jan. 19,1934 Photoperiodism and Hydrogen-Ion Concentration 
*35 
uppermost were used; for lower leaves the oldest available were taken, 
some of which had begun to yellow. The upper and lower leaves show 
much the same acidity relations as the corresponding portions of the 
axis and in all cases the acidity is lowest under the io-hour day. The 
acidity of the fibrous roots is about the same as that of the base of the 
stem. 
During the short days of winter electric light of intensity as low as 5 foot- 
candles used to prolong the day length is effective in preventing flowering 
in cosmos, and the observations recorded in Table IV (A) indicate that 
the acidity relations characteristic of the more advanced vegetative con¬ 
dition are thereby maintained. After flowering has been induced in 
cosmos by exposure to short-day conditions a return to the vegetative 
condition, involving phemomena of rejuvenescence, is eventually effected 
by exposure of the plant to long-day conditions. Cosmos which was 
planted in the greenhouse March 11 formed flower buds in response to 
the short seasonal length of day. The first blossom opened May 8. By 
May 21 it was evident that as a result of the increasing length of day the 
plants were passing definitely into the vegetative condition. Observa¬ 
tions on these plants are recorded in Table IV (B). Seedlings of cosmos 
which germinated May 15 under a io-hour day were showing flower buds 
by June 3. On June 13 these plants were placed permanently out of 
doors. By June 28 evidence could be seen of the change to the vegetative 
condition, though flowering had not entirely ceased. Transition stages 
in this type of rejuvenescence are shown in Plate 1, B. Observations on 
these plants are given in Table IV (B). It is obvious that change from 
short-day to long-day conditions after flowering has been initiated 
results in establishing the relatively high acidity of the apical structures 
which is characteristic of the advanced vegetative condition. 
EXPERIMENTS WITH BIEOXI SOY BEANS 
The type of growth in soy beans differs markedly from that in cosmos 
in that stem elongation is decidedly reduced, there is much branching, 
and development of foliage in proportion to stem is greater. Late 
maturing varieties of soy beans, however, show much the same responses 
to length of day as does late cosmos. For study of acidity relations the 
young, topmost leaves averaging 1 to 1 %. inches in length were commonly 
used. As the flowering condition is approached the formation of new 
leaves is checked, so that in such cases, while the leaves taken for study 
were of the usual size, they were probably somewhat older than those 
gathered during the more active vegetative period. In a series of 
observations on plants growing in the field during the months of June 
and July, which will not be reported here in detail, it was found that the 
acidity of the sap from the young, topmost leaves remained close to the 
P H value 6.50 from the earliest stages of growth up to the approach of 
the flowering condition, while thereafter the acidity increased up to a 
maximum of P H 5.97 after seed pods had appeared. Larger, more 
mature leaves showed a somewhat higher acidity. The acidity gradient 
of the stem and root during the vegetative period was found to be quite 
similar to that in cosmos. Typical P H values found for top and base of 
stem and fibrous roots are 5.97, 6.17, and 6.46, respectively. As flowering 
buds came into evidence there was a temporary rise in P ? value in the top 
of stem to about 6.25, followed by a return toward previous values as the 
flowers unfolded. 
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