Flora 226 (2017) 47–54 Contents lists available at ScienceDirect Flora journal homepage: www.elsevier.com/locate/flora Internal cambium and intraxylary phloem development in Ipomoea turbinata Lag. (Convolvulaceae) Kishore S. Rajput ∗ , Amit D. Gondaliya Department of Botany, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390 002, India a r t i c l e i n f o Article history: Received 19 July 2016 Received in revised form 5 October 2016 Accepted 3 November 2016 Edited by Alessio Papini Available online 15 November 2016 Keywords: Internal phloem Morning glory Vessel dimorphism Variant growth Xylem a b s t r a c t Strands of phloem that are present at the periphery of pith are known as intraxylary phloem. Its presence remains restricted to a small portion of eudicots and considered as a characteristic feature for certain families. In the present study, Ipomoea turbinata Lag. showed development of intraxylary protophloem on adaxial tips of protoxylem from the procambial derivatives. Subsequently, additional intraxylary sieve elements were added from the adjacent parenchyma cells that were morphologically different from the pith cells. In thick stems, thin walled cells located between protoxylem and intraxylary protophloem acquired meristematic characters and formed several small segments of internal cambium. Initially, these cambial segments produced only phloem derivatives. Soon after, these segments became bidirectional and began to produce secondary xylem centrifugally and secondary phloem centripetally. Interestingly, in some of the samples development of secondary phloem and xylem was observed in the same direction. The secondary xylem formed by the internal cambium was composed of wide and fibriform vessels, fibres and axial parenchyma. Phloem possessed sieve tube elements, companion cells and parenchyma cells while rays in both xylem and phloem were mostly uniseriate but multiseriate rays were also observed. © 2016 Elsevier GmbH. All rights reserved. 1. Introduction Phloem forms an important constituent of the vascular system and plays a pivotal role in the translocation of photosynthate syn- thesized by leaves (Pace et al., 2015). Recently transport of signaling molecules like mRNA, macromolecules, proteins, several amino acids and a number of unknown secondary metabolites (Golecki et al., 1999; Zhang et al., 2010) are also ascribed to it. In most of the eudicots and magnoliids, distribution of secondary phloem tissue remains restricted external to the cambium. However, in some of the plants it may be distributed/formed within secondary xylem referred as interxylary phloem or included phloem (IAWA Committee, 1989) while in some of the cases it may differenti- ate at the periphery of the pith is referred as intraxylary phloem or internal phloem (Carlquist, 2013). Beside these patterns of the phloem distribution, family Bignoniaceae stands distinct in which small segments of the normal cambium produce relatively less sec- ondary xylem as compared to secondary phloem. Therefore, such cambial variant results in the formation of four or multiple phloem wedges (Pace et al., 2015). In the present study, Ipomoea turbinata is ∗ Corresponding author. E-mail address: ks.rajput15@yahoo.com (K.S. Rajput). one of the few species that show development of secondary xylem in quantifiable amount from the internal cambium. Moreover, for- mation of intraxylary/internal phloem is unique i.e. in some of the samples development of intraxylary phloem and secondary xylem, takes place on the same direction. The occurrence of intraxylary phloem is reported in 19 fam- ilies (Carlquist, 1988) and the family Convolvulaceae is one of them (Carlquist and Hanson, 1991; Rajput et al., 2008, 2014). Recently, Carlquist (2013) reviewed diversity of inter- and intraxy- lary phloem and its functions. Further, he stated that in some examples of intraxylary phloem, cambial activity [referred here as internal cambium (Rajput et al., 2014)] takes place on the inner margin of protoxylem. The similar cambium (referred as per- imedullar cambium) has also been reported by Angyalossy et al. (2015) in Maripa glabra from Convolvulaceae). Internal cambium may be functionally unidirectional i.e. producing only phloem or it may become bidirectional and produce phloem centripetally and xylem centrifugally. Such internal cambium is reported by earlier workers (Fukuda, 1967; Lowell and Lucansky, 1986; Carlquist and Hanson, 1991; Terrazas et al., 2011; Carlquist, 2012; Rajput et al., 2008, 2014). However, no exhaustive list of such species is avail- able in which such cambial activity occurs (Carlquist, 2013). The thick stems of Ipomoea turbinata accumulate a considerable amount of intraxylary secondary xylem and phloem on the pith margin. http://dx.doi.org/10.1016/j.flora.2016.11.002 0367-2530/© 2016 Elsevier GmbH. All rights reserved.