154 
Journal of Agricultural Research 
Vol. XXVII, No. 3 
flowering. This form of development is associated with general increase 
in acidity. There appear to be some differences in detail, however, as 
to the acidity relations in the type of plant represented by summer 
radish, in which tuberization is a prominent feature, as compared with 
the type represented by Rudbeckia bicolor , in which tuberization ordi¬ 
narily does not occur. In radish, increase in the duration of the daily 
light period causes increased acidity in the leaf, and acidity is greater in 
the upper portion of the stem than in the lower even in the earlier stages 
of stem elongation. In Rudbeckia, increase in the light period causes 
decreased acidity in the leaf, and the upper portion of the stem is con¬ 
sistently less acid than the lower portion until after the blossom has 
unfolded. In both radish and Rudbeckia, however, the acidity of the 
upper portion of the stem increases progressively both before and after 
the appearance of flower buds. The acidity relations in the Rudbeckia 
type of plant when exposed to long days are strikingly similar to those 
of short-day plants when the latter are exposed to short days. In both 
instances the acidity of the stem increases moderately prior to the 
appearance of flower buds, and the reproductive structures themselves 
increase in acidity as growth proceeds, maximum acidity being found 
in the unfolded blossom. Moreover, the upper portion of the developing 
stem is less acid than the lower portion, in contrast with conditions in 
short-day plants when these are exposed to long days. 
There is a group of plants which occupy a position intermediate between 
the more typical short-day and long-day groups in their responses to 
differences in the length of day. This group shows some of the features 
of both short-day and long-day plants. Thus, in Helianthus annuus 
the time of flowering is not materially influenced by the natural range 
in length of day of spring and summer at Washington, but the stature 
attained is greatly affected. Increasing the daily light period to 18 
hours, moreover, causes considerable delay in flowering. Thus, under 
the relatively long days of summer this species shows much the same 
behavior as do short-day plants when they are exposed to short days or 
long-day plants when exposed to long days. It was found also that 
the same relationships hold as to acidity of the cell sap. On the other 
hand, when exposed to an 18-hour illumination period this sunflower 
approaches in behavior that of short-day plants when exposed to long 
days. Here, again, the rise in acidity of the sap resulting from the longer 
light period shows an approach toward the condition of high acidity 
found in the short-day plants when exposed to long days. This type of 
plant, therefore, forms a connecting link between the two groups of 
short-day and long-day plants. 
Preliminary studies, which are now being followed up with more 
extensive observations, indicate that changes in form of the carbohydrate 
content and in the degree of hydration of the tissues of the plant are 
among the earliest observable effects of change in the length of day to 
which the plant is exposed. Transfer of cosmos plants from a long to a 
short day resulted in a material increase in reducing sugar in the upper 
portion of the stem within 48 hours after the transfer had been made. 
Two days later the increase in sugar content was found to be in the form 
of polysaccharid, and apparently this was accompanied by a slight 
decrease in water content of the tissues. Twelve days later, after flower 
buds had appeared, the increased content of sugar was again in the form 
of monosaccharid. A notable increase in water content of the tissues 
also had taken place. In Biloxi soy beans exposed to natural length of 
