jan. i 9 ; 1924 Photoperiodism and Hydrogen-Ion Concentration 
143 
parts of the plants exposed to the shorter illumination period. The 
above results of the studies on acidity relations were confirmed by obser¬ 
vations on plants exposed to the full length of day of late spring and 
summer in comparison with plants exposed to a io-hour day, but it 
seems unnecessary to present the data in detail. In these latter experi¬ 
ments the plants germinated April 13. As late as June 7 the very young 
leaves of the rosettes in the plants exposed to a io-hour day showed a 
P H reading of 6.06 while the tops of the stems of the plants exposed to 
the natural length of day (nearly 15 hours), which were showing young 
flower buds, gave a reading of 5.57. By June 26 stem elongation was 
beginning in the plants under the io-hour day and the tops of the stems 
showed a P H reading of 5.67. 
EXPERIMENTS WITH RUDBECKIA BICOLOR. 
Seeds of this garden ornamental were planted in the greenhouse 
December 19 and on January 27 the seedlings were transplanted in flats. 
On February 6 one lot of the small seedlings was transferred from the 
natural length of day to a 7-hour day. A second lot was exposed to 
an 18-hour daily illumination, a 100-watt electric bulb being placed 2 
feet above the plants and the current turned on at sunset and turned 
off at midnight each day. On March 22 a portion of the plants which 
had been exposed to the 7-hour day were permanently transferred to 
the 18-hour day. Under the 7-hour day stem elongation was indefi¬ 
nitely inhibited, but under the 18-hour day development of the axis 
had begun within 2 weeks. Growth characteristics under the two light 
conditions are shown in plate 2,A. Of the plants under the 7-hour day, 
only the leaves, of course, were available for acidity studies. In samp¬ 
ling, all sound leaves excepting the smallest individual of the rosette 
were used in both the long-day and the short-day series of plants. The 
leaves of the plants exposed to the 18-hour day were decidedly longer 
than those of the plants under the 7-hour day. In sampling the stems 
of the plants under the 18-hour light period the entire stem was used 
when the length was less than 4 inches.. In all other cases 2-inch sec¬ 
tions were used. In sampling the flower bud the involucre was rejected. 
The results of the observations are summarized in part in Table X. 
Additional observations which need not be reported in detail have con¬ 
firmed the data here given. It may be added that samples of open 
blossoms and upper and lower portions of stems collected May 10 from 
plants which had been exposed to the 18-hour light period from February 
5 gave P H readings of 5.55, 5.41, and 5.68, respectively. 
The acidity relations of the leaf in Rudbeckia differ somewhat from 
those in radish under the two conditions of illumination. In radish 
the higher acidity occurred under the longer light period, while the 
reverse was true in Rudbeckia. Again, under the influence of the longer 
light period maximum acidity of the stem in radish is found in the upper 
portion even before the appearance of the flower buds. In Rudbeckia 
the topmost portion of the stem is consistently less acid than the basal 
portion up to and including the period of flower formation (fig. 7). 
This relationship is not reversed till the blossom has unfolded. In both 
radish and Rudbeckia, however, the acidity of the upper portion of the 
stem increases progressively prior to the appearance of flower buds, 
although in both the increase is more pronounced in subsequent de¬ 
velopment. 
