@ The coloration occurs only inthe 
areas that received light--there 
is § no) sitranslocation »* of/ the 
stimulus. 
Additional experiments can be de- 
signed to learn more about the light 
reaction «and: about «the chemical 
processes that result in pigment for- 
mation. Questions that one might ask 
are: How much energy is requiredto 
induce the formation of anthocyanin? 
As light energy is increased, does the 
amount of anthocyanin increase pro- 
portionately? Once the light require- 
ment is fulfilled, what is the rate of 
anthocyanin formation? What is the 
role of temperature? What is the role 
of sugar? Does the red, far-red re- 
versible photoreaction operate in the 
control of coloration? 
EFFECT OF DURATION OF LIGHT 
ON PLANTS 
Flowering of many kinds of plants 
is controlled by the relative length of 
the daily light and dark periods. This 
phenomenon is called photoperiod- 
ism. Some plants, such as certain 
varieties of chrysanthemum, poin- 
settia, morning-glory, cocklebur, and 
lambsquarter, are short-day plants 
and flower only when the days are 
short and the nights are long. Certain 
varieties of spinach, beet, barley, and 
tuberous-rooted begonia, are ex- 
amples of long-day plants which 
flower only when the days are long 
and the nights are short. Flowering 
of many other kinds of plants is 
hastened but not absolutely controlled 
by the appropriate daylength. For ex- 
ample, scarlet sage, variety America, 
flowers quickly on short days but 
eventually flowers on long ones. Many 
varieties of petunia flower most 
rapidly on long days but finally flower 
on daylengths as short as 8 hours. 
Bulbing and tuber formation are 
also controlled by daylength. Tuber- 
ous-rooted begonia, which is a long- 
day plant for flowering, produces 
tubers on short days but not on long 
days. Onions, on the other hand, 
produce bulbs on long days but not 
when the days are short. 
Dormancy, and thereby preparation 
of woody plants for the coming of 
winter, is another plant response 
regulated by photoperiod, Even in the 
warm greenhouse many woody plants 
stop elongation of stems, produce 
terminal buds, and ''harden off'' when 
the days begin to shorten in the 
autumn. However, if artificial light 
is used to keep the days long, plants 
in the warm greenhouse will continue 
growing during the naturally short 
days of winter and several years' 
"field'' growth is often obtained in 
only 1 year. 
These plant responses are regu- 
lated not. .by, the length of the light 
period but by the length of the dark 
period. Thus, a long-day plant is 
really a _ short-night plant, and a 
short-day plant is really a long-night 
plant. Therefore, a long-day plant 
will flower, dormancy will be pre- 
vented, and onions will produce bulbs 
when a long dark period is broken into 
two short periods byarelatively brief 
exposure to light near the middle of 
the dark period, Under these same 
conditions, short-day plants will re- 
main vegetative. 
Studies of the responses of green 
plants often require plants to be 
grown indoors, yet they should have 
the same healthy appearance as well- 
tended plants grown out-of-doors. 
Everyone knows that plants cannot 
survive without light of adequate in- 
tensity to operate the processes of 
photosynthesis. In the fieldand garden 
or in the greenhouse this high-in- 
tensity light is obtained from the sun, 
which often provides an illumination 
as high as 10,000 foot-candles, In the 
average home the light intensity is 
