An enzyme of the Phosphorylases Family that catalyzes the degradation of Starch, a mixture of unbranched Amylose and branched Amylopectin compounds. This phosphorylase from Plants is the counterpart of Glycogen Phosphorylase in Animals that catalyzes the reaction of inorganic phosphate on the terminal alpha-1,4-glycosidic bond at the non-reducing end of Glucans resulting in the release of Glucose-1-phosphate.