Formation of inter-and intraxylary phloem in some species of Argyreia Lour. (Convolvulaceae)

Abstract

The development of inter-and intraxylary phloem was investigated in nineteen species of Argyreia Lour. Among all species, differentiation of intraxylary protophloem was observed concomitant to the regular external protophloem from the procambial derivatives (except A. nervosa). However, A. nervosa, showed the formation of medullary bundles along with the regular protoxylem while the development of intraxylary phloem was observed only after the establishment of a complete ring of the vascular cambium and deposition of a few secondary xylem elements. In all the species (except A. nervosa), additional intraxylary phloem differentiated from the adjacent parenchyma and the intraxylary cambium initiated at the periphery of the pith. It divided uni-directionally and showed deposition of only phloem elements. In the transverse view, sieve elements of intraxylary phloem were oval to circular, rectangular, or multi-angled or sometimes in a cluster of three to several cells with simple and compound sieve plates oriented obliquely on their terminal ends. In the thick stems of A. cuneata, A. elliptica, A. lawii, A. mastersii, A. nervosa, A. sericea, A. sharadchandrajii and A. splendens, thin-walled unlignified axial parenchyma strands embedded within the secondary xylem divide and differentiated into isolated or groups of interxylary phloem elements. In the transverse view, these sieve elements were like regular phloem (oval to circular) in appearance but occasionally they may be narrow in diameter, tangentially flattened and arranged in clusters. In thick stems, earlier-formed sieve elements of the interxylary phloem collapse and get crushed due to the expansion of adjacent cells. The remaining species though showed the presence of parenchyma strands within the secondary xylem, no interxylary phloem was observed even in thick stems. Such parenchyma cells within islands always showed the presence of druses.