Phanerogams, or flowering plants, differ from Ferns and other vascular plants in the great modifications which the leaves have undergone in the construction of the flower. A complete flower consists of four sets of organs, termed respectively the calyx, corolla, stamens, and pistil. Each member of these sets or whorls of organs consists of a leaf, modified according to the function which it has to perform in the economy of the plant. The axis bearing these floral leaves ceases to lengthen when the flowering stage of the plant has been reached.