Vascular differentiation in seed and seedling of Phaseolus mungo

Abstract

No mature tracheary or sieve elements are present in the mature embryo Phaseolus mungo L. However, immature protophloem elements are found in the hypocotyl, and special cells, termed coprotoxylem cells, are found in the cotyledon. Upon germination protophloem sieve tubes differentiate basipetally into the root. Hyperchromatic phloem parenchyma cells are associated with protophloem sieve tubes in the root. Coprotoxylem cells differentiate basipetally into the hypocotyl. They are thick-walled, nonlignified and empty. Protoxylem first matures next to coprotoxylem poles in the upper hypocotyl when the seedlings are 21 hours old and then differentiates basipetally into the root. The root is tetrarch. In the lower hypocotyl, two strands of metaxylem differentiate along with each strand of protoxylem, forming a triad. In the upper hypocotyl where protoxylem splits into two parts, one strand of protoxylem associates with each strand of metaxylem. The two pairs of intercotyledonary strands split into eight just below the node and they all enter the epicotyl. The cotyledonary strands split in the cotyledon and produce several major longitudinal veins. Small transverse strands of xylem connect the longitudinal ones. The vascular bundles in the cotyledon are amphicribral.