It is not fully understood why leaves change color, but a complex interaction of chemicals, temperatures, photoperiod, and moisture are involved. During shorter photoperiods trees release a kind of hormone that restricts the flow of sap to the leaves, causing them to change color. The leaf's green color comes from chlorophyll, which is eventually used by the leaf but not replaced. This allows the other colors that were masked by the green to show. Browns, yellows, oranges, and similar colors become outstanding. Reds and purples, not present before the change in the photoperiod, develop in the sap of the leaf cells. Red, purple, and their combinations depend on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf dwindles. The brighter the light during this period the more brilliant the colors.