48 BULLETIN 1450, U. S. DEPARTMENT OF AGRICULTURE 
GROWTH UNDER LONG AND SHORT DAILY PERIODS OF ILLUMI- 
NATION 
It may be assumed that the partial delay which occurs in the time 
of elongation of the stem and the more definite delay in the date 
of heading and blooming on timothy plants which make continuous 
and vigorous vegetative growth in the greenhouse during the winter 
is due to the tendency for these processes to take place only when 
the days are relatively long, as in late spring and early summer, 
rather than when the days are shorter. In order to test this theory 
more carefully under conditions where length of day was the only 
varying factor an experiment was conducted in the winter of 1923-24 
in a greenhouse where the length of daily illumination could be con- 
trolled. Each one of a number of timothy plants was divided into 
two parts. Beginning on or approximately on November 10, 1923, 
one part of each was grown on a bench where the length of daily 
illumination was extended by means of electric lights so that the 
plants were illuminated for several hours after nightfall. The other 
parts of these plants were grown in the same greenhouse. Each day 
at 4 p. m., they were placed in a large box from which light was 
excluded and in which they remained until 9 a. m. the next day. 
The former group of plants received, during the time the experi- 
ment was conducted, approximately 16.5 hours of light daily; the 
latter group of plants were in the light for 7 hours each day. 
On February 18, 1924, a typical plant grown under each period of 
illumination was photographed; they are shown in Plate 12. Each 
one of these plants was propagated vegetatively from the same 
original plant ; any differences in them are therefore due to different 
conditions for growth and not to variations in the plants themselves. 
On all of the plants used in this experiment which received light 
for 16.5 hours each day the shoots made a comparatively rapid 
growth in length. On the plant shown in the illustration, four of 
the shoots produced heads. The total lengths of these shoots, in- 
cluding the heads, were, respectively, 23.75, 29.40. 29.50, and 35.20 
inches. Three of the shoots had six elongated internodes each and 
the fourth one had seven. The average number of elongated inter- 
nodes was 6.25 per shoot, which is practically the same as on shoots 
which produce heads in meadows at the normal time in the spring. 
At the base of each fertile shoot, and also at the base of each one 
of five elongated shoots on which heads did not develop, on the plant 
grown under relatively long-day conditions there was some enlarge- 
ment in the diameter of the stem, owing to the formation of a 
haplocorm. 
Florets on one of the heads of the plant shown in the illustration 
and also on one head of each of two other plants grown with 16.5 
hours of light each day were in bloom on or had bloomed prior to 
February 18, 1924. 
Although the plants which received light for seven hours each day 
made a vigorous vegetative growth, the internodes of the stem did 
not become elongated, nor was there any development of the inflores- 
cence ; their growth resembled that of plants in the field during the 
relatively short days of late fall. On plants exposed to light 16.5 
hours daily, on the other hand, the stems became elongated, heads de- 
