914 
Journal of Agricultural Research voi xxni, No.» 
of chilling. The extent to which these renewed activities may progress 
after they have been initiated in the absence of a favorable light period, 
however, is extremely variable, as was pointed out on page 907. There 
seems to be no question as to the importance of the relationship between 
the action of the seasonal decrease in length of day in forcing the plant 
into a state of relative inactivity, on the one hand, and the effect of the 
cold of winter in promoting successful emergence from the rest period 
by conserving and bringing into play the stored energy of the resting 
organ, on the other hand. 
RANGE OF RESPONSE TO LENGTH OF DAY 
It is apparent that species differ widely in their sensibility to change 
in the length of day and in the form of expression which the response 
may take. Cosmos continues elongation of the vegetative stem under 
any day length in excess of about 13 hours, while under all day lengths 
much below 13 hours flowering readily takes place. Mikania may 
maintain sterile vegetative growth under the extreme day length (in 
excess of 15 hours) of summer in high latitudes. It flowers readily 
under a daily light period of about 14 hours, while both growth and 
flowering are inhibited by a light period of 13 hours or less. Goldenrod 
forms a flowering stem under any length of day in excess of approxi¬ 
mately 8 hours, but with a shorter light period only the leaf-rosette 
type of development is possible. Sorrel (Rumex) forms aerial flowering 
stems when exposed to the long days of summer, but under the short 
days of winter only the underground type of stem is formed. Artichoke 
is unable to flower under a light exposure of 10 hours or less, but under 
these conditions there may be intense tuber formation. Under a io-hour 
day groundnut (Apios) is unable to flower, there is no elongation of 
aerial stems, the number of new underground tubers is greatly reduced, 
and the mother tuber is materially enlarged. Under a longer day length 
all of these forms of response in groundnut are reversed. The Mandarin 
soybean flowers under the longest summer days at Washington, while 
the Peking does not begin flowering till the latter half of July, the Tokyo 
about two weeks later, the Biloxi some three weeks later still, and, 
finally, the Otootan about September 15—all in response to the prevailing 
day length. In the violet the extreme seasonal range in day length from 
early spring to midsummer suffices only to change the type of blossom 
from the blue petaliferous to the cleistogamous form. Finally, in buck¬ 
wheat flowering occurs throughout the range of day length from 5 to 
18 hours, although under the shorter period it behaves as an ephemeral, 
and under the longer period it assumes the form of a giant everbloomer. 
It may be considered that there is a wide range or “scale” of response 
to change in the light period and that the portion of this scale covered 
by the individual species or variety is extremely variable. For example, 
buckwheat through all ranges in length of day prevailing in the temperate 
zone occupies only that portion of the scale lying between the strictly 
flowering condition and the sterile, giant type of development. The bean 
referred to on page 893 occupies the portion of the scale represented in the 
strictly sterile giant form, the flowering condition, and the phenomenon 
of tuberization. No single species has been observed which manifests all 
forms of response, but one might perhaps conceive of a sort of composite 
plant, combining the responses of several different species and thus 
covering the entire range. Beginning with one of the extremes of this 
