Phytochrome causes stems to elongate when they are shaded since more Pr is present, making stems reach upward for the sunlight while full sunlight increases Pfr, making more branches to spread out and capture more of the canopy space, and thus more light. In some ways, phytochrome acts like a homeostatic mechanism to keep leaves in the sun where they can capture the most light, though, of course, all plants are doing the same thing, making the competition go on and on.
Work Step by Step
See p. 851 for relevant facts. Then use your imagination to see that phytochrome signals different things in sun and shade but that both signals are intended to help intercept more light.