8 BULLETIN 623, U. S. DEPARTMENT OF AGRICULTURE. 



In order that the data might be secured from trees located in 

 orchards where the conditions were suitable for carrying out this 

 work, a careful study was made in 1909 of many orchards in those 

 districts of California where Washington Navel oranges were most 

 successfully grown. Some of the conditions considered most desir- 

 able in the location of the performance-record plats of navel-orange 

 trees for use in these investigations were (1) the production of success- 

 ful and profitable crops; (2) location on virgin land in order to elimi- 

 nate any influence of previous cultural treatments; (3) the absence 

 of any radical pruning or other tree treatments or cultural practices; 

 (4) the absence or effective control of diseases or insect pests; (5) 

 the location of plats on uniform soils where little or no fertilizer or 

 manure had been used and where uniform irrigation and other cul- 

 tural practices had been continuously followed during the entire his- 

 tory of the orchards; (6) a knowledge of the history of the buds used 

 in the propagation of the orchards and the character or kind of stocks 

 used; (7) protection from cold, high winds, or other climatic causes 

 of tree injuries; (8) the prospect of a continuous ownership of the 

 orchard for a series of years; and (9) as little as possible apparent 

 variability of strain, so that the results obtained would be conserva- 

 tive and fairly representative of the behavior of the individual trees 

 from the standpoint of the variety as a whole. 



The preliminary survey of the Washington Navel orange districts 

 made possible the selection of plats where these conditions were 

 unusually uniform and particularly suitable for securing reliable and 

 valuable data concerning individual-tree behavior. The ranches on 

 which these plats were located are bounded on one or more sides 

 by deep arroyos, thus providing good air drainage. In all cases 

 these ranches are in the foothills and no other ranches are located 

 above them, so that there was no possible influence on the perform- 

 ance-record trees from higher lying orchards or cultivated lands. 

 All possible precautions were taken to provide against causes of indi- 

 vidual-tree differences other than those resulting from individual- 

 tree variability. 



In the beginning it was considered most important to study the 

 individual-tree variability within the Washington or best strain of 

 the variety rather than to compare the behavior of the individual 

 trees of the different strains. Later it was found to be equally impor- 

 tant to consider the differences between strains arising from bud 

 variations; therefore, plats selected subsequently were located mainly 

 for the purpose of comparing the behavior of various strains of the 

 variety. Inasmuch as the individual-tree performance-record work 

 with the Washington Navel orange was, so far as known, the first 

 investigation of this character with any citrus variety, the location 

 of the plats was made without the knowledge which has since been 



