jan. i9,1924 Photo periodism and Hydrogen-Ion Concentration 
145 
ACIDITY RELATIONS IN PLANTS OCCUPYING AN INTERMEDIATE PO¬ 
SITION BETWEEN THE MORE TYPICAL SHORT-DAY AND LONG-DAY 
PLANTS 
There is a group of plants which may be regarded as standing between 
typical short-day and long-day plants in their response to differences in 
length of day. In these plants the height attained increases with increase 
in the daily illumination period, but the condition of indeterminate elon¬ 
gation of the vegetative stem is not readily attained under relatively long 
days, nor is stem elongation readily and completely inhibited by expos¬ 
ure to short days. Helianthus annuus L. is a representative of this 
group. The range in length of day during the open growing season at 
Washington greatly affects the height attained by this sunflower but does 
not materially influence the time required for flowering. On the other 
hand, a more extreme change in the light period causes appreciable 
change in time of flowering and, of course, further accentuates differ¬ 
ences in height of the stem. The effect of the light period on the height 
of the plant is shown in plate 2, B. 
A series of plantings of sunflower were made at intervals of two weeks 
through the summer months. Observations were made on heights at¬ 
tained by the plants, the dates of flowering and the hydrogen-ion concen¬ 
tration of the cell sap at different stages of development. The data on 
height attained and time of flowering are shown in Table XI. The 
maximum height was reached by the first two plantings, but unfortun¬ 
ately exact data were not secured on these plantings. The plantings 
made after August 25 failed to flower because of onset of cold weather. 
Table XI.— Number of days from germination to flowering and final height attained by 
Helianthus annuus when planted at two-week intervals during the summer months 
Date planted. 
June is 
June 20 
July 13- 
July 27. 
Aug. 11. 
Aug. 25. 
Date of 
germination. 
Date of first open 
blossom. 
Number 
of days 
from ger¬ 
mination 
to flower¬ 
ing. 
June 29. 
luly «:. 
Aug. 19. 
...do. 
51 
45 
49 
48 
45 
52 
July iq. 
Sept. 6. 
Aug. 2. 
Aug. 18. 
Sept. 2. 
Sept. 19. 
Oct. 2. 
Oct. 24. 
Average 
height of 
plants at 
time of 
flowering. 
Inches. 
66 
60 
So 
42 
Results of observations on acidity relations in the different plantings 
at given stages of development and of individual plantings at various 
stages of development are shown in Table XII. For the leaf samples 
the three topmost leaves, exclusive of the embryonic leaver of the bud, 
were used. A 2-inch section of the stem immediately below the apical 
bud and a similar section immediately above the ground level were used. 
The data are consistent and establish several points of interest. Under 
the seasonal range in length of day at Washington the response of this 
sunflower closely coincides with that of cosmos and Tithonia when these 
are exposed to short days. Exactly the same relationship holds with 
respect to acidity of the cell sap. In the first section of Table XII data 
are given for seven different plantings of sunflower at 2-wxek intervals, 
and it is apparent that the seasonal difference in length of day does not 
materially affect the acidity of the plant during the first 3 weeks of growth. 
