Jan. 19.1924 
Photoperiodism and Hydrogen-Ion Concentration 
121 
composition of light, there are no satisfactory data to show whether in 
any particular species the process proceeds at the same rate and in the 
same manner with a daily illumination period of, say, 15 hours as with 
a period of 10 hours. The same is true of transpiration, respiration, and 
various other plant processes of fundamental importance. The present 
paper deals with fairly extensive studies of hydrogen-ion concentration 
of tissue fluids and some preliminary data on carbohydrate content and 
water relations as affected by the length of the daily light period, and thus 
associated with alternative forms of expression. 
WATER RELATIONS IN PHOTOPERIODISM 
Thus far only a beginning has been made in direct experimental study 
of the influence of the light period on water relations in the plant, but in 
connection with the other studies discussed in this paper it seems desirable 
to direct attention to observed plant reactions which suggest a definite 
relationship between the length of the illumination period and the water 
content of the plant. That maintenance of maximum turgidity is in 
some way favored by a light period which is optimal for increase in stature 
is illustrated by the fact that in certain cases transfer from such light 
conditions to those which are suboptimum for increase in stature quickly 
results in a change from the upright toward the horizontal or prostrate 
position of the stem, although growth may be maintained. In other cases, 
this change in the light relations causes dying back of the stem, followed 
by development of new basal shoots. Moreover, increased pubescence 
may be a feature in the change from an optimal to a suboptimal 
light period for upward stem growth. These phenomena and other 
features of development commonly seen under relatively xerophytic 
conditions are observed when the plants are abundantly supplied with 
water and even under fairly wide ranges in the external water supply. 
It is well known that as a rule exposure to sunlight greatly increases 
transpiration and it seems possible that in those species for which an 
intermediate length of day is optimal for growth the check in rate of 
growth resulting from exposure to long days is due at least in part to 
excessive transpiration. For other species, however, the rate of increase 
in stature is more or less proportional to the length of the daylight period 
and loss of turgidity resulting from decrease in the daily light period 
would seem to be due to changes in internal conditions of the plant. 
From the investigations of Briggs and Shantz (4, 5) and Livingston (14), 
together with the work of earlier investigators, it might be assumed that 
in the first case the excessive transpiration (if such actually occurs) is 
probably due chiefly to external evaporative forces. In those species 
in which maximum turgidity is maintained by exposure to a long daily 
illumination period internal factors are apparently involved, for it is not 
clear as to how reduction in the period of illumination could through 
direct action cause decrease in turgidity as a result of excessive trans¬ 
piration. According to Livingston and Brown (*5), exposure to sunlight 
may result in incipient drying of the foliage leaves, which in turn may 
tend to check transpiration to some extent by causing partial closure 
of the stomata and decreased partial pressure of water vapor in the 
leaf. There seems to be no ground at present, however, for supposing 
that increased duration of the light period would tend to accentuate the 
effectiveness of these factors in checking transpiration to the point of 
overcoming external evaporative forces. Changes in osmotic relations, 
