Because sunlight is the foremost environmental requirement of higher plants, processes which help seedling shoots emerge from the soil into the air, and which direct further development so as to optimize the intensity of light received by the leaves, are critical for plant survival. It is thus natural that plants, in addition to using sunlight as a source of energy, use it as a sensory cue to guide growth. It is the shorter wavelengths of the photosynthetic action spectrum to which the guidance systems respond, whereas the longer wavelengths often modulate the sensitivity or effectiveness of guidance.