39L> 



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 



FIG. 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. S. 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 



