very little far-red is emitted by the lamp, the net 

 result is reasonably pure red light. 



Incandescent-filament lamps emit considerable 

 amounts of far-red and are thus good sources of far- 

 red. The visible light is removed by appropriate fil- 

 ters such as a combination of red and blue cellophane. 

 The red cellophane absorbs all the visible light except 

 red and the dark-blue cellophane absorbs red. How- 

 ever, neither color of cellophane absorbs far-red, so 

 the radiation passing through the filter is, therefore, 

 far-red. 



Seeds of trees, shrubs, ornamentals, vegetables, 

 grains, and grasses can be obtained from commercial 

 seed sources that range from special seed supply 

 houses to the local hardware store. Many weed seeds 

 are light-sensitive, and these can be gathered by the 

 investigator. After the seeds are gathered they should 

 be stored dry in a refrigerator (about 40° F.) until an 

 appropriate time to begin the experiments, because at 

 higher temperatures they often undergo change in 

 their light requirements. A good supply of seeds 

 should be gathered to assure an adequate amount for 

 possible additional experiments. Any difficulty in 

 identifying the plants can be resolved with the aid of 

 high school biology teachers, the botanists or horti- 

 culturists at State university, State agricultural ex- 

 periment stations, or agriculture extension specialists. 



Demonstrations A-l through A-5 tell us the 

 following facts: 



• Certain kinds of seeds require light in order to 

 germinate. 



• The light requirement is not something that 

 occurs only under a special set of experimental 

 conditions, but occurs when seeds are planted in 

 an ordinary way in pots of soil. 



• A light requirement can be 'induced in seeds that 

 normally do not require light for germination. 



• The photoreaction that allows germination to 

 proceed is reversible; red radiant energy drives 

 the reaction in one direction and far-red drives 

 it in the reverse direction. 



• The sensitivity of seeds to a given amount of 

 radiant energy changes with the period of 

 imbibition. 



Several questions should come to mind immedi- 

 ately. How much light is required to induce germina- 

 tion? What are the relative amounts of red and far- 

 red required to drive the reactions? Do all light- 

 sensitive seeds require the same amount of energy to 

 trigger germination? What is the effect of various 

 temperatures on the light requirements? Are there 

 other methods of inducing a light requirement in 



seeds that normally are not light-sensitive? Are there 

 any seeds that are inhibited from germinating by 

 light? Experiments can be designed 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 to a large degree con- 

 trolled 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 

 yellow and without visible chlorophyll. Daily ex- 

 posures of the plants to light of higher intensities or 

 for a longer duration may not change the size of the 

 leaves or internodes of the plants from that obtained 

 with brief exposures to light of low intensity, but the 

 plants turn green as 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 photo- 

 periodically sensitive plants, germination of light- 

 sensitive seeds, and many other plant responses. Red 

 is the most efficient portion of the spectrum in in- 

 hibiting stem .elongation and promoting leaf expan- 

 sion. A far-red irradiation immediately following the 

 red reverses the potential effect of the red irradiation 

 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 photo- 

 tropic response. 



Demonstrations B-l through B-4 show several 

 ways in which light influences plant growth and de- 

 velopment. These demonstrations tell us the follow- 

 ing facts: 



• Light inhibits stem growth and promotes leaf 

 expansion. 



• Plants bend toward the light. 



