LIGHT AND PLANTS 
A Series of Experiments Demonstrating 
Light Effects on Seed Germination, 
Plant Growth, and Plant Development 
By R. J. Downs, H. A. Borthwick, and A. A, Piringer, Plant Physiologists, 
Crops Research Division, Agricultural Research Service 
INTRODUCTION 
Bach) year scientists in the U.. S. 
Department of Agriculture receive 
many inquiries from students, 
teachers, and other interested biolo- 
gists for details of simple but dra- 
matic experiments to demonstrate the 
photomorphogenic effects of light on 
plants. To answer these requests for 
this specialized information detailed 
and systematic experiments and 
demonstrations on effects of light on 
seed germination, growth, flowering, 
and fruiting are outlined herein. 
References accompanying each ex- 
periment provide supplementary 
reading and additional details. Cer- 
tain references will not be readily 
available to all interested persons, 
but, in general, the cited papers can 
be obtained from college and other 
school libraries of most metropolitan 
areas, as well as the personal librar- 
ies of local plant scientists. 
LIGHT AND SEED GERMINATION 
Seeds of many kinds of plants 
germinate poorly or not at all when 
planted and covered with soil. Inmany 
instances these are seeds that require 
light for germination. Some seeds, 
such as those of peppergrass (Lepid- 
ium virginicum), do not germinate at 
all in darkness, Others, suchas seeds 
of Grand Rapids lettuce (Lactuca 
sativa), often germinate as much as 
30 percent in darkness, andsome lots 
of lettuce even higher. All the seeds 
of both peppergrass and Grand Rapids 
lettuce germinate following a single 
brief exposure to light. A single ex- 
posure to light, nevertheless, is not 
adequate to promote germination of 
all kinds of light-sensitive seeds. 
Seeds of the Empress tree (Paulownia 
tomentosa), for example, require one 
or more periods of light each day for 
several days. 
Germination of still other kinds of 
seeds, suchas those of henbit (Lamium 
amplexicaule), appears tobe inhibited 
by light. 
A favorable temperature is one of 
the requirements for germination and 
often a change or alternation of tem- 
peratures is more effective than a 
constant one in obtaining maximum 
germination, For example, only about 
30 percent of peppergrass' seeds 
imbibed in water and placed on 
blotters in petri dishes at a constant 
temperature of 70° F, may germinate 
invithe, light .If, the temperature is 
alternated, more seeds germinate. If 
the seeds are imbibed in a solution 
containing 0.02-percent potassium ni- 
trate (KNO3) and the temperature 
alternated, maximum germination is 
attained, Such as alternation of tem- 
peratures might be 77°F. for 8 hours 
per day and 60° for 16 hours per day. 
