Anomalous secondary growth in stems

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Anomalous secondary growth in stems Dr.Anurita Sharma Asst.Professor, Botany P.G.G.C.G.-11, Chandigarh.

Monocotyledons Dracaena (the Dragon's blood tree) is the only monocot which has been shown to also have secondary growth in roots.

Dracaena stem

The Dracaena stem shown here, contains a number of scattered vascular bundles towards the central region of the stem, as well as vascular bundles arranged within what appears to be secondary, predominantly parenchymatous tissue. Dracaena and a few other genera such as Cordyline, Beaucarnea (=Nolina) are anomalous because they have secondary growth from a secondary thickening meristem (STM). This is irrespective of the fibres around the bundles as this occurs in both those with an STM and those without an STM.

Dicotyledons

Cross section of Boerhaavia stem, showing successive rings of xylem and phloem

Detail, showing part of the stem, note that the xylem contains metaxylem (MX) as well as protoxylem vessels (PX).

The stem in Boerhaavia contains well-defined anomalous secondary growth, which is characterized by the presence of successive rings of xylem and phloem. According to Rajput and Rao, (1998) The cambium is composed of fusiform initials only, which give rise to rayless secondary vascular tissues. The cambium is described as being storied when cell division ceases. Each successive ring of cambium is originated from the outermost phloem parenchyma cells.

The cambial ring is functionally segmented into fascicular and interfascicular regions which produce mostly conducting elements of the xylem and phloem with some parenchyma, the latter to parenchyma cells. The xylem parenchyma cells develop into conjunctive tissue following thickening and lignification of cell walls. In two of the species (B. verticillata and B. rependa) phloem parenchyma cells also undergo lignification, but in B. diffusa parenchyma cells remain primary-walled.

Alternate bands of lignified and parenchymatous bands are distinct in the stem. The xylem vessels elements are short with a simple perforation plate on slightly oblique to transverse end walls. The phloem contains sieve tube members and associated companion cells.