896 
Journal of Agricultural Research 
Vol. XXIII, No. XI 
were small and appeared weak at all stages, but the two individuals 
together formed 35 tubers weighing 2 pounds and 4 ounces. Of the two 
controls, one reached 6 feet in height and the second 3 feet. The total 
number of tubers was 60, but these weighed only 1 pound and 1 ounce. 
Under the short-day exposure tuberization might be said to have been 
complete; that is, there were no elongated underground stems. As 
already stated, flowering could not be successfully accomplished so that 
there was no diversion of formative material toward seed formation. 
Under the natural range in length of day, on the other hand, there were 
many long underground stems with nodes only slightly tuberized, and 
in some instances no thickening of nodes at all could be seen. The 
tubers formed under the two conditions were strikingly different in 
appearance, as can be seen in Plate 7, A and B. 
In the second series of tests with blazing star previously described 
(p. 876), it was observed that under the shortened daily light exposures 
well-developed corms of considerable size were formed, while the control 
plants, though many times larger in size, formed small corms or none 
at all. 
Under the influence of a day length shortened to the point where 
successful flowering and fruiting is almost completely inhibited even 
the nodes of both primary and secondary stems of soybeans may be 
partially tuberized, as is shown in Plate 9, B. The arrested develop¬ 
ment of the flower buds accompanying the thickening of the stem is 
plainly seen. 
The behavior of radish and beet under different day lengths has already 
been rather fully discussed (p. 887), and it only remains to point out here 
that under a short daily illumination period these plants continue to 
store up carbohydrate for a more or less indefinite period, eventually 
forming enormously thickened roots but no primary stem. 
SIGNIFICANCE OF TUBERIZATION AS A FEATURE OF PHOTOPERIODISM 
The data which have been presented will suffice to show that the pre¬ 
vailing length of day is an important factor in the formation of bulbs 
and tubers, as well as in flowering and fruiting. These data clearly 
indicate that the length of day not only influences the quantity of photo¬ 
synthetic material formed but also may determine the use which the 
plant can make of this material. Under the optimal illumination 
period for apogeotropic development there is normally no accumulation 
of unused carbohydrate, this material being utilized as rapidly as formed 
in the development of structures or tissues incident to maximum increase 
in stature. Under these particular conditions it may well be that the 
quantity of carbohydrate produced is a chief limiting factor in rate and 
extent of stem elongation. Any departure from this optimal light 
period will check stem elongation; and the first outstanding result is 
initiation of flowering; that is, the growing point is transformed into an 
inflorescence. The natural result of checking stem elongation is a ten¬ 
dency toward accumulation of carbohydrate. In successful fruiting this 
carbohydrate (in many cases first converted into oil) is stored in the seed 
and its coverings. Thus, in a special sense, the seed may be regarded as 
a tuberized structure. From the standpoint of nutrition, apparently one 
of the functions of fertilization as a prerequisite for successful setting of 
the fruit is to aid in setting up a “center of attraction” about which 
nutrient materials will be mobilized. Since there may be an optimal 
