{ 
controls plant growth. Examples of 
such questions are as follows: Are 
bending (phototropism) and growth of 
internodes controlled by the same 
photoreaction? This question can be 
answered by using different regions 
of the spectrum (colors of light) and 
testing to see if bending and growth 
are controlled by the same colors. 
Does the duration of darkness fol- 
lowing the far-red irradiation of light- 
grown plants affect the ultimate length 
of the internodes? What is the opti- 
mum period of darkness and why is it 
optimum? 
How concentrated is the pigment 
that controls growth? How do we 
know it is not chlorophyll? These 
questions can be answered by com- 
paring the growth responses of albino 
and green corn or barley seedlings. 
LIGHT AND PLANT PIGMENTS 
The autumn coloration of leaves and 
stems of woody plants is in part 
caused by the formation of a red pig- 
ment called anthocyanin. The forma- 
tion of anthocyanin is alsoresponsible 
for the red color of apple fruits and 
for the red to purple color of milo, 
turnip, and cabbage seedlings. 
A common observation is that 
apples often do not turnred uniformly 
but that one side of the fruit is green 
or at least a lighter shade of redthan 
the other side. The reddest side ofthe 
apple is usually facing outward from 
the tree. The formation of the .red 
color (anthocyanin) in apple fruits is 
controlled by light. Detailed studies 
have shownthat anthocyanin formation 
in milo, turnip, and cabbage seedlings 
and in leaves of red maple and other 
trees is also regulated by light. 
Unlike many other light-controlled 
plant responses, anthocyanin forma- 
tion requires high-intensity light for 
a relatively long time. However, at the 
close of the high-intensity light period 
the low intensity, red, far-red photo- 
reaction may exert final control on 
anthocyanin synthesis. Thus, if the 
plant material is irradiated for a few 
minutes with far-red at the close of 
the high-intensity light period, the 
potential anthocyanin synthesis is in- 
hibited and very little is formed. Ifa 
brief irradiation with red follows the 
far-red, then anthocyanin is formed 
in an amount equal tothat produced by 
the high-intensity light alone. 
An example of a low-intensity, 
light-controlled coloration is the 
yellow color of the skin of the tomato 
fruit. Plant breeders recognize dif- 
ferences in the color of the skins of 
fruits of certain tomato varieties and 
have classified the skins as yellowor 
clear. The red flesh and a trans- 
parent or white skin give the fruit a 
translucent pink color, whereas the 
yellow skin and red flesh give the 
fruit an orange-red appearance, In 
many tomato varieties the formation 
of this yellow pigment is controlled 
by light. Moreover, the same red or 
far-red photoreaction that controls 
flowering of photoperiodically sensi- 
tive plants, germination of light- 
sensitive seeds, and many other plant 
responses also controls the formation 
of the yellow pigment in the skins of 
tomato fruit. 
Demonstrations C-1 through C-4 
concern light and its control of plant 
coloration. From these demonstra- 
tions we know: 
@® That light is required for the for - 
mation of the red color (antho- 
cyanin) of certain seedlings and 
apple fruits. 
@Light is required for the forma- 
tion of a yellow pigment in the 
skin of tomato fruit. 
