46 
BULLETIN 1450, U. S. DEPARTMENT OF AGRICULTURE 
to occur. Some varieties bloom and produce seed in midsummer, 
when the days are long and the nights are short, whereas other vari- 
eties of the same species may not produce seed until fall when the 
days are short. This response in the behavior of plants to variations 
in the length of the period of daily illumination is called photoperi- 
odism (ii, p. 871). 
When timothy plants are transferred in late fall or winter from 
the field to a greenhouse where temperature and soil conditions are 
favorable, growth occurs, but in a manner much different from that 
in the field during the spring and early summer. 
On January 5, 1922, a number of timothy plants were taken from 
a field. Some of them were transplanted again to a row in the field 
on the station; the remainder were transplanted to a greenhouse. 
During the period from January 5 until April 4 a new leaf appeared 
on each one of the shoots observed on the plants in the greenhouse 
at an average interval of 8.7 days. This record shows that the con- 
ditions there were such that a relatively vigorous vegetative growth 
occurred. 
Five of the plants transplanted to the greenhouse were retrans- 
planted to a coldframe on April 4; five other plants were retrans- 
planted to the field on May 20 in a row plat adjoining the one to 
which plants were transplanted directly from the field on January 
5. A record was obtained of the date when the first head appeared 
on each plant. When the shoots on the different plants had com- 
pleted their growth they were examined, and the number of leaves 
and of elongated internodes on one shoot marked for observation on 
each plant Avere counted. These data are presented in Table 19. 
Table 19. — Characteristics of timothy plants taken from a field January 5, 
1922, and grown under different conditions 
Treatment given the plants 
Number 
of plants 
used 
Average 
date 
when first 
head 
appeared 
Average 
number 
of leaves 
on shoots 
observed 
Average 
number 
of elon- 
gated in- 
ternodes 
in the 
culm 
7 
5 
5 
June 5 
June 9 
June 7 
17.3 
25.6 
23.2 
6.4 
Transplanted from field to greenhouse Jan. 5, 1922: 
11.4 
(b) Retransplanted to field May 20 
9.8 
In the experiment conducted in 1922, described in Table 19, the 
plants grown in the greenhouse were different from those grown in 
the field. There was, therefore, some opportunity for small experi- 
mental errors, owing to variation inherent in different plants. In 
1923 a similar experiment was again conducted. In the latter sea- 
son, however, three plants were taken from the field on January 19, 
each of which was divided into two parts ; one part was transplanted 
to a greenhouse and the other to the field, where it continued its 
growth under normal conditions. The results are presented in 
Table 20. 
