Journal of Agricultural Research 
Vol. XXIII, No ii 
916 
excess of the optimal for stem growth is the laying down of flower buds. 
It is possible, also, that resting vegetative buds of woody perennials 
result from these conditions, although this has not been definitely deter¬ 
mined. It has been shown.that bulb formation commonly results as 
one of the final effects of an illumination' period of excessive duration. 
No example of death from this cause has yet been encountered. 
These outlines of the series of responses initiated by graded departures 
from the optimal light period for stem growth are necessarily tentative. All 
responses are not included, and it is possible that the proper sequence of 
those which have been given will ultimately require modification in some 
instances. 
' NATURE OF THE REGULATORY ACTION OF LENGTH OF DAY 
Definite conclusion as to details of the internal processes concerned in 
the phenomena of photoperiodism is withheld, pending the completion 
of fairly extensive biochemical investigations now in progress in this 
office. For the present only the more general aspects of the problem as 
brought to light in the response of the plants as a whole will be considered. 
These various forms of response seem to indicate clearly that in some way 
the internal water supply, the degree of hydration, of the living cell 
content is subject to very delicate regulation by change in the ratio of 
the number of hours of light to the number of hours of darkness in the 
daily period of 24 hours. It will be observed that there is an optimal 
light-darkness ratio for maximum rate of stem growth, and Chandler (5) and 
Reed (19) have shown that maximum growth is correlated with a dilute 
cell sap or high degree of turgidity. In recent years MacDougall (14) 
has especially emphasized the significance of hydration in growth pheno¬ 
mena. The various responses of the plant coming into expression as the 
light period becomes progressively less favorable for upward stem growth 
are practically without exception those more or less associated with 
relatively xerophytic conditions. Tendency toward increased flowering 
and fruiting, increased branching, pubescence, abscission, and leaf fall, 
the general tendency toward stem shortening, tuberization, and increased 
underground stem development are phenomena in point. 
The behavior of Viola papilionacea under different light periods fur¬ 
nishes direct evidence on this subject. During the shortest days of 
winter the plants are in a semidormant condition, the young leaf buds 
becoming tuberized through loss of power to elongate the leaf petiole. 
Under the intermediate day length of spring leaves with short petioles 
and blue blossoms with relatively short stems make their appearance. 
During the long days of summer the petioles of the leaves are greatly 
lengthened. When plants bearing the summer type of leaf are abruptly 
transferred to short-day conditions partial collapse occurs within two 
or three days, obviously due to loss of turgidity in the leaf stem. In 
due course, also, the characteristic spring type of blue blossoms reap¬ 
pears. This behavior is shown in Plate 18, B. 
Without at this time raising the question as to the exact mechanism 
of the internal processes affected, it seems possible that highly refined 
regulation of the degree of hydration of the protoplasm furnishes a 
basis for the various responses of the plant to changes in the light period. 
It is believed, however, that this regulation is effected primarily through 
direct action on the internal mechanism which maintains hydration 
rather than by merely influencing external factors which modify relative 
