Anomalous Secondary Thickening in Dracaena, a Monocot

Dracaena is a typical example of Anomalous secondary thickening  in monocots.
Typically, secondary thickening is absent in monocots. Therefore, secondary thickening itself is an anomaly as Dracaena is a monocot.
Dracaena young stem cross section
Young stem has typical monocot structure
1.Epidermis single layered
2.Hypodermis is sclerenchymatous
3.Numerous closed, collateral vascular bundles scattered in the parenchymatous ground tissue
4.Xylem is endarch
Anomalous secondary thickening in Dracaena
In Dracaena, secondary growth is due to
a) Extrastelar cambial ring in a monocot stem at the cortex
b) Abnormal activity of cambium
Dracaena anomalous secondary thickening TS, cross section
During Secondary thickening
1.Formation of secondary meristem or secondary cambium occurs in the inner region of parenchymatous cortex
2.The activity of cambium is abnormal
3.It produces secondary vascular bundles on its inner side only and parenchymatous cells on the outer side.
4.The secondary vascular bundles are amphivasal where phloem is surrounded by xylem
5.Later cambium produces more parenchymatous cells to the inside that pushes newly formed vascular bundles to the centre. The activity and position of vascular bundles keep on changing and vascular bundles are arranged in concentric rings. The second ring of vascular bundles is alternating in position with the first ring. The vascular bundles in the last inner ring are embedded in a mass of lignified conjunctive tissue.
6.Cork cambium activity is normal and produces cork and secondary cortex at the outer region.
See notes on Anomalous Secondary Thickening in Bignonia
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