Mar. 17,1933 
Further Studies in Photoperiodism 
915 
range of response, there would be an optimal light period for elongation 
of the vegetative stem or increase in stature. Under this optimal light 
period, which in the first instance will be assumed to correspond with 
the longest days of summer, there would be more or less indefinite elon¬ 
gation of the primary axis, with no branching and a minimum of foliage 
development (Cosmos). Any departure from the optimal light period 
would check stem elongation; and, as a result, a chain of responses would 
come into expression as the light period became progressively less favor¬ 
able for increase in stature. The first effect of outstanding importance 
would be flowering and fruiting; and, as a result of partial loss of domi¬ 
nance in the apical bud, branching might occur at this stage (Cosmos). 
The change in the light period may be such as to direct the plant's 
energies more or less quantitatively toward flowering and fruiting, in 
which case senescence and death would be the logical outcome, the plant 
behaving as an annual. Even in this case, however, rejuvenescence may 
be effected under favorable conditions by a return of the optimal light 
period for vegetative growth, the plant thus escaping death (Bidens). 
Between the respective optimal light periods for stem growth and for 
sexual reproduction there would be a buffer zone in which combined 
growth and flowering would occur, thus representing the everblooming 
condition (Viola). 
With a further departure from the optimal light period there would be 
more or less complete inhibition of stem elongation, resulting in intense 
tuberization; that is, stem elongation would be replaced by stem thicken¬ 
ing. In this case flowering would be inhibited (artichoke) . If a.primary 
axis had previously devdoped, there would be a general downward 
transfer of activity to the ground level or even beneath the soil surface. 
The end result would be the formation of underground resting structures, 
as rhizomes or tubers. Here, again, between the respective optimal 
light periods for flowering and for tuberization there would be an inter¬ 
mediate zone in which important responses find expression. There may 
be combined flowering and tuberization (groundnut). Aerial and under¬ 
ground types of tuberization are alternative forms of expression (cin¬ 
namon vine). Aerial branching may give way to underground branching 
(sorrel). The stemless leaf rosette, with or without tuberization, is an 
important type of activity falling within this zone. Here would be 
placed the flowering herbaceous perennial which flowers and fruits so 
freely as to cause the death of the aerial parts but with sufficient regenera¬ 
tive energy remaining to organize underground perennating structures. 
The final response would be death as a direct result of starvation when 
the light period has become too short to furnish sufficient photosynthetic 
material to support any form of activity. The circumstances of death 
in this case are quite different, however, from those which induce intensive 
fruiting and thereby result in the death of the mother plant. It is to be 
understood, of course, that all forms of expression are conditioned on the 
existence of general environmental conditions falling within the limits of 
tolerance of the species in question. 
The above-mentioned series of responses would result from a progres¬ 
sive decrease in the duration of the light period. Assuming an optimal 
light period for stem elongation corresponding to the intermediate day 
length of spring and fall, a somewhat different chain of events, not yet 
fully determined, seems to result from exposure to light periods of exces¬ 
sive length. Probably one of the first notable effects of a light period in 
