9io 
Journal of Agricultural Research voi. xxm, No.» 
LIGHT PERIOD IN RELATION TO OTHER FACTORS OF THE 
ENVIRONMENT 
To arrive at a satisfactory conclusion as to the practical significance of 
the seasonal range in length of day in plant growth it is essential, of course, 
to know to what extent other environmental factors are likely to modify 
the action of the length of day. Because of the unvarying regularity 
of the annual cycle in day length it is apparent that any influence of this 
factor on plant behavior will find expression in the normal or usual sea¬ 
sonal periodicity rather than in unusual or occasional responses. In 
short, the change in length of day may be expected to bring about the 
usual rather than the unusual in the annual cycle of the plant’s activities. 
On the other hand, occasional severe drought, abnormally high or low 
temperatures, unusual chemical conditions of the soil, attacks by insects 
or diseases, may cause temporary disturbance either qualitatively or 
quantitatively in the usual seasonal development of the plant. 
The question of chief interest in the present connection is the extent to 
which the normal sequence of seasonal changes in other climatic factors 
may modify or nullify the influence of length of day on periodicity in 
plant activity. That various climatic factors, as well as the factor 
spoken of as soil fertility, may produce quantitative differences is known 
to all. To what extent the actual annual range in these factors may 
cause distinct qualitative differences in expression is not so well under¬ 
stood. In connection with the present investigations a limited number 
of observations have been secured with certain species on the significance 
of climatic factors other than change in the light period in phenomena of 
plant periodicity. 
As is well known, the seasonal change in the lengeh of day is accom¬ 
panied by important changes in the intensity of the sunlight and in its 
quality, that is, in the energy distribution of the visible portion of the 
sun’s spectrum. Hence, it is of special interest to consider these factors 
in relation to photoperiodism. In the earlier paper considerable data 
were presented (7, p. 581 et seq .) tending to show that reduction of the 
intensity of the sunlight in summer to as low as one-third the normal 
had no appreciable effect on the time of flowering of soy beans. These 
results have been confirmed in the experiments referred to on page 905. 
As a part of the tests a long, rectangular frame, with sides and top covered 
with cheesecloth, was erected over a portion of a steel track leading into 
the dark house so that a truck carrying one lot of the plants could be 
rolled under the shade each day while a second lot was exposed to the 
direct sunlight. A series of measurements of relative light intensity 
within and outside the shade from 10 a. m. to 3 p. m. on July 27 were 
made by R. A. Steinberg, of this office. For the purpose a Sharp-Miliar 
model F photometer, tested by the United States Bureau of Standards 
and found to be accurate to between 5 and 10 per cent, was used. In 
round numbers the average intensity thus obtained for the period indi¬ 
cated was 10,500 foot-candles in the open and 3,500 foot-candles under 
the shade. The results of this test of the comparative effectiveness of 
light duration and light intensity are given in Table VIII. The maximum 
effect of a reduction in light intensity to one-third the normal, with a 
light period of 10 hours, was to delay the date of first flowering by one 
to two days. In marked contrast, a reduction in the length of the daily 
light period from the average natural summer day length of about 14J? 
hours to 13 hours, a decrease of about 10 per cent, promptly initiated 
