J 44 
Journal of Agricultural Research 
Vol. XXVII, No. j 
Table X .—Showing the Ph 'value of the cell sap of Rudbeckia bicolor exposed to a 
7 -hour day and an 18 -hour day 
Date of 
sample. 
Plants exposed to 
a 7-hour day. 
Plants exposed to an 18-hour day.® 
Hour of 
sampling 
leaf. 
Ph 
value 
of leaf. 
Height 
of stem. 
Hour of sampling. 
j Ph values. 
} 
Deaf. 
Top of 
stem. 
Base of 
stem. 
i 
Deaf. 
! Top of 
1 stem. 
Base of 
stem. 
Feb.24. 
Do. 
12.40 p. m. 
6.62 
Inches . 
4 
6 
10 
17 
2 p. m. 
3 P- m. 
3.40 p. m.. 
i 
6 .93 
! 6 7 - 69 
j ? 7*32 
6 7.43 
7.89 
7.72 
7-66 
7.24 
Feb. 26. 
Do. 
10.05 a * m. 
6.61 
11.10 a. m. 
1.05 p. m.. 
2 p. m_ 
3.10 p. m.. 
6.94 
7.30 
10.30 a. m. 
11.40. 
11.50 a. m 
7*03 
7.04 
6-55 
Mar. 10. 
12.55 P* m 
Mar. 19. 
Mar. 21. 
10.10 a. m. 
6.58 
12.40 p. m. 
1.40 p. m.. 
3.40 p. m.. 
c 6.48 
Apr. 12. 
7 
18K 
20 
9.55 a. m.. 
12.55 P- m. 
10.55 a - m - 
11.10 a. m. 
7.17 
6.63 
6.27 
5-91 
5.82 
5-79 
Apr. 23. 
Apr. 27. 
11.25 a - m. 
9.50 a. m .. 
1.55 p. m.. 
12.45 p. m. 
e 6* 37 
c 6.01 
® The data for the period Feb. 24 to Mar. 19, inclusive, relate to plants exposed to 18 hours of light daily 
from the beginning of the experiment; the data for the period Apr. 12 to 27, inclusive, relate to plants exposed 
to the 7-hour day till Mar. 22 and thereafter exposed to the 18-hour day. 
ft Entire stem. c Flower buds. 
J.O 
S.5 
6.0 
/0-BOOR DAP 
FROM OUTSET 
FROM 7-/SR TO/S -//R 
/WK MARCO 22 
6.S 
7.0 
7.S 
3.0 
- A -\ 
f . 
BASE i 
0 —* 
BASE O 
% 
\ 
b- 
A 
\offr 
of $> 
iUsV 
TOP 
Till l.lil 1 
IJJ.J l.liLll 
111111 111 
1111 in n 
II1 11IIII 
11111II 11 
27 
It is of considerable interest to compare the acidity relations in Rud¬ 
beckia, as representative of the group of long-day plants which do not 
ordinarily become tuberized, with the acidity of the short-day plants. 
The course of devel¬ 
opment of long-day 
plants when exposed 
to relatively long 
days is more or less 
similar to that of 
short - day plants 
when exposed to rel¬ 
atively short days. 
By comparing the 
acidity in the stem 
of Rudbeckia under 
the 18-hour day 
(Table X) with that 
of cosmos (Table II) 
and Tithonia (Table 
VIII) under a io- 
hour day it will be 
apparent that in gen¬ 
eral the acidity rela¬ 
tions are very simi¬ 
lar. There is in all cases an increase in acidity of the stem till the flower bud 
appears. The flower buds themselves increase in acidity as growth pro¬ 
ceeds, maximum acidity being found in the unfolded blossom. In distribu¬ 
tion of acidity the upper portion of the stem is in all cases less acid than the 
lower portion, in contrast with conditions in the short-day plants when 
exposed to long days. Radish, as representative of the group of long- 
day plants which commonly become tuberized under short days, seems 
to differ somewhat from short-day plants (when exposed to short days) 
and the Rudbeckia group of long-day plants in distribution of acidity in 
the stem, particularly in the earlier stages of stem development. 
26 8 /8 28 7 /7 
FEB. MARCtf AFR/L 
Fig. 7.—Showing the effect of abrupt increase in the daily light period 
on the active acidity of the cell sap in the stem of Rudbeckia bicolor, 
which is a typical long-day plant (Table X). Increase in the light 
period from 7 hours to 18 hours initiates elongation of the axis which 
is followed by flowering. In this group of plants the upper portion of 
the stem is lower in active acidity than is the base, a condition which 
is found in short-day plants only during the early stages of develop¬ 
ment and in the initial stages of reproductive activity. Thus, in the 
typical long-day plants the level of active acidity characteristic of the 
flowering stage is attained by an increase rather than a decrease in 
acidity of the stem apex. 
