In Monocots, secondary thickening phenomenon is studied mostly in Dracaena and this is named “Abnormal Secondary thickening” in all Text books. But secondary thickening is seen in all plants which increase in girth, like palms and rhizomes of Zingiberales and Cyperaceae (More groups if studied will unearth more incidences of Secondary thickening). All the present studies prove that it is the Endodermis and/or pericycle which becomes the “Monocot cambium” to which evidences from Dracaena and Curcuma are provided here.
Dracaena stem is familiar to all Botany students. Stem had a central vascular cylinder (stele) enveloped by endodermis. The cortical region is devoid of any vascular tissue.(Fig A) During secondary thickening, the layer just below endodermis, i.e. pericycle divides in perclinally to produce a meristematic region, which is cambium, and provide vascular tissues in the form of vascular bundles towards centre and a few parenchymatous cells towards outside which we call as “secondary cortex” (Fig B). Therefore in this plant, the monocot cambium is pericyclic in origin
In Curcumaspp rhizome is differentiated to a central region delimited by endodermis and an outer region, both containing vascular bundles (Fig C). Here also the monocot cambium is pericyclic in origin. This cambium produces vascular bundles to the centre as that of Dracaena, but differs in that, there are no tissues produced between pericycle and endodermis (Fig D, E). In both Dracaena and Curcuma, cork cambium derived from hypodermis produces cork to the periphery and parenchyma inside
History
In monocots the secondary vascular system has been recognized within 22 genera only belonging to the Asparagales, whereas a protective tissue of secondary origin has been identified in Zingiberaceae, Bromeliaceae, Commelinaceae and Arecaceae (Schoute, 1902; Krauss, 1949; Tomlinson, 1961). Meristems responsible for secondary growth is named variously like “thickening ring “ “ Etagen cambium”, “ meristematic zone”, “ secondary thickening meristem”, “ anomalous cambium”, “vascular cambium”, “ accessory cambium”, “ cambium-like zone”, “monocot cambium”, “Etagen meristem”, “ storied meristem” (Fahn, 1967), and “ storied phellogen”. However, in the case of secondary protective tissues the meristem itself is rarely distinguished, and normally the term storied cork is used, covering both the meristematic cells and their derivatives.
The German term etagen and its English counterpart storied that appear frequently in descriptions of the monocot secondary growth, have been used to underline the temporary form of the monocot meristems.
Ontogenetically, the monocot cambium originates from the primary thickening meristem (PTM), which is a region of actively dividing meristematic cells, located around the apical meristem and extending down the periphery of the stem, where this continuation of PTM is referred to as STM. Thus, the occurrence of PTM is a pre-requisite for differentiation of the STM/monocot cambium.
According to Rudall (1991), some authors admit that the PTM corresponds to the pericyclic region of the stem. But the concept of "pericycle" in Monocots was debated. According to few, this is an entirely artificial concept, since in most monocotyledonous stems, the cortex and central cylinder each end where the other begins". Studies on origin of roots in monocots revealed that root vasculature (root trace) begins with the endodermis, distinguished with Casparian strips. Van Fleet (1961) refer to the endodermis as meristematic. Initially, the endodermis exhibits Casparian strips, and later, the walls of the endodermal cells in contact with the pericycle can be seen to be suberized. It is seen that percycle also starts dividing and thus become meristematic to become the cambium. These cambial cells produce vascular tissues to inside and remain inactive towards periphery. This proves the pericycle as the generative layer of secondary vascular tissues.
Manezes et al (2005) in a study of Cephalostemon riedelianus (Rapataceae), Cyperus papyrus (Cyperaceae), Lagenocarpus rigidus, L. junciformis(Cyperaceae), Echinodorus paniculatus(Alismataceae) and Zingiber officinale(Zingiberaceae), observed that the endodermis with meristematic activity in the root of all the species, in the stem of Cyperus, Cephalostemum and Lagenocarpus rigidus, and in the leaf trace of Cyperus and leaf of Echinodorus. Considering the continuity of tissues through the root, stem and leaf, the authors conclude that in the stem the pericycle remains active throughout the life of the plant as the generator of the vascular tissue. The "Primary Thickening Meristem" is in fact the pericycle plus the endodermis and its derivatives (or only the pericycle). Close to the stem apex, the assemblage of seems to be a unique meristem, giving rise to the inner cortex and vascular tissues.
References:
Meezes N.L. et al (2005) Meristematic activity of the Endodermis and the Pericycle in the primary thickening in monocotyledons: considerations on the "PTM" Biological Sciences • An. Acad. Bras. Ciênc. 77 (2)
Joanna J.M. (2015) Lateral Meristems Responsible for Secondary Growth of the Monocotyledons: A Survey of the State of the Art, Bot. Rev. 81(2): 150–161.
Mammen Daniel
