Jan. 19,1924 
Photoperiodism and Hydrogen-Ion Concentration 
147 
Ph 
6.4 
6.6 
6.8 
7.0 
Observations on plantings which germinated in the greenhouse on Jan¬ 
uary 23 and were exposed to the natural daylight period and to 18 hours 
of daily illumination, respectively, are recorded in Table XIII. The 
lengthened illumination period was maintained by use of 100-watt lamps 
from sunset till midnight, the lamps being placed about 1 foot above the 
plants. First flower buds were showing on the plants exposed to the 
natural light period on March 1, or 37 days after germination, and under 
the 18-hour period first flower buds were seen March 22, or 58 days after 
germination. For all observations 
prior to April 6, however, only plants 
which had not formed flower buds 
were used, except that one of the 
four individuals in the short-day 
series used on March 14 contained 
a very small bud. It is evident that 
18 hours of light daily, as compared 
with the natural daylight period 
ranging from about 9% to 
hours, materially delayed the time of 
flowering and increased the height 
of the stem. It is clear, also, that 
the longer light period maintained 
a higher level of acidity in the 
upper portions of the plant than 
did the shorter light period (fig. 9). 
There is, therefore, a tendency for 
the sunflower when exposed to an 18-hour light period to show the 
growth and acidity relations of typical short-day plants like cosmos and 
Tithonia when exposed to long days. The data in Tables XII and 
XIII, taken together, show that Helianthus annuus forms a connecting 
link between short-day and long-day plants, since it shows some of the 
characteristics of both types. 
72 
a . 
f - 
--0 
► ^ • ** 
1 
1 
1 
1 
1 
» 
LENGTH ^ 
OF PAY 
111111111 
1 11 1111 1 1 
-1 111 111 11 
© 
-! U-U 1 1 1 1 
_L 
23 
/5 
25 
4 
Fig. 9.—Showing the increase in level of active 
acidity of the apical structures in sunflower when 
the daily light period is increased to 18 hours. 
(Table XIII.) Under these conditions flower¬ 
ing is delayed, and in this respect as well as in 
the increased active acidity of the apex sun¬ 
flower approaches the behavior of typical short- 
day plants such as cosmos. Thus, sunflower 
may be regarded as a connecting link between 
the more typical short-day plants on the one 
hand andthe typical long-day plants on the other. 
Table XIII .—Showing the Ph value of cell sap of Helianthus annuus exposed to the 
natural length of day during the winter and early spring and to 18 hours of daily illumi¬ 
nation. 
PLANTS EXPOSED TO THE NATURAL LENGTH OF DAY. 
Date, 
| 
i 
Height. 
Hour of sampling. 
Ph values. 
Apical bud. 
Top of stem. 
Base of stem. 
Apical 
bud. 
Top of 
stem. 
Base of 
stem. 
Feb.23. 
Mar. 4. 
Mar. 14.; 
Apr. 6.! 
inches 
n J A 
17^4 
24K 
sM 
10.50 a. m. 
10.55 a - 
10.45 a - m - 
10.50 a. m. 
.do. 
1.15 p. m. 
1.30 p. m. 
1.55 p. m. 
2.55p.m. 
3.20 p. m. 
4 pm. 
7.04 
6.81 
06.44 
6.88 
6.99 
6.98 
6.91 
5-99 
6.09 
6.13 
6.18 
PLANTS EXPOSED TO 18 HOURS OF ILLUMINATION DAILY. 
inches 
Feb.23. 
20 
10.10 a. m. 
12.55 P- m - 
...... 
6.56 
5-78 
Mar. 4. 
31X 
9.50 a. m. 
1.05 p. m. 
2.55 p. m. 
6.82 
6.63 
6.10 
Mar. 14. 
45X 
9.55 a. m. 
12.15 p. m. 
2.20 p. m. 
6-73 
6.76 
6.16 
Apr. 6. 
86X 
9.45 a. m. 
12.35 P- ni. 
2.50 p. m. 
tt 6-5o 
6.65 
6.17 
° Young flower buds, i to xA inches in diameter. 
