SUMMARY. 43 



made subsequently failed, leaving the fruit trees without any water 

 other than the rainfall. 



The local rainfall of 3^ to 8 or 10 inches annually has proved 

 insufficient to maintain life in any of these plants except the olive, 

 which has been found in many instances green and flourishing after 

 six or eight 3'ears of abandonment and lack of irrigation. 



Under these conditions the olive has shown the characteristics of a 

 desert plant, competing with the mesquite, cat's-claw, and grease- 

 wood in their own territory. The plantations which have been 

 studied are the Bogart-Degolia grove near Casa Grande, Ariz., a grove 

 near Florence, Ariz., and "Las Palmas" trees in the olive belt north- 

 east of Phoenix, localities having a mean annual rainfall of 7 to 9 

 inches; and in California, the Pope olive grove near Palm Springs, 

 in the upper end of the Colorado Desert, where, with an annual 

 average rainfall of only 3^ inches, 20 acres of olives have survived six 

 years without irrigation and are still growing. 



The soils of the localities are sandy and gravelly clays derived 

 from the disintegration of the soft granitic rocks of the adjacent 

 mountains. They are low in organic matter, but fairly rich in avail- 

 able phosphoric acid and potash. The soil at Palm Springs is a 

 nearly pure granitic sand and gravel, very low in silt, clay, and 

 humus, but showing by analysis percentages of potash and phos- 

 phoric acid equal to the better agricultural soils of the Mississippi 

 Valley. 



A study of the olive trees growing under these conditions has 

 shown that unlike the mesquite and some other desert trees they do 

 not survive by sending roots down to subterranean supplies of mois- 

 ture, but develop instead a very elaborate system of roots occupying 

 the soil at from 2 or 3 to 18 inches in depth and adapted to gathering 

 moisture from the lightest rainfall. 



The remarkable drought resistance of the olive is made possible 

 (1) by the power these trees possess of extending their roots so as to 

 gather moisture from a large area; (2) by their habit of growth in 

 forming low spreading tops which protect the trunk and main branches 

 from the burning heat of the sun; and (3) by the character of their 

 leaves, which are constructed in a manner calculated to check evap- 

 oration and conserve the moisture obtained by the roots. 



The plantations studied were made according to irrigation stand- 

 ards and contained originally from 75 to 114 trees to the acre. These 

 plantings have proved too thick for successful growth without 

 irrigation. 



The varieties usetl in these orchards are the ones commonly grown 

 under conditions of sufficient rainfall in France and Italy or with an 

 abundance of irrigation in California. 



192 



