Vol.:(0123456789) 1 3 Journal of Plant Growth Regulation https://doi.org/10.1007/s00344-017-9771-x Structure and Ontogeny of Intraxylary Secondary Xylem and Phloem Development by the Internal Vascular Cambium in Campsis radicans (L.) Seem. (Bignoniaceae) Kishore S. Rajput 1  · Amit D. Gondaliya 1  · Manoj M. Lekhak 2  · Shrirang R. Yadav 2 Received: 10 May 2017 / Accepted: 9 November 2017 © Springer Science+Business Media, LLC, part of Springer Nature 2017 Abstract 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. Keywords Bignoniaceae · Cambial variant · Internal phloem · Medullary phloem · Xylem · Phloem Introduction Bignoniaceae is one of the families of the order Lamiales Bromhead (APG III 2009). The family is characterised by the presence of a wide range of habits, in which trees and lianas are dominant growth forms whereas shrubs and herbs are rare (Fischer 2004). Climbing members of the tribe Bignonieae are symbolised by a typical cambial variant in which a small segment of the cambium starts producing less secondary xylem; thus resulting in furrowed xylem and formation of phloem wedges (De Bary 1884; Schenck 1893; Dobbins 1971; Pace and others 2009, 2011, 2015) whereas climbers from other tribes of the family lack a fur- rowed xylem (Pace and others 2015). However, Sphingiphila tetramera A. H. Gentry is the only member from the tribe Bignonieae that lacks furrowed xylem (Pace and others 2016). De Bary (1884) reported the occurrence of phloem wedges in Bignoniaceae (compare Philipson and others 1971), which has been known for the last 133 years. There- after, several studies have been conducted on various species of this family (Schenck 1893; Pfeiffer 1926; Philipson and others 1971; Dobbins 1971; Pace and others 2009, 2011, 2015). Amongst them, significant contributions particularly on climbing species of Bignoniaceae are those from Dr. Pace Marcelo (University of São Paulo, São Paulo, Brazil), who studied 181 species (of which 103 species were a part of * Kishore S. Rajput ks.rajput15@yahoo.com 1 Department of Botany, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390 002, India 2 Angiosperm Taxonomy Laboratory, Department of Botany, Shivaji University, Kolhapur 416 004, India