PAPERS OX BIOLOGY AND AGRICULTURE 161 



oils, these factors interact among themselves, they are 

 difficult to isolate, and, to associate correctly any one of 

 them with observed phenomena is possible only through 

 close and prolonged study. 



With all varieties there are marked irregularities in the 

 alternation of long and short blooming periods, and, 

 further, there are conspicuous departures from those 

 relative lengths of periods of different varieties that 

 might easily be assumed to be constant: thus, where two 

 varieties, in any one year, have blooming periods differ- 

 ing in length by several days, one, say twice as long as 

 the other, it would not be unreasonable to suppose that 

 the varieties in question would hold to an approximation 

 of the difference in other or all seasons. As matter of 

 fact the relative lengths of the periods in one season may 

 be reversed in the succeeding season. 



This occurrence indicates plainly that temperature and 

 general atmospheric conditions, commonly regarded as 

 the chief determiners of lengths of blooming periods, do 

 not act equally on all varieties in all seasons, or that 

 varieties develop within themselves qualities that render 

 them less susceptible to stimulation, or more resistent to 

 adverse conditions in one season than in another. 



In illustration, the records of Tolman and Whitney for 

 the years 1903 and 190S may be cited. The flowering 

 period for Tolman in 1903, a year having a long average 

 flowering period (9.2 days) for all varieties, was six 

 days, while in 1908, when the average period for all va- 

 rieties was short (5.94 days) the period recorded was 

 16 days. In contrast with this record, Whitney, with a 

 variety average for 16 years of 7 3 4 days, had its flowering 

 period lengthened to 14 days in 1903 and contracted to 5 

 days in 190S. Bringing these records side by side for 

 ready comparison they appear as below : 



1903 1908 



Blooming period for Tolman 6 days 16 days 



Blooming period for Whitney. . . .14 days 5 days 



The trees compared were of the same age, grew but a 

 few rods apart and had received the same care each year. 



