Morphology and Anatomy

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The ginger plant is a herbaceous perennial grown as an annual crop. The plant is erect, has many fibrous roots, aerial shoots (pseudostem) with leaves, and the underground stem (rhizome). The roots of ginger are of two types, fibrous and fleshy. After planting, many roots having indefinite growth grow out of the base of the sprouts. These are the fibrous roots, and the number of such roots keeps on increasing with the growth of

Plant Morphology And Anatomy Photos
Figure 2.1 A ginger plant showing aerial shoots and inflorescence.

tillers. These fibrous roots are thin, have root hairs, and their function is mainly absorption of water and nutrients. As a ginger plant grows further, several fleshy roots of indefinite growth are produced from the lower nodes of the mother ginger and primary fingers. These roots are thicker, milky white in color, with few root hairs, and no lateral roots. Such roots carry out the functions of support as well as absorption (Figure 2.4).

During the initial growth, the apical bud of the rhizome piece planted grows out and becomes the main tiller or mother tiller. As this tiller grows, its base enlarges into a rhizome. This is the first formed rhizome knob and is often called the mother rhizome. From either side of the mother rhizome, branches arise and they grow out and become the primary tillers (Figure 2.5). The bases of these tillers become enlarged and develop into the primary fingers. The buds on these primaries develop in turn into secondary tillers and their bases into secondary fingers. The buds on the secondary fingers in turn can develop into tertiary tillers and tertiary fingers.

The aerial shoots have many narrow leaves borne on very short petioles and with sheaths that are long and narrow, and the overlapping sheaths produce the aerial shoot. A pair of ligules is formed at the junction of leaves and sheath. The leaves are arranged in a distichous manner.

Ginger is a subterranean stem (rhizome) modified for the vegetative propagation and storage of food materials. The stem has nodes with scale leaves and internodes. Except for the first few nodes, all the nodes have axillary buds. When the rhizome bit is used for planting ("seed rhizome" or setts), there may be one or more apical buds on it; however, normally only one bud becomes active. When large pieces are used, more than

Study Part Sketching Turmeric Root
Figure 2.2 Sketch of the ginger plant showing the origin of shoots, inflorescence, and flower. AS: Aerial shoot, R: Rhizome, Fl: Flower, P: Peduncle (scape), S: Spike.

one bud may develop simultaneously. If more than one branch from the parent rhizome is responsible for the ultimate growth and development of the adult rhizome, the branches of the mature rhizome lie in the same plane (Shah and Raju, 1975a).

The pattern of rhizome branching is illustrated in Figure 2.6. The main axis developing from the apical bud, which is the first developing branch, has 7 to 15 nodes, which later

Floral Diagram Zingiber

Figure 2.3 Floral diagram of ginger flower.

Figure 2.3 Floral diagram of ginger flower.

Imbir Rysunek
Figure 2.4 Ginger rhizome showing two types of roots—thick, white fleshy roots and the fibrous roots with root hairs.
Ginger Anatomy Plant
Figure 2.5 Ginger rhizome showing the conversion of branch apices into aerial shoots.

becomes an aerial shoot. Once this axis becomes aerial, the subsequent growth of the rhizome is due to the development of the axillary buds situated above the first two to three nodes of the underground main axis. These axillary branches are plagiotropic and then they quickly show orthotropic growth at their distal region and subsequently become aerial shoots (see Figure 2.5). The same pattern of growth is continued for successive branches to form a sympodial growth pattern. A few axillary buds at the distal end of the branch remain dormant. The number of primary branches may be two, three, or four. These primary branches arise on either side of the main axis. Subsequent development of the secondary, tertiary, and quaternary branches are on the abaxial side of the respective branches. Irrespective of the number of primary branches, the subsequent branches lie in the same plane, although alteration of this scheme is seen sometimes. A mature rhizome may consist of 6 to 26 axillary branches with foliage leaves or only with sheath leaves and they show negative geotropic response (Shah and Raju, 1975a).

The number of nodes in each rhizome branch varies. The main axis (mother rhizome) and the subsequent branches (primaries) have 6 to 15 nodes. The internodal length of the rhizome branches ranges 0.1 to 1.5 cm, and varies even in a single branch. The internodal length is more in secondary, tertiary, and quaternary branches, and in the aerial stem it ranges from 3 to 7 cm. In the underground stem the nodes have scale leaves that ensheath and protect the axillary buds. These scale leaves fall off or may be lost, so that in mature rhizomes only the scars remain. Young scale leaves have pointed tips that help in penetration of soil.

The distal few nodes of the rhizome have sheath leaves. At the early stage of development they lack any apparent slit due to the overlapping of their margins. Later a longitudinal slit is formed through which the shoot tip projects. After the development of 6 to 12 scale leaves and 3 to 5 sheath leaves, the foliage leaves are produced. A foliage

Ginger Plant Anatomy

Figure 2.6 Growth pattern of the ginger rhizome.

A. Habit. B. Typical mode of growth pattern (nodes are represented by dark horizontal lines and dormant buds by a black spot). C. Two main axes developing from the seed rhizome, and their subsequent branches developing in the same plane. D. A main axis with four primary branches and their subsequent branches developing in the same plane. (Source: Shah and Raju, 1975a.)

Figure 2.6 Growth pattern of the ginger rhizome.

A. Habit. B. Typical mode of growth pattern (nodes are represented by dark horizontal lines and dormant buds by a black spot). C. Two main axes developing from the seed rhizome, and their subsequent branches developing in the same plane. D. A main axis with four primary branches and their subsequent branches developing in the same plane. (Source: Shah and Raju, 1975a.)

leaf consists of a leaf sheath, a ligule, and an elliptical—lanceolate blade. The leaf sheath is about 15 to 18 cm and lamina about 12 to 15 cm long. Above its region of insertion, the sheath encircles the internode; and from the side opposite to its origin up to the ligule, the sheath is open longitudinally. A distinct mid rib is present only in the lamina. The phyllotaxy of the scale leaves on the rhizome and foliage leaves on the aerial stem is distichous, with an angle of divergence of about 180°. Within the bud, leaves have imbricate aestivation (Shah and Raju, 1975a).

Rhizome Anatomy

The early studies on the anatomy of ginger were carried out mainly by the pharmacog-nosists, and they concentrated on the officinal part, the rhizome, either dry or fresh (Futterer, 1896). A comprehensive survey on the anatomy of the plants belonging to Zingiberaceae was that of Solereder and Meyer (1930), in their classical work Systematische Anatomie der Monocotyledonen (Systematic Anatomy of the Monocotyledons). They provided anatomical notes on 18 genera and some 70 species (Tomlinson, 1956). Later Tomlinson (1956) supplemented the information and filled in the gaps. However, no information was available on the developmental anatomy. Some studies were carried out by Pillai et al. (1961), Aiyer and Kolammal (1966), and Shah and Raju (1975b). More recently, Ravindran and colleagues investigated the developmental anatomy of rhizomes, oil cells, and associated aspects (Remashree et al., 1997; 1998, 1999; Ravindran, 1998). The following discussion is based on the studies of the above workers.

The transection of a fresh, unpeeled rhizome is almost circular or oval, about 2 cm in diameter, with the outline almost regular. The TS shows a light-brown-colored outer border and a central zone 1.2 cm in diameter marked off by a yellowish ring from an intermediate cortical zone. A distinct continuous layer of epidermis is generally present, consisting of a single row of rectangular cells; in some cases, it may be ruptured. Within this is the cork, varying in thickness from 480 to 640 ^m and differentiated into an outer region 300 to 400 ^m in thickness, composed of irregularly arranged, tangentially elongated, slightly brown-colored cells, and an inner zone of 6 to 12 regular rows of thin-walled rectangular to slightly tangential elongated cells arranged in radial rows. They measure 30 X 30 to 114 X 48 ^m. (Note: Cork tissue develops after the harvest and during storing. So when a rhizome is cut soon after harvest, one may not encounter much cork tissue.) A cork cambium is not evident. Inner to the cork is the cortex that is about 4 mm in thickness, composed of thin-walled large hexagonal to polygonal parenchymal cells. The cortical cells are heavily loaded with starch grains. These grains are large, simple, and ovoid, in length varying from 15 to 65 ^m. Scattered within the cortex are numerous oil cells that contain large globules of yellowish-green color. The outermost three to five rows of cortical cells are not rich in oil contents. Many scattered, collateral, closed vascular bundles are present, of which the greater number is seen in the inner cortical zone. The large bundles are partially or entirely enclosed in a sheath of septate fibers, whereas the smaller bundles are devoid of any fiber. Each vascular bundle consists of phloem, composed of small thin-walled polygonal cells with well-marked sieve tubes and xylem composed of one to nine vessels with annular, spiral or reticulate thickenings. These vessels have a diameter varying from 21 to 66 ^m. In the enclosing sheath of fibers the number of cells varies very much. There are 4 to 48 fibers or occasionally more. These fibers are very long, but less than 1 mm, have a diameter from 10 to 40 ^m, and are not straight, but undulate in character. The inner limit of the cortex is marked by a single-layered endodermis composed of thin-walled rectangular cells, much smaller than the cortical cells, with their radial walls slightly thickened and free from starch grains. The endodermis is lined by a pericycle composed of a single row of thin-walled slightly tangentially elongated cells devoid of any starch grains.

The stele that forms the bulk of the rhizome consists of parenchymal cells similar to those of the cortex, with starch grains and oil globules and a large number of irregularly scattered vascular bundles. Just within the pericycle a number of very small vascular bundles are arranged in a ring. These bundles have only one to three vessels and a small phloem. No fibers are present enclosing these small bundles. Generally, the vascular bundles present within the stele are slightly larger than those present in the cortex. The stele contains more oil cells and starch grains than the cortex (Aiyer and Kolammal, 1966).

Rhizome Enlargement

Rhizome enlargement in ginger is by the activity of three meristematic zones. Very early in the development of the rhizome, a zone of meristematic cells is formed at the base of a young scale leaf primordium of developing rhizome. These meristematic cells develop into the primary thickening meristem (PTM) and procambial stands. The meristematic activity of the PTM is responsible for the initial increase in the width of the cortex. The second type is the actively dividing ground parenchyma. The third type is the secondary thickening meristem (STM), in which fusiform and ray initials are clearly visible. The STM develops just below the endodermoidal layer.

At a lower level in the rhizome from the shoot bud apex, the PTM can still be identified. The scattered vascular bundles are developing from the PTM or procambial cells. Such groups of cells can be identified by the plane of cell division. The differentiation of procambial cells into vascular tissue takes place at different stages of rhizome growth. Unlike in many monocots, in ginger rhizome there is a special meristematic layer along with the endodermoidal layer, and this layer consists of cambium-like cells. The cells are thin-walled and arranged in a biseriate manner. In certain loci, where the vascular bundles develop, these cells are elongated with tapered ends and appear similar to the fusiform initials with an average of 62.34 ^m length and 8.12 ^m width in mature stages. Between these fusiform initials, some cells show transverse divisions to form ray initials. The presence of the cambium-like layer is an important feature in rhizome development. From this layer inverted and irregularly distributed groups of xylem and phloem are formed along the intermediate layer. The cells outer and inner to the cambial layer become filled with starch grains.

Development of Oil Cells and Oil Ducts

Oil cells are present in the epidermis or just below the epidermis of the leaf, petiole, rhizome, and root. In the rhizome, oil cell initials are present in the meristematic region. They are spherical and densely stainable. The initiation of oil cells and formation of ducts occurs in the apical parts of shoots and roots and starts much before the initiation of vascular elements. Secretory ducts are formed both schizogenously and lysigenously (Remashree et al., 1998; Ravindran et al., 1998).

Schizogenous Type

The schizogenous type of secretory duct originates in the intercalary meristem of the developing regions. The ducts are initiated by the separation of a group of densely stained meristematic cells through dissolution of the middle lamella. Concurrent separation of the cells leads to the formation of an intercellular space bordered by parenchymal cells. These ducts anastomose and appear branched in longitudinal section. Further separation of the bordering cells along the radial wall leads to widening of the duct lumen.

Lysigenous Type

The lysigenous type of duct formation is more frequent in the meristematic region, but occurs in mature parts too. There are four stages involved in its development: initiation, differentiation, secretion, and quiescence. These steps are a gradual process that occurs acropetally (Figure 2.7).

Initiation and differentiation: In shoot apex, the meristematic cells are arranged in tiers. In between these cells, certain cells in the cortical zone are distinguishable from the rest by their large size, dense cytoplasm and prominent nucleus (see Figure 2.7A). Such cells act as the oil cell mother cell. Anticlinal and periclinal divisions of these cells result in

Cork Cells Zingiber

Figure 2.7 Ontogeny of oil cell in ginger: lysigenous development.

A. Oil cell mother cell. B—D. Division of mother cell. E. Nuclear disintegration of central cell. F. Nuclear disintegration (note the deformed cell). G. Cytoplasmic condensation. H. Darkening of cell contents and increase in vacuolation. I. Mature oil duct with scanty cytoplasm (lc, lysing cell; n, nucleus; oc, oil cell; sd, secretory duct; s, starch grain; v, vacuole).

Figure 2.7 Ontogeny of oil cell in ginger: lysigenous development.

A. Oil cell mother cell. B—D. Division of mother cell. E. Nuclear disintegration of central cell. F. Nuclear disintegration (note the deformed cell). G. Cytoplasmic condensation. H. Darkening of cell contents and increase in vacuolation. I. Mature oil duct with scanty cytoplasm (lc, lysing cell; n, nucleus; oc, oil cell; sd, secretory duct; s, starch grain; v, vacuole).

a group of oil cell initials (see Figure 2.7B—E). Cyloplasmic vacuolation initiates in the oil cells at a distance of about 420 ^m from the shoot apex. Subsequently the surrounding cells also enlarge in size, showing cytoplasmic and nuclear disconfigurations (see Figure 2.7E, F). Further development leads to the disintegration of nuclear content of the central cell, which stretches toward the intercellular space. Later the central cell disintegrates and the contents spill into the cavity thus formed (see Figure 2.7I). This process that takes place in adjacent cells leads to the formation of a duct. The duct can be either articulated or nonarticulated, and becomes gradually filled up with the cell contents of the lysed cells. Once the lysogeny of the central cell is completed, the adjacent cells also lyse gradually in a basipetal manner, resulting in the widening of the duct lumen. These stages occur between 1500 and 3000 ^m from the apex.

Secretion: The differentiated oil cells start a holocrine type of secretion and expel their contents into the duct. Then the next cell (in acropetal order) becomes differentiated into an oil cell and starts elimination of its contents followed by lysis. Simultaneously the primary tissues continue to become differentiated into new oil cells and reach the secretory stage. The secretion fills the duct in young stages, but the quantity becomes reduced gradually, and finally the ducts appear empty. This could happen because of the diffusion of oil basipetally and radially; such oil particles are deposited in the cells and can be seen as black masses inside cells as well as in the intercellular space. Such stages are noticed about 3,250 ^m from the shoot tip (Ravindran et al., 1998; Remashree et al., 1999).

Quiescence: In the mature rhizome the ground parenchyma does not undergo further division and differentiation into the duct. In this stage the cells adjacent to the duct become storage cells, containing numerous starch grains and large vacuoles. An empty cell or cells with distorted cytoplasm appear along the duct lumen. Quiescence and secretory stages are visible from the third month onward after planting. In primary tissues the oil duct development is schizogenous, whereas further development proceeds both schizogenously and lysigenously.

Root Apical Organization

The root apical organization in ginger together with many other zingiberaceous taxa was first reported by Pillai et al. (1961). They found that the structural organization of ginger root apex differs from that of other taxa (such as Curcuma, Elettaria, and Hedych-ium). In ginger, all zones in the root apex are originated from a common group of initials. From the rim of this common group, calyptrogen, dermatogen, periblem, and plerome become differentiated. Raju and Shah (1977) also reported a similar observation in ginger and turmeric. The following discussion is adapted from Pillai et al. (1961).

The root cap is not differentiated into columella and a peripheral zone, and hence there are no separate initials for these regions. The cells in this region are arranged in vertical superimposed files. The cells arise by the activity of a meristem, which can be easily differentiated from the rest of the region. Pillai et al. (1961) named this meristematic region columellogen. In transections, the cells of the columella form a compact mass of polygonal cells in the center with the cells of the peripheral region arranged in radiating rows around it.

In the root body two histogens could be distinguished: (1) the plerome concerned with the formation of stele and (2) the protoderm—periblem complex concerned with the formation of the outer shell to the stele including periblem and dermatogen. The protoderm—periblem complex is located outside the plerome and is composed of a single tier of cells. The cells of this zone located on the flanks exhibit T-divisions, which help the tissue to widen out. Periblem consists of the initials of the cortex extending from the hypodemis to the endodermis. The hypodermis arises from the inner layer of the protoderm—periblem initials. The cells composing this tissue vacuolate earlier than the outer cells of the cortex.

Endodermis differentiates from the innermost periblem cells. Outside the plerome dome all cells of the periblem exhibit T-divisions initially but later in development show anticlinal divisions, and the endodermis is differentiated at that time.

Plerome has at its tip a group of more or less isodiametric cells. On the sides of the plerome dome is the uniseriate pericycle. Near the dome, cells take less stain because of their quiescent nature. The metaxylem vessel elements with wider lumens can be seen near the plerome dome. The isodiametric cells at the very center of the plerome divide like a rib meristem to give rise to the pith. In transections passing near the tip of the plerome dome, the initials can be distinguished as a compact mass of isodiametric cells surrounded by radiating rows of periblematic cells.

Cytophysiological Organization of Root Tip

The root tip can be distinguished into two zones on cytophysiological grounds:

1. The quiescent center: This zone is found at the tip of the root body, characterized by its cells having (a) cytoplasm highly stained with pyronin-methyl green and hematoxylin, (b) smaller nuclei and nucleoli, (c) cell divisions less frequent, and (d) vacuolation noticeable in most.

The median longisection of this group of cells is in the shape of a cup with the rim forward. The above characteristics show their state of rest and are called the quiescent center. This zone includes cells belonging to all the structural histogens of the root body (i.e., not structurally delimitable). It gradually merges with the zone outside, the meristematic zone. Raju and Shah (1977) studied the root apices of ginger, mango ginger, and turmeric with azure B staining to localize DNA and RNA contents in order to identify the quiescent center. A quiescent center was present in all the three cases as indicated by the light stainability of its cells. In longisection the quiescent center resembles an inverted cup.

2. The meristematic zone: This zone is shaped like an arch surrounding the quiescent center on the sides of the root body. The cells of this zone have the following features:

a. cytoplasm deeply stained with pyronin-methyl green and hematoxylin.

b. divides more frequently c. have larger nuclei and nucleoli d. vacuolation is absent or not prominent

The meristimatic zone includes the cells of all the structural histogens of the root body.

The percentage of cell division is much lower in the quiescent center compared to the meristematic zone. This character combined with the response of these cells to stains such as pyronin-methyl green indicates that these cells are in a state of comparative repose and hence are not synthesizing nucleic acids (Pillai et al., 1961). The distance between the tip of the root body and the nearest mature phloem element, which carries the metabolic products required by the active cells, was reported to be 480 to 490 ^m. This led to the suggestion that the cells at the tip of the root body go into quiescence because of the dearth of sufficient metabolites (Pillai et al., 1961).

Ontogeny of Buds, Roots, and Phloem

The ontogeny of ginger was studied by Shah and Raju (1975b), Remashree et al. (1998), and Ravindran et al. (1998). In a longisection, the shoot apex is dome shaped with a single tunica layer, below which the central mother cell zone is present. The flank meristem is situated on either side of the central mother zone. The leaf is initiated from the outer tunica layer and from the flank meristem. The shoot apical organization and acropetal differentiation of procambial strands are closely related to the phyllotaxy. At an even lower level basipetally in the rhizome axis, additional inner cortical cells are produced by a lateral PTM or procambium in which the resulting cells are radial rows.

The nature of the shoot apex: Shah and Raju (1975b) investigated the nature of the shoot apex in ginger. In the shoot apex in all stages, a single layer of tunica occurs, showing only anticlinal divisions. Cytohistological zonation based on staining affinity is not observed at any stage. The distal axial order (cr) includes the central group of corpus cells dividing periclinally and anticlinally and the overlying cells of the tunica (Figure 2.8). The peripheral zone (pr1) is concerned with the initiation of the next leaf primor-

Figure 2.8 Ontogeny of shoot apex: (A) dormant rhizome with stage 1 root apices; (B) rhizome with stage 2 shoot apex; (C) rhizome with stages 5, 6, and 7 root apices; (D) aerial apex showing topographical zonation; (E) rhizome apex showing topographical zonation. ab, axillary bud; as, aerial shoot; cr, central zone; lba, leaf base; lp1, lp2, leaf primordium; pr1, pr2, peripheral zone; rr, inner aerial zone; sr, seed rhizome; sz, shell zone. (Source: Shah and Raju, 1975.)

Figure 2.8 Ontogeny of shoot apex: (A) dormant rhizome with stage 1 root apices; (B) rhizome with stage 2 shoot apex; (C) rhizome with stages 5, 6, and 7 root apices; (D) aerial apex showing topographical zonation; (E) rhizome apex showing topographical zonation. ab, axillary bud; as, aerial shoot; cr, central zone; lba, leaf base; lp1, lp2, leaf primordium; pr1, pr2, peripheral zone; rr, inner aerial zone; sr, seed rhizome; sz, shell zone. (Source: Shah and Raju, 1975.)

dium and formation of the leaf sheath on the opposite side. It is delimited by the shell zone on the rhizome apices, which appears as an arc of narrow cells in median longitudinal section. The peripheral zone (pr2) is associated with the initiation of the next leaf primordium. In the rhizome apices it is also associated with the initiation of the axillary buds. As the phyllotaxy is distichous, this zone is opposite to pr1 in median longisections. Pith cells differentiate in the inner axial zone (rr).

Shah and Raju (1975b) recognized seven developmental stages of the apical bud. In stage one (dormant apex), the shoot apex lies in a shallow depression, the apex measures 116 to 214 ^m by 45 to 70 ^m. A few cells toward the flank showed increased concentrations of DNA as evidenced by dense staining. Some cells of pr1 and pr2 (see Figure 2.7) showed dense stainability for C-RNA (cytoplasmic RNA). The outer corpus cells show peripheral divisions. In stage two, the apex is dome shaped and its width and height are 94 to 165 ^m and 35 to 75 ^m, respectively. Zones pr1 and pr2 show denser histological staining than cr and rr zones. A biochemical zonation is present at pr2 that shows deep staining for DNA. The apex at stage three measures 76 to 140 ^m in width, and 53 to 86 ^m in height and is dome shaped. The cells of the inner axial zone are vacuolated. The shoot apex dome at stage four is 140 to 160 ^m high and 90 to 116 ^m wide. Outer corpus cells are vertically elongated. At stage five, the apex is a low dome having 214 to 248 ^m height and 53 to 75 ^m width. Cells of the pr2 zone show dense staining. The apex of stage six is prominently dome shaped having a width of 169 to 200 ^m and height of 87 to 96 ^m. During stage seven, the underground branch reaches the soil level. The shoot apex is 91 to 112 ^m in width and 134 to 167 ^m in height.

In ginger all the underground branches show a negative geotropic response. Two kinds of apices are found in ginger: (1) the apices are low dome and surrounded by either scale leaves or leaf bases, and (2) they are dome shaped and raised on an elongated axis. In the base of the rhizome apices, cells derived from the inner axial zone elongate tangen-tially and contribute to the widening of the axis. In certain cases these cells extend up to the base of the axillary buds. In a dormant apex they are thick walled and contain starch grains. These cells are distinct in the dormant or early active rhizome apex and constitute latitudinal growth meristem. During vascular differentiation a few cells of this meristem develop into procambium. During subsequent development of the rhizome apex the cells derived from the inner axial zone elongate and contribute to the pith.

Procambial differentiation: The peripheral or flank meristem divides periclinally and produces parenchymal cells. Some of the cells are distinguishable from the rest by deeper stainability, smaller size, less or no vacuolation, and darkly stained nuclei. These are the procambial initials and each such group contains 15 to 20 cells. Later these cells elongate, vacuolation increases, and they develop gradually into sieve tubes. Protophloem differentiation precedes that of protoxylem. The collateral differentiation of phloem and xylem with parenchymal bundle sheaths becomes distinct after an intermediate stage of random differentiation of the bundles. Ultimately the vascular bundles are found scattered in parenchymal ground tissue. In transection, an endodermoidal layer is also visible during the development (Remashree et al., 1998; Ravindran et al., 1998).

Axillary Bud

The development of leaves and scale leaves that encircle the shoot apex in ginger rhizomes is in a clockwise direction. The axillary bud meristem is first discernible in the axillary position on adaxial sides of the third leaf primordium from the apical meristem as a distinct zone by the stainability of the constituent cells and multiplane division of the cells in the concerned peripheral meristem sectors. The axillary buds thus originate as a cellular patch in the adaxial side of a leaf or scale leaf of the node. In a fully developed axillary bud the cytohistological zones akin to the main shoot apex can be distinctly observed. The development of a new rhizome is by the enhancement of a dormant axillary bud, which acts just like the main shoot apex. The procambial cells and the ground meristem cells divide and parenchyma as well as vascular tissues add thickness to the newly enhanced axillary bud. Likewise, many buds become active during favorable conditions, each of which produces secondary or tertiary rhizomes. The axillary buds show vascularization by the activity of the procambial strands of the mother rhizome and procambial cells originated from the differentiation of parenchymal cells.

Development of the Root

The adventitious root primordia become differentiated endogenously from the endoder-moidal layer of the rhizome. Roots always develop just below the nodal region. Transec-tion of the rhizome reveals that the endodermoidal layer and the pericycle become meristematic and undergo periclinal and anticlinal divisions resulting in a group of root initials. This is in direct connection with the vascular ring situated beneath the endo-dermoidal layer. The root primordia are of the open type, having common initials for the cortical meristem, root cap, and protoderm. The actively dividing and deeply staining central cylinder shows vascular connections with the rhizome vasculature. As the enlarging root primordia emerge through the cortex, the cortical cells are crushed and torn apart. Normally, these roots originate from the lateral or opposite side of the axillary bud and scale leaf.


As a rule there is no secondary growth in monocots. However, the rhizome structure of ginger gives evidence of both primary and secondary growth having a well-developed endodermoidal layer and cambium. The vascular bundles are collateral, closed, and scattered in the ground parenchyma. The phloem element consists of the sieve tube, companion cells, parenchyma and fiber.

Sieve tube: Phloem cells originate from a group of actively dividing procambial cells of PTM. These cells can be distinguished from the surrounding cells by their meristem-atic activity, stainability, and size of the nucleus. During development, a procambial cell elongates and becomes thick walled with cytoplasm and a prominent nucleus; this is the sieve tube mother cell. It undergoes a longitudinal unequal division, and the resulting smaller cell gives rise to the companion cell. This cell continues to divide, forming four to eight cells. The large cell is the sieve cell. It has cytoplasm and nucleus in early stages, which degenerate during its development into the sieve tube. During further development, the vacuolation increases and the cytoplasm shrinks and appears like a thread along the wall. At the same time, the nucleus disintegrates and the cell assumes the features of the enucleated sieve tube element. The transverse wall of the sieve tube changes to simple sieve plates with many pores and with very little callose deposition.

The first sieve tube element can be distinguished at a distance of 720 to 920 ^m from the shoot apex.

In the ginger rhizome, four to eight companion cells per sieve tube element are arranged in vertical lines with transverse end walls. They may vary from 18 to 32 ^m in length and 7 to 19 ^m in width. The sieve tube elements are arranged end to end to form columns of sieve tubes. The length of a sieve tube element varies from 57.5 to 103.8 fxm, the average being 76.8 fxm. The width varies from 5.29 to 10.35 ^m, the average being 8.76 ^m (Remashree et al., 1998). At the early stage of development, the slime body is present in the sieve tube, which appears to be amorphous but homogeneous. Later the slime body disintegrates.

In ginger, development of sieve tube is pycnotic, similar to the second type of nuclear degeneration reported by Esau (1969) and Evert (1984). The sieve element passes through a "fragmented multinucleated stage," a unique feature in the ontogeny of the multinucleated sieve tubes as reported by Esau (1938).

Phloem parenchyma: The phloem parenchymal cells are comparatively larger than the companion cells and smaller than normal cortical parenchymal cells. The increase in size of the phloem element is proportional to the growth of the rhizome. Some older phloem parenchymal cells become lignified into thick phloem fibers.

Anatomical Features of Ginger in Comparison with Related Taxa

Many species-specific anatomical variations were noted in the genus Zingiber. These variations were presented in a comparative study of ginger and three other species (Ravindran et al., 1998). The salient features are given in Table 2.1, which presents the important anatomical similarities and differences among the four species: Z. officinale, Z. roseum, Z. zerumbet, and Z. macrostachyum. Ginger has distinct anatomical features compared to other species, such as the absence of periderm, short-lived functional cambium, the presence of xylem vessels with scalariform thickening, helical and scalar-iform type of xylem tracheids, scalariform perforation plate, outer bundles with a col-lenchymatous bundle sheath, and high frequency of oil cells. The oil cell frequency was found to be 17.8/mm2 in ginger, whereas the corresponding frequency in the other species was 9.5, 5.3, and 2.8/mm2 in Z. zerumbet, Z. macrostachyum, and Z. roseum, respectively. Species differences were also noticed in fiber length, fiber width, and fiber wall thickness. Histochemical studies indicated that Z. zerumbet has greater amount of fibers than the others.

In general, xylem elements in Zingiber consist mainly of tracheids and rarely of vessels. The secondary wall thickening in the tracheids of ginger is of two types, scalariform and helical. The rings, or helices, are arranged either in a loose or dense manner. The helical bands are found joined in certain areas giving ladder-like thickening. The width of helical tracheids is less than that of scalariform tracheids. Similar tracheids are present in Z. macrostachyum, whereas in Z. zerumbet and Z. roseum, only scalariform thickening occurs (Ravindran et al., 1998). Xylem vessels occur in ginger and not in other species. Snowden and Jackson (1990), while studying the microscopic characters of ginger powder, recorded that the vessels are fairly large, reticulately thickened, less commonly spirally, and annularly thickened.

Table 2.1 Comparative anatomy of four species of Zingiber


Z. officinale

Z. roseum

Z. zerumbet

Z. macrostachyum

Epidermis Periderm

Cortex (outer cylinder) Endodermis Casparian strips Cambium Central cylinder

Number of vascular bundles

Nature of vascular bundles Vascular bundles distribution

Single layered Absent

Not wide

Present Present Present

Wider than the outer zone

Less in the outer cylinder than in the inner zone

Collateral closed

More toward inner cortex and scattered in the central zone

Single layered Periderm with lenticel Not wide

Present Present Not found Wider than the outer zone

Less in the outer zone than in the inner zone

Single layered Periderm present

Not wide

Present Present Not found Comparatively less wider than the outer zone Less in the outer zone than in the inner zone

Collateral closed Collateral closed

More toward inner cortex and scattered in the central zone


Xylem elements Vessels


Vessels, tracheids, and fibers

Vessels few with scalariform/reticulate thickening


Tracheids, fibers Not found

More bundles in the middle cortex and number of bundles is very less compared to other 3 species


Tracheids, fibers Not found

Single layered Absent


Present Present Not found Not wider than the outer zone

More in the outer cylinder than the other 3 species but lesser than the inner zone Collateral closed

Bundles are arranged in two rows in the middle cortex and only a few bundles in the inner cortex and the bundles are uniformly distributed in the central zone. Present

Tracheids, fibers Not found

Xylem tracheids

Helical and



Helical and


scalariform type

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