Stem Anatomy and Development of Intraxylary Phloem in Vallaris Solanacea (Roth) Kuntze (Apocynaceae)

A single ring of vascular cambium increases the stem thickness of Vallaris solanacea (Roth) Kuntze (Apocynaceae). During the ontogeny, procambium becomes distinct in the third visible internode and complete ring of the cambium is observed in fifth internode. Discrete strands of intraxylary phloem develop prior to protophloem in the fourth visible internode while additional intraxylary phloem derivatives are added subsequently from the adjacent parenchyma cells. Earlier formed phloem became nonconducting and accumulated centripetally due its obliteration. In fully grown thick stems, cambial action occurs external to the intraxylary phloem and differentiation of secondary xylem and phloem takes place in centrifugal and centripetal direction respectively. Diffuse porous secondary xylem with distinct growth rings consisted vessels, tracheids, fibres, axial and ray parenchyma cells. Dimorphic vessels showed drastic difference in their lumen diameter. The narrow vessels were thick walled and similar to tracheary elements with small and obliquely placed perforation plate on sub-terminal end. Vessels with wide diameter are mostly solitary while smaller vessels either appear in clusters or arranged in radial or diagonal multiples. Xylem rays are uni-to biseriate while multiseriate rays observed occasionally. Presence of radially arranged non-articulated laticifers in thick walled rays is frequent in all the samples investigated.