Plants have multiple hormones which asist the plant in its daily functions. One hormone which promotes growth is called Cytokinins. This hormone stimulates cell division and stem formation which will make the plant grow taller. Another regulator hormone called Abscisic Acid. This regulator works with the chloroplast to moderate light intake.
Through a combination of both of these plants, the plant can and will actually grow towards the light. The Abscisc Acid will first detect the crack of light and send signals to grow with Cytokinins. Through photosynthesis, the plant will have no choice but to bend and attempt to have its fullest part growing in the light so it will actually bend and grow towards the light!