140 
Vol. XXVII, No. 3 
Journal of Agricultural Research 
under these circumstances a slight increase in acidity as compared with the 
plants remaining under the full length of day was found on the sixth day 
after the transfer. A week later, however, the plants exposed to the 
longer daylight period had become the more acid. Transfer of the plants 
to the io-hour day after the advanced vegetative stage had been reached 
had the effect of bringing about a transitory decrease in acidity about the 
fifth day after the transfer had been made. This was soon followed by a 
moderate increase in acidity prior to the appearance of the flower bud. 
5.0 
s.s 
e.o 
as 
7.0 
76 
s.o 
— ■ ■ 1 NATURAL LENGTH OF DAY. 
~ — NATURAL LENGTH OF PAT UNT/L UULY/2, WHEN 
TRANSFERRED TO /O-HR. DAK 
NATURAL LENGTH OF PAY UNT/L AUG. /7. WHEN 
TRANSFERRED TO /O-HR. DAK 
FLO, 
1 
1- 
i 
< _ 
FLOWS* 
P SUDS Fry , 
4 
Ayfl) . 
1 1 1 1 ! 1 1 iT 
1 l 
■ 
IIIIII.IM 
1 H II ! 11 1 
1 m 1 m M 1 
Lli.Ui.LLL 
■ l-LLLLLLLL. 
/2 22 
uult 
// 2/ 
AUG. 
3 / 
/o 
sept. 
20 
Fig. 6. —Showing the active-acidity relations in the uppermost portion of the stem in Tithoniarotundifolia 
when exposed to the long days of summer and to a io-hour day, respectively (Table VIII). These rela¬ 
tionships are very similar to those found in cosmos. Abrupt decrease in the light period causes a decrease 
in active acidity, followed by rise during the development of the flower bud. 
TabbF VIII .—Showing the Ph value of cell sap in topmost and basal portions of the 
stem in Tithonia rotundifolia as affected by the duration of the daily illumination 
|4 period 
Date of sam¬ 
ple. 
Plants exposed to natural length of day. 
Plants transferred from the natural length 
of day to a io-hour day. a 
Hour of sampling. 
Ph value. 
Hour of sampling. 
Ph value. 
! 
Top of stem. 
Base of stem. 
Top of 
stem. 
Base of 
stem. 
Top of stem. 
Base of stem. 
Top of 
stem. 
Base of 
stem. 
Tiilv 12 
8.45 a. m_ 
11.5s a. m— 
9.05 a. m.... 
9.10 a. m.... 
10.20 a. m... 
2.25 p. m- 
12.20 p. m... 
1.35 p. m.... 
7.64 
7. 78 
7-34 
6.87 
6.46 
6.78 
5 * 96 
5-63 
J lay xa .••«««* 
July 18. 
July 26. 
Aug. 16. 
Do 
1.10 p. m.... 
10.50 a. m... 
10.25 a. m... 
12.20 p. m... 
3.25 p. m- 
1.35 p. m.... 
3 P- m. 
I 7 - 79 
j 7.96 
| & 6.44 
\ c 6 .34 
6-57 
6.46 
6.32 
,6.43 
6 6. 36 
b 6. 23 
6. 26 
6.04 
5-74 
Aug 27 
T 7 .TO p. m. . . 
6.30 
6. 60 
6. 59 
6. 76 
1.25 p. m.... 
Aug ?9 
T .90 p. m. . . . 
12.20 p. m... 
pppf f\ 
T 9 .25 p. m... 
1.30 p. m.... 
Spnt tt 
^-<>5 p. m.... 
1.20p. m.... 
Spnt to 
3 p. m. 
Sppt O T 
1 p. m. 
a The data for the period July 18 to Aug. 16, inclusive, relate to plants permanently transferred from the 
natural length of day to a 10-hour day on July 12; subsequent data relate to plants similarly transferred 
on Aug. 17. 
b Flower buds about one-half inch in diameter, 
c Upper portion of flower bud stem. 
EFFECT OF THE TIGHT PERIOD ON ACIDITY RELATIONS IN TONG-DAY PTANTS 
In plants of this group exposure to relatively long days results in 
stem elongation, which is invariably coupled with flowering; that is, the 
condition of indeterminate elongation of the sterile vegetative stem is 
wanting. Exposure to relatively short days tends to inhibit stem 
