Jan. 19.1924 Photoperiodism and Hydrogen-Ion Concentration 
141 
elongation and flowering, thus resulting in the leaf-rosette type of 
development, either with or without tuberization. Radish (Raphanus 
sativus L.) may be taken as an example of this type in which a short 
daily illumination period results in marked tuberization without stem 
elongation, while Rudbeckia bicolor L- may be taken to represent the type 
in which tuberization does not result under these conditions. 
EXPERIMENTS WITH SUMMER RADISH 
Radish of the horticultural variety Scarlet Globe was planted in 
wooden boxes and galvanized-iron cans in the greenhouse on September 
29 and had germinated on October 3. One lot of plants was exposed 
to the natural length of day while the other lot received, in addition, 
illumination from 50-watt electric bulbs without reflectors, placed 
about 18 inches above the plants. The electric lights were turned 
on at sunset and turned off at midnight, the plants thus receiving about 
18 hours of illumination daily. By December 6 development of the 
primary axis had begun in 46 individuals, or 82 per cent, of the plants 
exposed to the 18-hour day, and under the natural length of day 9 plants, 
or 17 per cent, of the total were developing a stem. Of the two lots 
the plants under the longer illumination period were much the taller. 
While the natural length of day of winter at Washington is not short 
enough to permanently inhibit stem elongation in this radish, the process 
is much delayed and restricted. 
Results of observations on the hydrogen-ion concentration of the sap 
of the plants under the two conditions are shown in Table IX. For the 
leaf material the entire midribs of the larger leaves were used after 
removal of the lamina. In sampling the tuber or thickened root approxi¬ 
mately the upper third was taken for the first sample and for the second 
sample a central horizontal section constituting approximately a third 
of the total was used, the lower third with tap root being rejected. In 
sampling the primary stem, in the case of the plants exposed to the longer 
illumination period, the topmost section of 1 inch in length and a section 
one-half inch in length immediately above the thickened root were used. 
For the samples taken on December 15 plants were used which had 
developed stems 9 inches high, while the plants used four days later 
had stems only 6 inches high. The plants used January 4 had developed 
stems 32 inches high which were showing flower buds. It may be added 
at this point that unopened flower buds collected January 5 from some¬ 
what further advanced plants showing open blossoms gave a P H reading 
of 5.51. In the case of plants exposed to the natural length of day only 
those were used which showed no evidence of developing flowering stems. 
In this instance the acidity values corresponding to those of the top of 
the stem in the plants under the lengthened light exposure relate to the 
tissues in the region of the growing point immediately above the thick¬ 
ened root after removal of tlie leaves of the rosette. The data in Table 
IX show that under the relatively short daily illumination period, which 
is unfavorable for stem elongation, the acidity of the plant as a whole is 
relatively low and is lower in the region of the growing point than else¬ 
where. Under the longer light period the acidity of the plant is higher, 
especially in the upper part of the stem, which increases in acidity as 
growth proceeds. It may be added here that while the average weight 
of tops of the plants under the longer daily illumination was much greater, 
the average weight of roots was decidedly less than the corresponding 
