Jan. i9,1924 
Photoperiodism and Hydrogen-Ion Concentration 
139 
seeds show a much lower and decreasing acidity till about the time 
maximum size is attained. During the later stages of ripening the seeds 
appear to increase somewhat in acidity. 
Table VII.— Showing the Ph value of cell sap of seed pods and seeds in the Peking variety 
of soy beans at different stages of development 
Date of sample. 
Aug. 31. 
Sept. 1.. 
Do. 
Sept. 9. 
Sept. 19. 
Sept. 28 
Oct. 5.. 
Oct. 7.. 
Oct. 28. 
Material. 
Entire seed pods, less than inch long. . . 
Entire seed pods, about 1 inch long. 
Entire seed pods, about 1 % inches long.. 
Entire seed pods, about 1^ inches long.. 
Hulls only, to i )4 inches long. 
Seeds only. 
Hulls only, i }4 to 1% inches long. 
Seeds only, just beginning to turn red 
Seeds only, full size, seed coats red in color 
Seeds only, black, nearly dry. 
Seeds only, dry and mature. 
Ph value. 
6.29 
6.09 
6. IO 
6.24 
5-94 
6.63 
5-9 1 
6.83 
6.63 
6.33 
6.34 
EXPERIMENTS WITH MARYLAND MAMMOTH TOBACCO 
This variety of tobacco behaves like the late varieties of cosmos and 
soy beans in response to length of day, and observations made on acidity 
relations gave results similar to those already presented for cosmos and 
soy beans. Typical P H values obtained for the terminal bud and the 
topmost and basal portions of the stem, respectively, are 5.25, 5.53, and 
5.30 during the vegetative stage of the plant, and 5.46, 5.51, and 5.29 just 
after the flower bud became visible. The acidity gradients of the stem 
under the two conditions are similar to those obtained for cosmos, though 
not quite so pronounced. 
EXPERIMENTS WITH TlTHONIA ROTUNDIFOUA (MIEE) BEAKE 
This tropical weed attains giant proportions without flowering when 
grown during the summer months in the latitude of Washington, but 
flowering is readily induced by shortening the length of day. Studies 
were made on the acidity of the cell sap as affected by the daily light 
period. Seed were planted May 16 and the seedlings transferred to boxes 
on June 20. One series was exposed to the natural length of day through¬ 
out the test, a second series was permanently transferred from the natural 
length of day to a io-hour day on July 12, and a third series was similarly 
transferred to a io-hour day on August 17. Determinations of hydrogen- 
ion concentration in the topmost and basal portions of the stem were made, 
using for the purpose sections 1 % to 2 inches in length. Under the natural 
length of day no flower buds could be seen as late as October 9, while 
they were first visible July 26 in the plants transferred to the io-hour day 
on July 12 and were first seen on September 6 in the plants similarly trans¬ 
ferred on August 17. The results of the acidity measurements are sum¬ 
marized in Table VIII. Under the long days of summer the acidity of 
the stem increased steadily up to August 22 but thereafter the decreasing 
length of day caused progressive decrease in acidity (fig. 6). The first 
transfer of plants to the short day was made before the acidity had in¬ 
creased to that associated with the advanced vegetative condition, and 
