In: Phyton 57 (2017): S. 79-89 DOI: 10.12905/0380.phyton57-2018-0079 Published online on 20th February 2018

Published online on 20th February 2018

Structure of secondary xylem and development of intraxylary phloem in Beaumontia jerdoniana (Apocynaceae)

Amit D. Gondaliya, Manoj M. Lekhak, Shrirang R. Yadav & Kishore S. Rajput

Key words: Internal cambium, intraxylary phloem, internal phloem, laticifers, radially arranged laticifers, second­ary growth, secondary xylem, secondary phloem, Beaumontia jerdoniana, Apocynaceae.

Summary
Gondaliya A. D., Lekhak M. M., Yadav S. R. & Rajput K. S. 2018. Structure of secondary xylem and development of intraxylary phloem in Beaumontia jerdoniana (Apocynaceae). – Phyton (Horn, Austria) 57 (1–2): 79–89, with 4 figures.

The presence of intraxylary phloem is characteristic of certain plant groups including the family Apocynaceae. In the present study, Beaumontia jerdoniana Wight, a member of Apocynaceae is investigated to elucidate the development of intraxylary phloem. Intraxylary phloem was found on the pith margin but also within the pith region, 350–841 µm in­wards from the protoxylem. Development of intraxylary phloem initiates from the ground meristem concomitant to nor­mal external phloem, and well developed discrete strands appear before the development of protoxylem elements. With the advancement of secondary growth, additional strands of intraxylary phloem develop from the adjacent pith cells while thick stems showed formation of internal cambium. Functionally, the internal cambium was unidirectional and showed formation of only intraxylary phloem at least in our samples. Radial increase in the stem diameter is achieved by a single ring of bidirectional vascular cambium that produces secondary xylem centripetally and secondary phloem cen­trifugally. However, formation of conducting elements, particularly wide vessels remains restricted to a small segment of the cambium (i.e. few fusiform cells) while the rest of them differentiate only into xylem fibres. Radially arranged secre­tory ducts (laticifers) were also present in some of the xylem rays. Development of intraxylary phloem and structural details of secondary xylem and phloem are discussed in detail.