138 
AGRICULTURE: REED AND HOLLAND 
part of the growth period. It increased very rapidly at first, but remained 
fairly constant after the 28th day. The variability thus appeared to reach a 
constant value which changed but slightly as the plants approached maturity. 
The decline in the growth rate of a plant began to appear as the flower bud 
on the apex of the stalk began to be differentiuted. As the 'head' developed, 
the growth of the stalk became slower, showing agreement with the condition 
accompanying tassel formation which Pearl and Surface (1915) found in maize. 
After the flowers of the composite 'head' had been pollinated there was no fur- 
45 
30 
25 
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80 
a I 28 35 42. 49 5b 70 17 %\ 
Number o\ days 
FIG. 2. INCREASES IN MEAN HEIGHT OF HELIANTHUS AT 7 DAY INTERVALS 
Observed mean height 
Theoretical mean height 
Temperature summations 
ther elongation of the stalk. It is evident that from this time on the growth 
forces of the plant are devoted to seed-formation instead of stalk-elonga- 
tion. Thus, variabiHty in the time of blossoming may, and undoubtedly 
does, influence the grand period of growth and the total growth of this plant. 
When the flowers have been pollinated the head which previously stood 
erect becomes pendant. The floral surface which is uppermost during the 
prepoUination period, is lowermost in the post-pollination period. 
