892 
Journal of Agricultural Research 
Vol. XXIII, No. 11 
May 21 and were exposed to regulated day lengths by use of the dark 
houses. The exposures were 5, 10, 13 hours, and the full daylight period 
of the season. The plants were harvested August 28. The results of 
the test are summarized in part in Table V. The conditions in general 
and the soil in particular were unfavorable for the best development of 
tubers, but for comparative purposes the results are of considerable 
interest. 
Table V. — Results with McCormick variety of potato exposed to different daily light 
periods 
Duration of daily light 
period. 
Date of 
first 
blossom. 
Average 
height 
when 
harvested. 
Average 
weight 
of tubers. 
Average 
weight 
of tops, 
air-dried. 
Ratio of 
tubers to 
air-dried 
tops. 
Condition of tops 
when harvested. 
5 hours. 
10 hours. 
13 hours. 
Full daylight period.. 
July 2 
...do. 
...do..... 
July *3 
Inches. 
20 
12 
25 
32 
Gm. 
80 
320 
664 
471 
Gm. 
2.8 
1.9 
30-9 
32.0 
12. I 
168. I 
22. I 
IS-1 
All dead. 
Mostly dead. 
Green. 
Do. 
With exposures of 5 and 10 hours of light daily the blossoms unfolded 
but no seed balls were formed. Taking the two experiments together 
it appears that a very long daylight period tends to direct the activities 
of the plant toward vegetative development to the exclusion of other 
alternative forms of expression. With a somewhat shorter daylight 
period the tendency is toward successful sexual reproduction and moder¬ 
ate tuber formation. With further shortening of the daylight period 
seed development again fails and there is a tendency toward tuber forma¬ 
tion to the exclusion, as far as may be, of other alternative types of 
expression. Finally, with an extremely short daylight period it is seen 
that there is some tendency toward return to exclusive vegetative 
development. With a light duration of 10 hours or thereabouts there 
is a condition of intense tuber formation; in other words, there is an 
optimal light duration for tuberization as well as for flowering and 
fruiting and for purely vegetative development. Since one of the 
limiting factors in tuber formation is the quantity of photosynthetic 
material formed, the actual or absolute weight of tubers is greatest under 
a somewhat longer daylight period, namely 13 hours. Under natural 
conditions, however, the seasonal range in day length becomes an impor¬ 
tant factor; and it may well be that in northern latitudes the long days 
of summer leading to extensive foliage development, followed in autumn 
by a correspondingly precipitate decline in day length, constitute a very 
favorable situation for tuber development. It may be added that 
nothing in this discussion should be construed as minimizing the signifi¬ 
cance of relatively low temperatures in tuber development, particularly 
with respect to size of tuber. 
Tubers of ground nut (Apios tuberosa Moench) planted in boxes in the 
greenhouse early in March germinated April 6. Beginning April 20, 
one lot was exposed to 10 hours of light daily while a second lot was 
exposed to the full seasonal day length. Flower buds were showing on 
both lots when the light treatment began. On the control plants exposed 
to the full-day length the first open blossoms appeared June 1, and these 
plants were in full bloom till late in August. Under 10 hours of light 
