Mar. 17, 2933 
Further Studies in Photoperiodism 
873 
phenomena of periodicity seem to be more or less influenced by the 
seasonal change in length of day. Moreover, some features of plan 
development not so definitely periodic in character appear to be related 
to the duration of the daylight period. Because of the wide field covered 
a great deal of experimental data must be accumulated before a full 
discussion of these various responses can be undertaken, and it is not 
practicable at this stage to attempt to correlate the extensive literature 
on the different features of plant development with the preliminary 
observations which have been made. In the present paper a somewhat 
more detailed discussion of the formation of tubers, bulbs, and thickened 
roots in relation to sexual reproduction will be undertaken. Only briefer 
mention of other features of the photoperiodic response will be made, 
reserving fuller consideration for the future. Perhaps the principal 
value of the limited number of observations which have been made on 
/arious forms of response, viewed as a whole, is that they may be made 
.he basis of a working hypothesis as to the nature of some of the internal 
processes involved in photoperiodism, thus suggesting more systematic 
biological and biochemical studies along definite lines. The wide extent 
and great variety in form of the photoperiodic response verifies in most 
striking manner the soundness of the modem view, the most consistent 
supporter of which perhaps has been Klebs, to the effect that environ¬ 
ment through its action on internal conditions governs the form of 
expression in the plant, subject only to the specific limitations imposed 
by heredity. 
FURTHER OBSERVATIONS ON FLOWERING AND FRUITING 
Woody perennials, particularly the larger-growing forms, do not 
lend themselves so readily to the methods of study which have been 
followed, so that the data thus far accumulated have reference mostly 
to annuals and herbaceous perennials, though a number of shrubs and 
dwarf trees are now under observation. In the following experiments 
the methods followed were essentially the same as those described in 
the former paper; namely, the use of light-proof houses into which the 
plants could be placed for a portion of the day during the open growing 
season and the use of electric light in the greenhouse for a portion of the 
night during the winter months. The average intensity of the electric 
illumination maintained in the vicinity of the plants in the greenhouse 
was approximately 3 to 5 foot-candles and the lights (40-watt mazda 
bulbs) were on from sunset till midnight each day. In reducing the 
daylight period the plants in most experiments received sunlight only 
from 10 a. m. to 3 p. m. for the 5-hour exposures; from 5.30 a. m. to 3.30 
p. m. for the io-hour exposures; from 5.30 a. m. to 5.30 p. m. for the 
12-hour exposures; from 5.30 a. m. to 6.30 p. m. for the 13-hour exposures. 
Seed of Amaranthus hybridus L. which was sown in metal buckets 
July 8 had germinated by July 11. Under daily light exposures of 10,12 
and 13 hours the first blossoms opened August 6 while the average 
heights of the plants, 20 to 30 in.number, were 13 to 14 inches, 16 to 17 
inches, and 15 to 16 inches, respectively. With 5 hours of light daily, 
first blossoms were open August 14 and the average height was, 8 to 9 
inches. The controls exposed to the full natural day length first showed 
blossoms August 23 and the average height was 26 to 27 inches. 
