@The sensitivity of seedstoa given 
amount of radiant energy changes 
with the period of imbibition, 
Several questions should come to 
mind immediately. How much light is 
required to induce germination? What 
are the relative amounts of red and 
far-red required to drive the reac- 
tions? Do all light-sensitive seeds 
require the same amount of energyto 
trigger germination? What is the 
effect of various temperatures on the 
light requirements? Are there other 
methods of inducing a light require- 
ment in seeds that normally are not 
light-sensitive? Are there any seeds 
that are inhibited from germinating 
by light? Experiments can be de- 
signed to answer these and many more 
questions relating to the mechanism 
by which light controls germination, 
LIGHT AND PLANT GROWTH 
Vegetative growth of plants is toa 
large degree controlled by light. 
Plants grown in total darkness have 
very long internodes, small leaves, 
and are yellow in color because no 
chlorophyll is formed. If the dark- 
grown plants are exposed to weak light 
for a minute or two each day, the 
plants have shorter internodes and 
normal-size leaves, although they 
may still be yellowand without visible 
chlorophyll. Daily exposures of the 
plants to light of higher intensities 
or for a longer duration may not 
change the size of the leaves or inter- 
nodes of the plants from that obtained 
with brief exposures to light of low 
intensity, but the plants turn greenas 
chlorophyll is formed, 
The formative effects of light, but 
not chlorophyll formation, result from 
the same red, far-red reversible 
photoreaction that also controls 
flowering of photoperiodically sensi- 
tive plants, germination of light- 
sensitive seeds, and many other plant 
responses. Red is the most efficient 
portion of the spectrum in inhibiting 
stem elongation and promoting leaf 
expansion, A far-red irradiation im- 
mediately following the red reverses 
the potential effect of the red irradi- 
ation and the stems become long. 
Far-red at the close of each light 
period causes stems of light-grown 
plants to elongate. If the far-red is 
followed by a brief exposure to red, 
the effect of the far-red is reversed 
and the stems remain short, 
If light is directed at either light- 
grown or dark-grown plants from 
one side, the leaves tend to bend and 
the leaf petioles twist until the plane 
of the leaf blade is perpendicular to 
the light. The stems tend to curve in 
such a way that the tip of the stem is 
directed toward the light source. This 
phenomenon is called phototropism 
and is caused by a different photo- 
reaction than the red, far-red one, 
Blue light is the most effective kind 
of light to promote the phototropic 
response, 
Demonstrations B-1 through B-4 
show several ways in which light 
influences plant growth and develop- 
ment, These demonstrations tell us 
the following facts: 
@Light inhibits stem growth and 
promotes leaf expansion. 
@Plants bend toward the light. 
eChlorophyll formation requires 
light and the light must be of 
higher intensity than that which 
controls stem length. 
@The red, far-red reversible pho- 
toreaction that controls seed 
germination also controls stem 
length and leaf size. 
Additional experiments can be de- 
signed to answer many other questions 
relating to the manner by which light 
