Structure and Ontogeny of Intraxylary Secondary Xylem and Phloem Development by the Internal Vascular Cambium in Campsis radicans (L.) Seem. (Bignoniaceae)

Bignoniaceae is known for the presence of different growth forms like trees, shrubs, lianas and herbs that show a wide range of variations in their wood structure. Trees, shrubs and lianas are characterised by the presence of normal secondary growth except the lianoid forms of the tribe Bignonieae that have a unique type of the cambial variant (that is, wedged/furrowed xylem) whereas other lineages lack it. Campsis radicans belongs to tribe Tecomeae and thus is expected to lack the cambial variant but it develops a quantifiable amount of intraxylary secondary xylem and phloem in the pith. Therefore, the main aim of the present investigation is to elucidate the ontogeny of intraxylary phloem and the secondary xylem formed by internal vascular cambium. Development of intraxylary phloem is delayed significantly as compared to other families of eudicots that show the presence of intraxylary phloem. Subsequently, a functionally bidirectional complete ring of internal vascular cambium initiates along the pith margin that produces secondary xylem centrifugally and secondary phloem centripetally. In thick stems, an internal vascular cambium forms quantifiable amounts of inversely oriented secondary xylem and phloem that completely replace the pith. Structurally, this secondary xylem remains similar to the xylem formed by the regular (external) cambium. It is composed of vessels, fibres, axial and ray parenchyma cells while the secondary phloem consists of sieve elements, companion cells, axial and ray parenchyma cells. The development of internal vascular cambium completely replaced the parenchyma of the pith by producing intraxylary secondary xylem and phloem. The reason for the occurence of intraxylary phloem in C. radicans remains unknown. However, to the lesser extent it may be playing a secondary role in conductive safety of the conduit.