456 
Journal of Agricultural Research 
Vol. XXVIII, No. 5 
The variations in the growth of the plants receiving a 10-hour day length 
would seem to indicate that, in addition to length of day, some other factor or 
factors influenced growth, such as difference in atmospheric moisture or tempera¬ 
ture at the different seasons, or difference in the quality of sunlight, the sun being 
much higher in the sky at the hour at which the plants were brought from the 
dark chamber in summer. 
Two factors account for the difference in height, namely, length of internode 
and number of nodes. In 22 of the 24 instances, the internodal length of the 
control plant exceeded that of the treated. In the graph illustrating this (fig. 
7), the lines do not cross except in the last group, the measurements being for 
the period of shortest day length. That the difference was very pronounced for 
growth made in the period of greater day length was demonstrated by the growth 
of plants in cans No. 1 to 18, inclusive. In the case of plantings in cans No. 8 
PATE AfEASttXEP 
CAN A/a. 
NUMBEK OF WEEKS 
FKOM PLA/V77N6 
ON PATEMEASOAEP 
Fig. 6.—Height of tallest plant in cans No. 1 to 24, A and B series. A, day length reduced to 10 hours 
B, normal days 
to 18, inclusive, made between the last of March and the middle of August, the 
control in each instance had more nodes than the treated plant, the average 
difference between control and treated being 2.8 nodes per plant. The others 
varied, their total measurements averaging only 0.7 node in difference between 
control and treated. In leaf measurements, the midrib of plants in the B series 
was longer than was true in case of the A series in 23 of the 24 instances, plants 
in can No. 19 being the exception. 
Measurements were made of the fifth expanded leaf below the apex, this leaf 
presumably having developed, in most instances, approximately 4 to 6 weeks 
previously. Measurements of the midrib, made in May and August, showed a 
difference in length of 46 and 53 per cent, respectively, between the A and the B 
series. In the November measurement this difference was a trifle less pronounced 
amounting to 42 per cent. In the January measurements of leaves developed in 
the period of shortest days, this difference fell to 8 per cent. 
