Mar. 17,1923 
Further Studies in Photoperiodism 
885 
SPRING FLOWERING AND FALL FLOWERING 
The facts brought out in the preceding discussion of the relation 
between long-day and short-day plants and the balance between the vege¬ 
tative and the reproductive types of activity throw light on the relation¬ 
ships existing between spring-flowering and fall-flowering plants. As is 
well known, many plants normally flower only during the spring, while 
many others regularly flower only in the fall. There are a number of 
plants, however, which flower in both spring and fall, although usually 
there is a marked difference in the extent of the flowering during the two 
seasons. Finally, there is a large group of plants which blossom in mid¬ 
summer and a smaller group which under favorable conditions flower in 
winter. It is well known that in a very large number of perennials the 
flower buds are organized during the summer or early fall of the year 
preceding that in which the blossoms finally open, so that, in considering 
the effects of light duration, it is necessary to avoid confusing the action 
on the laying down of primordia and that on the final unfolding of the 
blossom. 
Broadly speaking, in cool temperate regions short-day plants will flower 
chiefly in the fall rather than in the spring because of the lag in tempera¬ 
ture rise in spring as compared with the lengthening of the day. In other 
words, in spring the day length is likely to become too long for flowering 
of short-day plants before the temperature has risen sufficiently to permit 
plants to become active. This is true more particularly of the annuals 
and those herbaceous perennials which require considerable vegetative 
development as an antecedent to flowering. That plants of these types 
which regularly flower in the fall will actually flower in the spring when 
the obstacle of low temperature is removed has been demonstrated in a 
number of cases. As a specific illustration Peking soybeans germinating 
in the greenhouse March 31 showed first open blossoms on May 12, or 42 
days after appearance above ground, while a second lot germinating April 
13, just two weeks later than the first planting, did not show open blos¬ 
soms till July 16, 90 days after germination. Plantings of the Biloxi 
soybeans made on the same dates failed to flower till September, since 
the day length was already above the critical for flowering in this variety. 
The Biloxi quickly flowers, however, if planted in winter or early spring 
in the greenhouse. Indirectly, the many experiments already described 
in which various plants have been forced into flowering out of season by 
shortening the light period or inhibited from flowering in the normal 
season by increasing the light period furnish proof of this relationship of 
spring and fall flowering in the short-day plants. As the critical light 
period for flowering becomes longer the chances for spring flowering of the 
species are increased till finally spring flowering merges with summer 
flowering in those species which are intermediate between the more typical 
short-day and long-day plants. Again, sparse flowering in spring is to be 
looked for in those of the short-day plants which are able to flower at all, 
for the reason that the change in day length is toward the optimum for 
vegetative activity and away from the optimum for flowering. It is under 
these conditions, also, that various modifications and abnormalities in 
flowering and fruiting, quantitative as well as qualitative, are most likely 
to occur. On the other hand, the short-day plants as a whole will have 
their energies thrown into flowering and fruiting more or less quantita- 
