122 
Journal of Agricultural Research 
Vol. XXVII. No. 3 
permeability of the protoplasmic layer and the hydration capacity of the 
hydrophilic colloids, might be regarded as possible factors in the reduced 
water-holding capacity of the plant tissues when the plant is exposed to 
illumination periods which are too short for maximum rate of increase 
in stature. 
Prompt initiation of sexual reproduction when certain species are ex¬ 
posed to a reduced daily illumination period is perhaps the most striking 
phenomenon of photoperiodism, and in the case of the violet, at least, this 
response has been shown to be associated with partial loss of turgor. 
Here again is seen a possible relationship with the well-known fact that 
flowering and fruiting are favored by conditions of comparative drought, 
as has been especially emphasized by Mobius ( 20 ). That darkening spe¬ 
cies of this type for several hours in the middle of the day fails to initiate 
flowering possibly may be due in part to resultant check in rate of trans¬ 
piration, thus enabling the plant to maintain higher turgidity. There 
is the possibility that the reduced rate of growth under these conditions 
is explainable on the basis of reduced photosynthesis. 
There can be no doubt that the water relations of the plant are pro¬ 
foundly affected by the duration of the daily illumination period, but it 
is impossible to state whether change in water content stands in a direct 
causal relation to observed responses of the plant or merely follows as a 
sequel to other internal processes more directly controlled by the light 
period. 
ACIDITY RELATIONS IN PHOTOPERIODISM 
The theory advanced by Liebig (15) that organic acids are intermediate 
products in the photosynthesis of carbohydrate has not met with favor 
among plant physiologists and it has come to be the generally accepted 
view that these acids result from partial oxidation of carbohydrate and 
fat. While formation of organic acids thus seems to be more directly 
related to respiratory activity than to photosynthesis, there is no doubt 
of the fact that light is in some way a dominant factor in the origin of 
these catabolic products, as will be developed more fully in the present 
discussion. Some of the acids formed are neutralized by mineral 
bases derived from the soil, and to the extent that insoluble salts are 
formed these tend to accumulate in the plant tissues. According to de 
Vries (25), however, the total quantity of acids formed in a period of 100 
days may greatly exceed the total dry weight of the leaf, so that obviously 
the greater portion of the acids formed must undergo decomposition, 
the final products being carbon dioxide and water. It is generally 
believed that light is a very important factor in the breaking down of 
organic acids in the plant cell. It is apparent that the total acidity at 
any particular moment is dependent, on the one hand, on the rate of 
formation of acids and, on the other hand, on the rate of their decomposi¬ 
tion and, to some extent, perhaps, on the quantity neutralized by bases 
derived from the soil. In addition to light, temperature has been found 
to be one of the important external factors influencing relative rate of 
formation and decomposition of acid, high temperatures favoring net 
decrease in acidity. 
With respect to comparative total acidity of different organs of the 
plant and of particular organs at different stages of development the 
most extensive previous observations are those of Astruc (1) who inves¬ 
tigated the total free acid content of a number of species. This inves- 
