Root Apical Organization in Monocotyledons—

ROOT APICAL ORGANIZATION IN MONOCOTYLIgDONS--MARANTACE2E BY S. K. PILLAI AND (MRS.) AMBUJA PILLAI (Department of Botany, Birla College of Science, Pilani; Raja~than) Received January 9, 1961 (Communicated by Dr. T. V. Desikachary, F.A.SC.) ]~NTRODUCTION THE authors have reported elsewhere the root apical organization of some members of the families Musace~e, Cannace~e and Zingiberace~e of the order Scitamine~e (Pillai and Pillai, 1961a, b; Pillai etal., 1961). This contribution embodies the results of their investigations of the root apices of seven species belonging to four genera of the family Marantace~e of this order. MATERIALS AND METHODS The techniques described by the authors in their previous reports were followed in investigating the root apices of Calathea zebrina Lindl., Stro- manthe sanguinea Sond., Phrynium capitatum Willd., Maranta variegata Lodd., Maranta major, Maranta zebrina Sims. and Maranta arundinacea Lindl. OBSERVATIONS Structural Organization This is essentially similar at the root apices of these species, except that in Maranta arundinacea which is described separately. Root-cap.--This is separate from the root body and is composed of two regions, the columella in the middle and the peripheral region around. The central columella does not extend far towards the tip in some roots like those of Phrynium capitatum (Text-Fig. 1). Where distinct, it is composed of vertically arranged files of cells. The cells of the meristem, the columellogen, concerned with the production of the columella, divide transversely like a rib meristem (Schopf, 1943; Allen, 1947; Spurr, 1949; Pillai and Pillai, 1961 b, Pillai etal., 1961) (Text-Figs. 2-4). The cells of the peripheral region of the cap can be distinguished from the central columella by the oblique orientation of its cell-rows which are 302 Root Apical Organization in Monocotyledons--Marantacece 303 I [- 304 S.K. PILLAI AND (MRS.) AMBUJA PILLAI curved from the sides towards the columella. T-type divisions are frequent at the sides with the head of the 'T' directed towards the flanks as at t and tk in Text-Figs. 1, 4 and 5. Such divisions enable this region to become wider towards the front. In transections the cells of the columella appear as a compact mass of polygonal cells in the centre with the cells of the peripheral region around it in radiating rows. The columella maintains long vertical files of cells. Furthermore, the number of cells across the diameter of the columella is essentially the same at different levels. When development of the lateral roots is studied, the endodermis is found to give rise to the root-cap and the pericycle to the root body. The middle cells of the endodermis which turn meristematic exhibit periclinal divisions and become distinct from the rest of the meristem very early (Text-Figs. 9-11). Histogens of the Root Body.--There are two histogens of the root body: The protoderm-periblem complex.--As in the roots of members of the Zingiberace~e and Cannace~e (Pillai and Pillai, 1961 b; Pillai etal., 1961), this is located in front of the plerome dome and is composed of one tier of cells in all the species listed here, except in Maranta arundinacea, where it is composed of two superimposed tiers of cells (Text-Figs. 2 and 7). The single tier is one to five cells across in the various species. The state of quiescence of the cells in this region makes it slightly difficult to recognize this layer in some cases. The cells of this tier of initials located on the flanks exhibit T-divisions, with the T-head pointing to the centre, by which the tissue widens out on the flanks. Repetition of such T-divisions results in the formation of a slight concavity at the front of the root body in some cases into which the columellogen fits (see Text-Figs. 3 and 4, for instance). Dermatogen.--This differentiates from the outermost layer of the pro- ducts of division of the protoderm-periblem complex at some distance on the flanks. Once distinct as a tier, it divides anticlinally and is concerned with the development of the epidermis only. In the quiescent centre the cells are comparatively elongated radially, while a little behind it, i.e., in the meristematic zone, they are thin and short and appear like a stack of coins. Behind this zone the cells again elongate along the long axis. Periblern.--The cells of this region undergo repeated divisions with the T-head directed to the plerome dome resulting in obliquely oriented Root Apical Organization in Monoeotyledons--Marantacece 305 files of cells, i.e., the Kiirper complex (Schiiepp, 1917), proceeding from outside the tip of the plerome dome towards the sides. After the periblem has become wide enough the T-divisions give place to anticlinal ones, the cell-rows exhibiting the rib meristem type of activity. Hypodermis.--This is composed of a few layers of cells. It originates from some of the inner twin daughter cells produced by the T-divisions of the protoderm-periblem complex near the plerome dome before the separa- tion of the dermatogen. These cells undergo further T and periclinal divisions to form the multilayered hypodermis. The cells of the hypodermis become vacuolated earlier than the other cells of the cortex. In Calathea zebrina and Stromanthe sanguinea these cells develop into a velamen-like tissue (Text-Fig. 8). The cells show a crumpled appearance even near the meristematic zone. These velamen-like cells are mycorrhizal, i.e., they have fungal hypha~, and algal cells too, inside them. On the inner side of this velamen-like hypodermis there are a few layers of narrow-lumened cells which are much elongated along the long axis of the root. These show thickenings like sclerenchyma in the mature portions of the root. The crumpled cells of the hypodermis, delimited by longitudinal sclerenchymatous cells towards the inside, were found in the roots of Maranta variegata, M. zebrina and Phrynium eapitatum also (Plate XIV, Fig. 1). Cortex has three distinct regions, the hypodermis outside, the longi- tudinally elongated ring of cells just inside the hypodermis and the rest of the cortex. The cells of the last-mentioned region are polygonal, contain deeply staining cytoplasm and exhibit the rib meristem type of division. Endodermis.---Near the plerome dome all the cells of the periblem exhibit T-divisions enabling the region to become wide (Text-Figs. 1-3). After a few T-divisions, the innermost of these initials divide only anticlinally. This is the endodermis. The endodermis can, as in members of Zingiberacea~ and Cannacea~, be said to arise from an endodermis-periblem complex. P.lerome.--This is clearly distinguishable except at the very tip, where due to the cells going to quiescence, the demarcation is not very clear. But close to the dome the outer boundary of this, nam.ely, the pericycle, can be distinguished by its cells with densely staining cytoplasmic contents. The pericycle is composed of a single layer of cells which divide only anticlinally. It is also characterised by the cells retaining their meristematic activity long after the other cells have lost it. The elements which go to compose the metaxylem vessels can be distinguished very close to the plerome dome by the cells becoming wider and 306 S.K. PILLAI AND (MRS.) AMBUJA PILLAI m- q¢- 6 @ • I TExT-FIGs. 5-11 starting vacuolation early. The cells destined to develoi~ into phloem elements are found, particularly in the transections, by their topography, as Root Apical Organization in Monocotyledons--Marantacece 307 groups of narrow-lumened ceils with densely staining cytoplasmic contents and exhibiting longitudinal divisions. From these develop the sieve tubes. In the centre, the cells exhibit the rib meristem type of division and give rise to the medulla. The organization of the tip of the roots of Maranta arundinacea differs from the others in having a two-tiered protoderm-periblem complex (Text- Figs. 2 and 7). Of these two tiers, the inner is concerned with the formation of the cortex and endodermis while the outer gives rise to only the dermatogen and hypodermis. T-divisions in the outer tier (Text-Figs. 2 and 7) give rise to the initials of the dermatogen and hypodermis. The outer daughter cell of the T may undergo one or two more T-divisions contributing the inner daughter cells to the hypodermis. After this, the outer daughter cell divides only anticlinally and gives rise to the dermatogen. The inner daughter cells undergo further T-divisions forming the multi-layered hypodermis, but they do not contribute cells to the cortex. This can be desig- nated as the "Dermato-hypodermis complex". Thus, the structural organization of the root apices of these species falls under two categories: (i) with independent plerome and root-cap and a single-tiered protoderm-periblem complex, and (ii) with independent plerome and root-cap and a two-tiered protoderm- periblem complex. Cytophysiological Organization The observations on this aspect correspond to those on members of the other families described already. There is a group of cells characterised by: (i) lightly stained cytoplasm with pyronin and methyl green and ha~mato- xylin (Text-Figs. 2, 4 and 6), (ii) vacuolation (Text-Figs. 5 and 7), (iii) smaller nucleoli and nuclei (Table II), (iv) lesser nucleolus/nucleus ratio (Table II), and (v) lesser frequency of mitotic figures (Table I). These features indicate that these cells are in a state of comparative repose. Behind this, as seen in longisections and around it as seen in transections, can be seen cells which have B3 308 S.K. PILLAI AND (MRS.) AMBUJA PILLAI TABLE I The number of cells and the number of dividing cells in tiw quiescent centre and the meristematic zone (as seen in one transection of each root) and the percentage of dividing cells Quiescent centre Meristematic zone Species Total No.

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