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

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.