DEVELOPMENT OF INCLUDED PHLOEM IN THE STEM OF COMBRETUM NIGRICANS (COMBRETACEAE)

The development of diffuse included phloem strands in Combretum nigricans stems is described. During a short period of time, a small phloem strand is cut off locally in an inward direction by an otherwise normal bidirectional vascular cambium. This contrasts with previous descriptions and interpretations because these strands are not formed after redifferentiation of secondary xylem parenchyma. A complementary cambium formed at: the inner border of the young strand somewhat enlarges the strand and, during a relatively long period, produces secondary phloem outwards. Finally this complementary cambium stops functioning as a cambium and merges with the secondary phloem it has produced. Radial rows of cells are present within the included phloem strands which continue into the later-formed secondary xylem; rays transverse the strands. Crushing of the phloem takes place near the outer border of the strand, forming dap-like tissues of disorganized cells. The strands form a three-dimensional secondary phloem network. Because sieve tubes are rare in the external secondary phloem, and internal phloem as the periphery of the pith only temporarily functions, the functional assimilate transporting system of older C. nigricans stems consists of this network of included phloem.