 and heat of Palm Springs, Cal. These were placed in the hands of 

 Dr. Theodore Holm, whose study of them, illustrated in Plate V, 

 figures 1 and 2, and five text cuts, is given in the Appendix to this 

 bulletin. 



192 



