Jan. 19,1924 
Photoperiodism and Hydrogen-Ion Concentration 
129 
stems when exposed to long days but quickly flower when exposed to 
short days. Late-flowering varieties of Cosmos bipinnatus Cav., Biloxi 
soy beans, the Maryland Mammoth variety of tobacco (Nicotiana taba- 
cum L-), and Tithonia rotundijolia (Mill) Blake are examples of this 
group. In this type of plant the height attained is usually proportional 
to the duration of the daily light period, whether or not flowering occurs. 
In most cases, however, it is not possible to completely eliminate stem 
elongation by shortening the light period, so that only the two first- 
named forms of development need be considered. A rather extreme 
illustration of these two contrasted forms of development in Bidens 
jrondosa L., which behaves in the same manner as Cosmos bipinnatus, is 
shown in plate 1, A. 
EXPERIMENTS WITH COSMOS BIPINNATUS 
Late-flowering white cosmos was planted in boxes on May 2 and had 
germinated on May 6. One lot of plants received only 10 hours of illu¬ 
mination daily from the beginning of the experiment, a second lot was 
exposed to the natural length of day up to and including July 11 and 
thereafter was exposed to 10 hours of illumination daily, while a third 
lot was exposed to the natural length of day throughout the test. Very 
small flower buds could be seen on the plants of the first lot by June 1 
and on the second lot by July 24, while no flower buds were visible on the 
third lot of plants till September 19. The hydrogen-ion concentration of 
the cell sap of the apex and the topmost and basal portions of the stem 
under the two conditions of illumination is shown in Table I. The ma¬ 
terial designated as “apex” includes the growing point, the extreme tip 
of the stem and the attached leaflets. For “top of stem” a section 
to 2 inches long immediately below r the apex was used and a section im¬ 
mediately above the ground level of the same length was employed as 
“base of stem.” The data presented in this table include observations 
at fairly wide intervals for the plants exposed to a 10-hour day from the 
outset and for the earlier stages of development of the plants exposed to 
the natural length of day. More frequent observations are given for the 
plants transferred from the natural length of day to the io-hour day after 
considerable growth had already been attained and for later stages of 
development of the plants under the full length of day of summer. In 
order to complete the data, a second series of observations was made in 
such way as to supplement the data of Table I. For this purpose a sec¬ 
ond planting of the white cosmos was made on June 5 and the seed had 
germinated on June 8. One series was grown from the beginning under 
a io-hour day and a second series was exposed to the full length of day of 
summer throughout the test. Under the shortened length of day flower 
buds were found on June 30, but none appeared during the test under the 
long-day conditions. The observations were made in the same manner 
as before, and the results are shown in Table II. 
