1920] OVERHOLSER &* TAYLOR— TEMPERATURES 275 



Pears in the Vaca Valley, near Vacaville, California, have 

 behaved in a way to indicate that high temperature may retard 

 ripening. Although the Vaca Valley is famous for its early fruits, 

 especially cherries and apricots, it is a well established fact that 

 Bartlett pears grown there are notably longer in reaching maturity 

 than those from any other section of northern California, unless 

 it be from the mountain sections where the seasons are very late 

 in opening, owing to their high elevation. One of the writers 1 

 has often seen a full crop of immature Bartlett pears still hanging 

 on the trees in this valley when practically the entire crop was 

 gone from orchards in both coast and interior valley sections. 

 In the spring the pear trees blossom comparatively early, as do 

 the other fruits. The young pears develop normally until the hot 

 summer weather predominates, when they apparently almost 

 cease growth, or at least grow slowly until cooler fall weather 

 comes. Then the pears seem to commence growth again, often 

 increasing noticeably in size and ripening in the normal way. It 

 should be noted, however, that while the summer temperatures in 

 Vaca Valley are generally unusually high, the relative humidity 

 is practically always comparatively low. 



In discussing Shamel's interesting results and the results 

 obtained by the writers, later recorded in this paper, Whittex, of 

 the Division of Pomology, University of California, recalls observa- 

 tions which apparently bear upon this subject. He comments as 

 follows: 



summer 



most severe drought and the highest temperatures recorded for that section 

 since the United States Weather Bureau was established. During that 

 season pears remained firm on the trees much later than in normal years. In 



numerous 



Similar 



ordinarily keep for exhibition 



same section, has been observed to occur during occasional subsequent dry, 

 hot summers. 



The casual explanation, usually offered at that time, was, that the develop- 

 ment of the pears was retarded by unfavorable conditions for growth, and that 



■ 



Taylor. 



during 



