Development of lateral meristem and pattern of secondary growth in the stems and roots of Spinacia oleracea Linn (Amaranthaceae) were studied anatomically. Maturing stems and roots increased in thickness by developing successive rings of cambia which produce secondary vascular elements embedded in the groundmass of thin walled parenchymatous conjunctive tissues. Second ring of the cambium was originated outside the first ring of the cambium and gave rise to parenchyma cells and sieve tube elements on the outer side whereas it gave rise to conjunctive tissue and secondary xylem on the inner side. Prior to the development of second ring of cambium, parenchyma cells located outside to the previous cambium dedifferentiated and acquired meristematic property which led to the formation of meristematic bands of radially arranged cells. Later on each successive ring of cambium developed in similar fashion as described above. The groundmass of the stems and roots was mostly composed of thin walled conjunctive tissue on both the outer and inner side of each cambium, whereas formation of thick walled conducting elements of secondary xylem and secondary phloem remained restricted to the small segments of the vascular cambium. Secondary xylem was composed of vessel elements, tracheids and libriform fibres whereas secondary phloem comprised of sieve tube elements, companion cells and axial parenchyma with no typical rays even in the mature stems. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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