Std. deviation divides to form a pair of guard cells. The epidermal cells that are lying at the polar region of the guard cell may divide and occasionally completely abut the stomatal complex and appear as subsidiary cells (Raju and Shah, 1975).
In commercial ginger rhizome (peeled dried rhizome), the outer tissue consisting of cork, epidermis, and hypodermis is scraped off. So the transections of processed rhizome consist of cortex, endodermis, pericycle, and the central cylinder or the vascular zone. The epidermis (of dry unpeeled ginger) is frequently disorganized, consisting of longitudinally oblong rectangular cells; the hypodermis consists of a few layers of parenchymal cells. The cork consists of several layers of oblong—rectangular, thin-walled suberized cells. The cortex is made of (1) thin-walled parenchymal cells containing plenty of starch grains, (2) brown-colored oleoresin and oil cells scattered throughout the cortex, and (3) fibrovascular bundles. There is an unbroken endodermis made of tangentially elongated cells with thickened suberized radial walls. Below the endodermis there is a pericycle that consists of an unbroken ring of tangentially elongated cells.
The central cylinder consists of an outer and an inner zone. In the outer zone adjoining the pericycle there is a vascular bundle zone without fibers. Fibrovascular bundles and oleoresin cells occur in the central zone of the central cylinder. The ground tissue of the central cylinder consists of thin-walled parenchymal cells containing starch.
The fibrovascular bundles are large. In longisections the fibers are long with moderately thick walls and a wide lumen. The vessels are large and scalariform, except in the vascular bundle zone adjoining the pericycle, where large reticulate vessels, scalariform vessels, and some special vessels occur.
Starch grains are present in abundance. The granules are ovate and many are characterized by a protuberance at one end. They vary in size to about 45 ^m in length and 24 fxm in width. Under polarized light the granules exhibit a distinct cross through the hilum at the tapering end (Parry, 1962).
Ginger rhizome powder is pale yellow or cream in color with a pleasant, aromatic odor and a characteristic and pungent taste. The diagnostic characteristics of ginger powder given by Jackson and Snowden (1990) are:
1. The abundant starch granules are mostly simple, fairly large, flattened, oblong to subrectangular to oval in outline with a small pointed hilum situated at the narrower end; infrequent granules show very faint transverse striations. Compound granules with two components occur very rarely.
2. The fibers usually occur in groups and may be associated with the vessels; they are fairly large and one wall is frequently dentate; the walls are thin and marked with numerous pits, which vary from circular to slit shaped in outline; very thin transverse septa occur at intervals. The fibers give only a faint reaction for lignin.
3. The vessels are fairly large and usually occur in small groups associated with the fibers; they are reticulately thickened, frequently showing distinct, regularly arranged rectangular pits, and are often accompanied by narrow, thin-walled cells containing dark brown pigment; a few smaller, spirally or annularly thickened vessels also occur. All the vessels give only a faint reaction for lignin.
4. The oleoresin cells in uncleared preparations are seen as bright yellow ovoid to spherical cells occurring singly or in small groups in the parenchyma.
5. The abundant parenchyma is composed of thin-walled cells, rounded to oval in outline with small intercellular spaces; many of the walls are characteristically wrinkled; the cells are filled with starch granules or oleoresin. Occasionally, groups of parenchyma are associated with thin-walled tissue composed of several rows of collapsed cells.
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