Inter- and Intraxylary Phloem in Vascular Plants: A Review of Subtypes, Occurrences, and Development

Abstract

Phloem is one of the vital tissues of the vascular system that plays a crucial role in the conduction of photosynthates. In vascular plants, it develops external to the vascular cambium but in a small fraction of eudicots (formerly known as dicots), it occurs within (interxylary) and inside (intraxylary) the secondary xylem. Ontogenetically, it is classified as Strychnos, Combretum, Azima, and Calycopteris types. In all four cases, phloem islands remain enclosed within the secondary xylem but each has unique origins. Similarly, the deposition of the phloem at the pith margin is common in several plants. It develops from procambial derivatives or adjacent pith cells or by initiating an intraxylary phloem cambium. Functionally, this cambium can produce only phloem or both secondary xylem and phloem. In some instances, the deposition of the secondary xylem and phloem in the same direction has also been documented. Some experimental evidence is available on the role of phloem but is it applicable to inter- and intraxylary phloem? The presence of inter- and intraxylary phloem is attributed to a defence mechanism against insects or plants that show sudden and enormous flowering or it can correlate with high temperatures or an unconducive climate in a desert region where sieve tube elements have become nonfunctional due to high temperatures. The present review is an attempt to analyse the role of interxylary and intraxylary phloem.

Vascular variants in seed plants—a developmental perspective

“…There are distinct patterns that originate from developmentally diverse processes and which are described with several names, for example, ‘internal phloem’ ( Patil and Rajput 2008 ; Rajput and Gondaliya 2017 ), bicollateral bundles ( Hayden and Hayden 1994 ; Patil et al 2011 ), perimedullary phloem ( Araújo and Costa 2006 ) and medullary cambia ( Philipson and Ward 1965 ). In the context of vascular variants, these overlapping yet disparate phenomena ( Carlquist 2012 ) have been placed under the term ‘ intra xylary phloem’ ( Carlquist 2013a ; Angyalossy et al 2015 ; Rajput et al 2022 ), and considered a synapomorphy of some lineages such as Myrtales and core Convolvulaceae ( Stevens 2001 onwards; Stefanovic et al 2002 ). Since many patterns deviate from the typical eustele, for instance, through the formation of bicollateral bundles (e.g.…”

“…Apocynaceae), they are better described as cases of procambial variants. In some cases, perimedullary pith cells may differentiate into phloem followed by a de novo cambium, which may produce large amounts of vascular tissue in the pith ( Rajput et al 2018 , 2022 ). Yet, in some cases, it may be difficult to distinguish whether perimedullary phloem originates from procambial-derived cells or from a de novo cambium, in which cases even vascular variants categories may be difficult to apply.…”