88 o 
Journal of Agricultural Research 
Vol. XXIII, No. « 
The important point is that the inhibition of flowering in the first case 
by a long light period is not of the same sort as the inhibiting effect of a 
short light period in the second case; that is, the inhibition in the two cases 
results in different alternative forms of vegetative activity, as will be 
shown in later paragraphs. There are, moreover, plants which may be 
said to occupy an intermediate position in that it is possible to have a day 
length too long as well as one too short to induce flowering. Mikania 
scandens L., briefly discussed in our former paper (7), is an example of 
this type. In the wild state Mikania regularly flowers in late July and 
through August, and if kept in the greenhouse through the winter it also 
usually flowers very sparsely in the spring. Seedlings from a planting 
made December 10 were unable to flower under the influence of the 
lengthened daily light period in the electrically illuminated greenhouse, 
while similar plants in the control greenhouse flowered at the usual time 
in August of the following summer. On the other hand, this species was 
unable to flower through the summer or fall under day lengths of 5, 10, 
and 13 hours’ duration. The Biloxi variety of soybeans (Soja max (L.) 
Piper), discussed in detail in the former paper (7), occupies a position 
slightly below the intermediate position of Mikania in response to day 
length. In this case flowering is readily inhibited by a day length in 
excess of 13 hours. On the other hand, with very short day lengths 
only a very few cleistogamous flowers and fewer seed are developed but 
the reproductive phases apparently can not be entirely suppressed. 
Thus, there are species occupying various positions within the annual 
range of day length with respect to the initiation of reproductive activi¬ 
ties, and there is no definite line of division between long-day plants 
and short-day plants. An illumination period on one side of the critical 
duration for flowering promotes certain forms of vegetative activity, 
while a light period lying on the other side of the critical promotes other 
forms of vegetative activity, as will be shown under the discussion of 
apogeotropism (p. 886). 
Not only do plants differ markedly as to the particular length of day 
most favorable for flowering but they also differ widely as to the nar¬ 
rowness of the range in day length which will permit of flowering. Ex¬ 
periments with Mikania, referred to above, indicate that under ordinary 
conditions this plant will readily flower only under a day length ranging 
but little beyond an hour on either side of 14^ horns, which is approxi¬ 
mately the optimum for flowering. Buckwheat flowers readily under a 
daily illumination period ranging at least from 5 hours up to 18 or 20 
hours and probably even under continuous illumination, notwithstand¬ 
ing the fact that the extent of vegetative development and life duration 
are profoundly affected by this range in the length of the light period. 
While there is a certain degree of antagonism or incompatibility between 
the vegetative and reproductive phases of activity, the two obviously 
are not necessarily affected to the same degree by a given change in 
duration of the light period. Viola papilionacea Pursh will flower under 
all lengths of day met with in temperate regions except for a short period 
in midwinter. In the broadest sense the plant will flower continuously 
for about 10 months of the year, a conspicuous example of everblooming. 
During the summer months, however, only the cleistogamous type of 
flower is seen while in spring and fall only the showy blue, chasmoga- 
mous blossoms develop. By maintaining a light period equivalent to 
the long summer days the cleistogamous flowering continues indefinitely 
and, similarly, a considerably shorter light period will maintain the 
