1074 
, Journal of Agricultural Research 
Vol. XXVIII, No. 11 
rate of hemp which is grown in a daily light exposure of 12 or more hours is con¬ 
tinued up to the time of flowering, which is about 90 days from planting, and is 
then increased during anthesis. As a result such plants attain a relatively great 
height. 
The minimum length of daily exposure to light required for continued growth 
of hemp in this experiment was 3 hours. In an exposure of 1 hour daily the plants 
were all dead in 20 days, and those which received light for 2 hours daily were all 
dead in 34 days from the start of the experiment. In these two exposures the 
growth was fairly rapid during the first 7 days but after this time it rapidly de¬ 
clined to almost zero. At the end of one week from the start these lots began to 
lose their rich green color and at the time of death the leaves were yellow. In 
3 hours of light daily the rate of growth was low and fairly constant. Weakened 
development was clearly evident in the few, small, greenish yellow leaves and the 
very few flowers. At the end of 34 days the mean height of the lot was only 11.5 
cm. as compared with 44 cm. in the 7-hour lot. It seems quite clear that it is 
not possible to grow hemp in a daily exposure to light which is short enough to 
prevent flower development and yet allow the plants to live. In this respect 
hemp belongs to the same class as Cosmos and Bidens. 
It is unfortunate that the analysis of growth in this experiment had to be dis¬ 
continued at the end of 34 days because it leaves the answers to several interesting 
points incomplete. The writer has not observed a difference between the growth 
rate of the sexes until near the time of flowering. With the approach of anthesis 
the staminate plants increase in height much more rapidly than the carpellate 
forms. After this point in the life history is reached there is therefore a differ¬ 
ential growth rate among the sexes. Since it is impossible to determine the'sexes 
in advance, there is no sure way of selecting equal numbers of each sex in each 
lot. In order to compare the growth rate from planting to maturity in different 
light exposures it would be necessary to determine the growth rate of each sex 
in each light exposure and use this information in working out a correction factor 
to be applied in cases where the sex ratios were not the same. So in an experi¬ 
ment of this kind with hemp there are two important factors to be considered— 
a differential growth rate of the sexes, and a second differential rate among the lots 
due to differences in time of flowering. 
EFFECT OF LENGTH OF EXPOSURE ON SEXUAL DEVELOPMENT 
Although marked alterations in the vegetative development were produced 
by various changes in the length of daily exposure to light, equally interesting, 
and of much greater importance to this investigation, were the effects on the 
reproductive processes. The length of time between planting and the appearance 
of flower buds increases as the length of exposure to light is increased from 7 
hours daily, and an exposure of less than 7 hours daily also delays the time 
of blooming. The graphical representation of the data in figure 3 shows the 
relation between the time of flowering and the length of daily exposure to light. 
The criterion of time of flowering used here is that time when the sexual nature 
of the flower buds can be determined with certainty. The actual opening of 
the flowers does not occur until several days later. In 3 hours of light daily, 
which is the minimum for growth to maturity in hemp, flower buds appear in 
about 46 days from date of planting. As the length of daily exposure to light 
becomes greater, the time required for reaching the flowering stage becomes less 
until an exposure of 7 hours is reached. From this point on, the time between 
planting and flowering increases rapidly until it is about 90 days in a 15-hour 
day. Since the experiment in which hemp was exposed to continuous illumina- 
