Absorption  of  red  light  causes  P[r]  to change to the P[fr] form.
   Far-red light reverses this conversion.
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   Shade-avoidance: In the shade of a forest, chlorophyll pigments in the
   canopy  absorb red light and allow far-red light to pass. Trees in the
   shade shift the phytochrome ratio in favor of P[r] and grows taller.
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   Direct  sunlight  increases  the proportion of P[fr], which stimulates
   branching and inhibits vertical growth.