sept. 15,1925 Response of Plants to Length of Day and Night 
565 
ing the action of the light period, exposure of the upper portion of the 
axis to continuous darkness apparently did not in itself favor forma¬ 
tion of flower buds, for these appeared only when the lower portion 
of the axis was exposed to daylight for 10-hour periods, and failed to 
form when the lower portion was exposed to a long day. On the 
other hand, mere absence of light seemingly does not inhibit the 
laying down of flower buds in response to the transmitted influence 
of a short daily light period acting on an adjoining portion of the 
A short daily illumination period may exert the same local 
axis. 
effect in initiating sexual reproduction, without material inter¬ 
ference from the action of total darkness or of a long daily illumin¬ 
ation period on other parts of the plant, as when acting upon the 
green portion of the plant as a whole. The same is true of the local 
action of a long daily light period in maintaining vegetative develop¬ 
ment and inhibiting flowering and fruiting. 
SUMMARY 
In earlier investigations it was seen that when two coordinate 
branches of an individual plant of Cosmos bipinnatus are exposed 
to light for daily periods of different length each branch responds in 
characteristic manner to its particular light period more or less inde¬ 
pendently of the other branch. Continuing the investigations, ex¬ 
periments have been made with Cosmos sulphureus, a typical short- 
day plant, in which different portions .of the primary stem were 
exposed to different daily periods of illumination, and, in some 
instances, to continuous darkness. 
When the upper portion of the plant was exposed to the full length 
of day of summer while the lower portion received only 10 hours of 
light daily, the latter promptly flowered while the former remained 
vegetative. These localized responses might, it is supposed, con¬ 
tinue for several months under favorable conditions. 
Thus each portion of the axis has responded to its particular 
light period, in much the same manner as if it had been a separate 
plant. 
Definite localization of the response was likewise obtained when 
the upper portion was subjected to a short daily period of illumina¬ 
tion, and when the lower was exposed to a long one, and the former 
flowered readily while the latter continued to develop vegetatively. 
When the central portion of the axis was exposed to a long day 
while both the lower and uppermost portions were subjected to the 
action of a short day, flowering was initiated in both the latter por¬ 
tions of the axis while the central portion continued to develop only 
vegetative shoots, further illustrating the possibility of sharply local¬ 
izing the response to length of day. 
On the other hand 1 , when the central portion of the axis was ex¬ 
posed to a short day and the upper and lower portions to a long 
day, flower buds appeared in the former while both of the latter 
remained vegetative. 
When the upper portion of the stem was excluded from light corn 
tinuously for a period of 3 to 5 weeks while the lower portion received 
the light of a short day and consequently was forced into flowering, 
formation of flower buds was induced in the upper, darkened portion, 
and open blossoms appeared after the top had been returned to the 
66850—25f-5 
