germination in some seed that normally germinate in darkness. The 

 tests were extended to many kinds of plants in growth stages, and showed 

 that the same reversible action controls stem-lengthening, flowering, and 

 other changes in plant development. 



This evidence necessitated extending the band of the spectrum that 

 governs photoperiodic responses. Photoperiodic researchers assign the 

 name "red" to light wave lengths from about 5800 to 7200 A., and "far 

 red" to wave lengths fronn about 7200 to 8000 A. Far red is a borderland 

 including the longest waves of faintly visible light and extending beyond to 

 include some infra-red waves that man's eye cannot see. 



PREDICTING THE NATURE OF THE GROWTH-TRIGGERING 



CHEMICAL 



At this point, the hidden light-sensitive chemical's characteristics 

 and action could be predicted to a rennarkable extent. The main clues and 

 deductions were these: 



The chemical is in all kinds of plants , working reversible shifts to 

 control orderly growth changes. 



The chemical is a pigment because it reacts to light in color fashion. 

 It is blue, or just possibly green-blue, because only such a pigment would 

 absorb as it does the complementary red and far -red waves of white 

 light. It would not be closely related to chlorophyll because green chloro- 

 phyll reacts strongly to blue as well as to red. 



The triggering pigment is in extremely tiny amounts, because albino 

 plantTj that have a bleached look for want of normal color, respond 

 reversibly to red and far red, showing presence of the blue pigment. 



The chemical is a dual form- -a type that occurs widely in nature. 

 In such chemicals, each molecule contains some atoms arranged in 

 reverse pattern, like a right hand and its mirror image or a left hand. 

 Red light absorbed by the molecule would move these atoms in the blue 

 pigment into the right-hand or active form that triggers a growth change. 

 Far red, and also darkness, would push the pattern into the left-hand 

 form that inhibits a change. In sunlight both red and far red would be 

 constantly pushing the atoms opposite ways, but with red more commonly 

 prevailing. 



The concentration of the pigment's active form has to reach a definite 

 level before a growth change can start. Awaiting the right concentration, 

 the plant would keep to its current activity, such as bigger leaf production 

 in the vegetative stage. 



BRINGING PHYTOCHROME FROM HIDING 



The outstanding advantage of getting the growth-triggering pigment 

 into the open was achieved in 1959. One of the research group named it 

 phytochrome. Within a few nnonths the group was able to announce how 

 this basic part of a plant was located and separated, and to describe new 

 information gained by working with it directly. 



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