392 GENETICS IN RELATION TO AGRICULTURE 
many of the growers are keeping records of the annual yields of part 
or all of the trees in their orchards. 
Bud Mutations in Citrus——What has been accomplished through bud 
selection in citrous fruits has been made possible by the relatively high 
frequency with which bud mutations occur. A dozen distinct types of 
Washington Navel orange are now known to occur more or less fre- 
quently in California orchards (Fig. 161). This fact is of especial sig- 
nificance in the light of the history of the Washington Navel orange which, 
as it originally existed in Southern California, consisted of only a few trees 
Fic. 161.—Fruits of the Washington Navel orange (1), and four forms that have 
originated from it by bud mutation; (2), Thomson Navel; (3), Yellow Navel; (4), Corru- 
gated; (5), Ribbed. (Courtesy U.S. Department of Agriculture.) 
(possibly only two) which were propagated from navel orange trees 
that were introduced from Brazil by the U. 8S. Department of Agriculture. 
According to Coit the evidence from early California horticultural 
literature indicates that the Washington Navel variety was recognized 
as a distinct and at least fairly uniform type of orange. That a strong 
tendency to mutation characterizes this variety is evidenced by the 
frequent origin of new forms or reappearance of old ones as bud sports. 
In some cases the aberrant type differs not alone in fruit characters but 
also in habit of growth or leaf-shape and frequently in yield. In fact high 
yield is said to be correlated with superior fruit at least in some types. 
Similarly in the Eureka lemon the so-called “shade-tree type” makes 
Digitized by Microsoft® 
