1072 
Journal of Agricultural Research 
Vol. XXVIII, No. 11 
Table II .—Effect of length of exposure to light on growth 
Exposure 
Date 
planted 
Height 
July 6 
Aug. 15 
5 hours _ _ ; 
May 13 
Meter 
0.22 
.46 
.65 
.71 
Meters 
0.47 
.67 
.90 
2.28 
7 hours. 
10 hours........ 
_do. 
Full day........... 
...do_ 
EFFECT ON VEGETATIVE DEVELOPMENT 
Although exact measurements of the details of vegetative development in the 
dark-house were not taken, the mean heights give a certain measure of the effect 
of day length on the rate and the extent of growth. Plants in the 5-hour 
exposure group grew slowly at first which accounts for the small amount of 
growth made by the time of the first measurements on July 6. The data show 
that the extent of growth in all lots is roughly proportional to the duration of 
daily exposure to light. This does not mean, of course, that the rate of growth 
is proportional to the length of the daily exposure to light. The second series 
of measurements which were taken on August 15, show the same general effect 
of day length on height as the first measurements; the plants in the 5-hour day 
had the lowest mean height and those in the full daylight had the greatest mean 
height. Although it is realized that such results are not critical, they are very 
suggestive and agree with the work of Garner and Allard (8) who found that the 
increase in height of soy beans is proportional to the length of daily exposure to 
light. 
Since the length of the growing period was greatly reduced by the accelerating 
action of the short exposure to light on the reproductive processes in the case of 
the plants which were grown in a short day, the amount of growth made by these 
lots was necessarily somewhat limited. The r61es played by the rate and the 
extent of growth, respective^, is a question which can not be answered from the 
data obtained in the dark-house. Another carefully controlled experiment has 
yielded results which show very clearly the relation between rate and extent of 
growth in hemp. Nine lots of 8 plants each were exposed to light daily for 1 
hour, 2 hours, 3 hours, 4 hours, 5 hours, 7 hours, 12 hours, 18 hours, and 24 
hours, respectively. Healthy seedlings of about the same height were selected 
and the treatment begun at the time of first potting. All seedlings were dark green 
in color and the mean height of each lot was 5 cm. The growth of each lot is 
represented graphically in figure 2. The rate of growth is not constant at all 
times but varies considerably at different periods. All lots began with a fairly 
low rate which gradually increased but this increase was not the same in all lots 
and it is due to this differential increase in growth rate that the extent of growth 
is not the same in all exposures at the end of a certain length of time. The lot 
which received 24 hours and that which received 18 hours of light, respectively, 
grew at so nearly the same rate that the data are represented by one curve. 
Their rate of growth was high from the start and it is due to the rapid growth 
during the first 10 days of the experiment that the height attained is greater 
than that of any of the other lots. Plants which were exposed to light for 12 
and 7 hours daily were correspondingly slower in attaining a growth rate 
comparable to that of the 24-hour lot and their mean heights at the end of 34 
days was correspondingly less. 
