388 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
with short blooming periods. In the plants with the longest 
blooming periods, the greatest decrease comes between 40 and 60 
days, while in the others it comes between 20 and 40 days. In 
the last column are given the average differences for the successive 
twenty-day periods. It will be seen that the greatest difference 
is between the first and second period, so that here the rate of 
decrease must be the greatest. Between the third and fourth 
period there is an increase in flower number rather than a decrease, 
and after that there is only a slight decrease, if any. The diagram 
of TABLE 18 illustrates this point. The differences between the 
averages of the different periods have been plotted and it will be 
seen that there is a marked change in the rate of decrease after 
the first eighty days. Plants that bloom eighty days and less 
will show a greater rate of decrease than those that bloom longer 
for the reason that there is a slight increase or no further decrease 
in flower number after the first eighty days. This accounts for 
the large negative correlation between rate of decrease and length 
of blooming period. This discussion also illustrates the fact that 
the range of variability in respect to high and low flower number 
per head is not very closely related to length of bloom. 
The variabilities have also been calculated for the successive 
twenty-day periods (TABLE 18). Тһе o’s show a decrease, then 
an increase, and following this they again decrease. Since all the 
factors influencing flower number are not known, one cannot define 
the different variabilities exhibited by the different periods. We 
know, however, that there are sources of variability during the 
first period which do not exist later on, as, for instance, the coming 
into bloom of the different branches. The decrease in variability 
towards the end of the blooming season follows necessarily from 
the fact that there is a limit to the lowest number of flowers per 
head produced by a plant. The high numbers which appeared 
early in the season do not as a rule appear late in the season; the 
low numbers, however, appear early and do not decrease further 
as the season advances. 
The range of variability for three-year-old F; plants and one- 
year-old F; plants for successive twenty-day periods is given in 
TABLE I9. These plants are reported in detail in TABLES 18 and 
20. : 
