458 
Joumal of Agricultural Research 
Vol. XXVIII, No. 5 
In order to measure a leaf which would show as fully as possible the effect of 
the 10-week lighting period and at the same time be well-developed, the second 
expanded leaf below the apex was measured on each plant, and also the length of 
stem from the node below this leaf to the node above it. That the day length 
very decidedly affected internodal length is indicated by the data presented in 
Table V. 
Table V.— Internodal length produced under daily light exposures of 10 to 1S\ 
hours for A series , and normal day length (11 to 11.2 hours), for B series a 
Can No. 
Plant 
No. 
Length 
of daily 
light 
exposure 
Combined length of 
two internodes 
below uppermost 
expanded leaf 
A 
A 
B 
16...... 
1 1 
Hours 
1 10 
Inches 
| 2* 
1 3i 
l 2* 
Inches 
21 
1* 
2* 
Total...... 
8 
1.3 
6* 
1.1 
Average internodal length... 
14....... 
1 
2 
3 
1 
2 
3 
12* 
12* 
( 4 * 
1 5* 
/ 6* 
\ 4* 
(6) 
2i 
(&) U 
l\ 
3* 
17........... 
Total....... 
26 
2.6 
11 
1.1 
Total internodal length.... 
15... 
1 
2 
3 
1 
2 
3 
[ 13* 
| 13* 
6* 
7 * 
(6) 
7 * 
6 
8 
2* 
2* 
2* 
3f 
3* 
5* 
18... 
Total....... 
35* 
3.5 
19* 
1.6 
Average internodal length. 
a Measured Jan. 19, 1923. 6 This plant was lacking. 
Since the plants were few in number, the figures should not be too closely 
interpreted. The greatest average internodal length for growth made by any 
group receiving normal day length amounted only to 1.6 inches, while under a 
123^-hour day length, it amounted to 2.6 inches, and under a 13J^-hour day 
length, to 3.5 inches. Although growth made under* the 10-hour day length 
exceeded its own control, it fell within the range of the three controls. The 
longer days produced longer internodes. 
The effect of day length on midrib length was most pronounced, as shown by 
Table VI. 
Under daily light exposures of 10, 12J^, and 13^ hours, the average midrib 
lengths were 5, 7, and 9 inches, respectively, while the average for their controls 
ranged from 4.6 to 5.5. inches. With the exception of one dying plant, every 
individual in the 133^-hour group exceeded in length of midrib every plant in the 
12J^-hour group, each of which in turn exceeded each of the plants of the 10- 
hour group. The longer days produced leaves with longer midribs. 
The indication seen in the earlier measurements of the development of a greater 
number of pinnae in the longer day was not borne out by the count on these 
leaves. However, the number of plants was small. The treated plants averaged 
19 pinnae for each group, while 2 controls averaged 20, and the third control, 21. 
The effect of day length on pinna length was most pronounced, as shown by 
Table VII. 
