Boerhavia diffusa, commonly known as ‘punarnava’ (meaning that rejuvenates the body) is a highly medicinal plant belongs to the family Nyctaginaceae. It is a prostrate herbaceous plant. Plants belonging to Amaranthaceae, Chenopodiaceae, Begoniaceae, Nyctaginaceae, Piperaceae etc. have many vascular bundles in the medulla. Such vascular bundles are called Medullary Vascular Bundles. Majority of them also show anomalous secondary thickening in their stem. In this post, we discuss the Anomalous Secondary Growth in Boerhaavia Stem.
Primary Structure of Boerhaaia Stem
Ø The epidermis is single-layered and parenchymatous.
Ø Numerous multicellular hairs are present over the epidermis.
Ø The hypodermis is two to three layers thick and composed of thick-walled collenchymatous cells.
Ø The cortex composed of parenchymatous cells with plenty of intercellular paces.
Ø Endodermis is present, but not distinct.
Ø Pericycle is one to two-layered.
Ø Vascular bundles are arranged in three rings.
(1). Two large centrally placed medullary vascular bundles
(2). A middle ring of six to fourteen loosely arranged and medium-sized vascular bundles
(3). An outer ring of fifteen to 20 small vascular bundles.
Ø All the vascular bundles are conjoint and open.
Ø The primary xylem is endarch.
Anomalous Secondary Growth in Boerhaavia
Ø As we discussed in the previous post about the anomalous secondary growth in Bougainvillea stem, here in Boerhavia also, the secondary thickening occurs by the formation of successive rings of accessory cambia.
Ø After the primary growth, the vascular bundles of middle ring increase in thickens to a limited extend b the activity of their fascicular cambium.
Ø No secondary thickening takes place in the medullary vascular bundles.
Ø Most of the secondary growth occurs in the outer ring of vascular bundles.
Ø The fascicular and inter-fascicular cambium of the outer ring of vascular bundles joins to form a continuous cambial ring.
Ø This cambial ring produce:
$. Internally: Secondary xylem in the intra-fascicular region and lignified conjunctive tissue in the inter-fascicular region.
$. Externally: Secondary phloem in the intra-fascicular region and parenchyma from the inter-fascicular region (above the sclerenchymatous conjunctive tissue)
Ø The activity of this cambium is short, after some time, the cambial activity ceases.
Ø A new cambium then arises by the joining of secondary parenchyma cells lying opposite to the phloem.
Ø This cambial ring also behaves similar to the previous cambium.
Ø It produces secondary xylem alternating with lignified conjunctive tissue on the inner side and secondary phloem opposite to the secondary xylem and parenchyma above the conjunctive tissue.
Ø After some times, the activity of this cambium also ceases.
Ø Additional cambia are differentiated external to the previous one as described previously.
Ø This process of accessory cambium formation repeated till four or more concentric rings of vascular bundles are formed.
Ø The concentric rings of vascular bundles embedded in the conjunctive tissue separated by thin walled parenchymatous zone give the appearance of growth rings.
Ø Phellogen is initiated in the hypodermal region.
Ø Phellogen produces phellem towards the outer side and phelloderm towards the inner side.
References
Prakash, E.J.J., 2000. Textbook of Plant Anatomy. Emkay Publishers, India
Rajput, K.S. and Rao, K.S., 1998, January. Cambial anatomy and absence of rays in the stem of Boerhaavia species (Nyctaginaceae). In Annales Botanici Fennici (pp. 131-135). Finnish Zoological and Botanical Publishing Board.
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