132 
Journal of Agricultural Research 
Vol. XXVII, No. 3 
The results recorded in Tables I and II, taken as a whole, are quite 
consistent and leave no doubt as to the conclusions to be drawn. There 
is a striking difference in the course of the acidity relations during the 
progress of development of the plant under the long-day and short-day 
conditions and these differences are clearly associated with differences in 
form of expression caused by the two conditions of illumination. The 
seat of maximum change in acidity is found in the region of the growing 
point. Under the longer daily illumination period the plants at the age 
of 2 to 3 weeks show a relatively low acidity, particularly in the apical 
structures, but with continued elongation of the stem there is a marked, 
progressive increase in acidity, the P H number for the apex declining 
from a value well above 6 to approximately 5.2, at which point it remains 
comparatively stationary (fig. 2). The advanced stage of vegetative 
development of the stem, therefore, is characterized by relatively high 
acidity of the apex as compared with early stages of develpment. The 
Fig. 2. —Showing the characteristic progressive increase in active acidity in the sap of the apical tissues of 
late-flowering cosmos when exposed to the long days of summer (Table I). Cosmos is a typical short- 
day plant and in this group indeterminate elongation of the vegetative stem under a long-day exposure 
is associated with a progressive increase in active acidity in the region of the apex. The relatively high 
active acidity of the advanced vegetative stage is maintained during the period of active vegetative 
growth. 
portion of the stem immediately below the apex shows greater acidity than 
the apex in earlier stages of development, but as growth proceeds this 
relationship is reversed. The base of the stem is initially the most acid 
portion of the plant, but shows only moderate subsequent change, and 
hence in later stages of growth tends to become the least acid part of the 
plant (fig. 3). 
When the plant is exposed to a 10-hour day from date of germination, 
which has the effect of promptly initiating flowering, there is an appreci¬ 
able but transitory decrease in hydrogen-ion concentration in the tissue 
fluids of the apex, which takes place about 2 weeks after germination. 
The probable significance of this phenomenon is later referred to more 
fully(p. 148.) This temporary decrease is followed by increase in acidity 
till the P H number of approximately 6 is reached, a level of acidity which 
is characteristic of the flowering condition and represents the acidity of 
the young flower buds themselves. The flower buds and adjacent tissues 
increase in acidity till the unfolding of the blossom is completed. This 
latter stage marks the culmination of the increase in acidity of the repro- 
