Cells in stems, leaves and flowers contain phytochromes

The cells in stems, leaves, and flowers contain phytochromes, tiny molecules that detect light. When that happens, phytochromes initiate a chain of chemical reactions, passing the message down into the cellular nuclei. There, transcription factors trigger the manufacture of proteins required to carry out light-dependent processes, like photosynthesis. These phytochromes not only sense the amount of light the plant receives, but can also detect tiny differences in the distribution of wavelengths the plant takes in.