Cell Contents: In pharmacognosy, we are concerned with the cell contents which can be identified in plant drugs by microscopical and physical tests. These are either food storage products or the by-products of plant metabolism and include carbohydrates, proteins, lipids, calcium oxalate, calcium carbonate, tannins, resins, etc. Some of these cell contents of diagnostic importance can be briefly described as follows.
Starch is present in different parts of the plant in the form of granules of varying sizes. Starch is found abundantly in fruit, seed, root, rhizome, and as smaller grains in chlorophyll-containing tissue of the plant such as a leaf. Starches of different origins can be identified by studying their size, shape, and structure, as well as, the position of the hilum and striations. Chemically, starches are polysaccharides containing amylopectin and β-amylose. Starch turns blue to violet when treated with iodine solution.
Starches of pharmaceutical interest are obtained from maize, rice, wheat, and potato. These starches can be differentiated from each other by microscopical examination. A comparative account of their macroscopical, microscopical and physical characteristics is given in Table.1. For purpose of microscopical studies, the powder should be mounted in Smiths starch reagent containing equal parts of glycerin, water, and 50% acetic acid.
Table.1: Characteristics of some starch grains
A systematic description of starch grains should include:
- Shape—Ovoid, spherical, sub-spherical, ellipsoidal, polyhedral, etc.
- Size—Dimensions in μm.
- Position of hilum—Central, eccentric, pointed, radiate, linear, etc.
- Aggregation—Simple, compound; several components present in a compound grain.
- The appearance between crossed polaroids.
- Location—Loose, present in the type of cell and tissue.
- Frequency—Occasional, frequent, abundant.
Protein is stored in the form of aleurone grain by plants. Aleurone grain consists of a mass of protein surrounded by a thin membrane and is found abundantly in the endosperm of the seed. The groundmass of protein, however, often encloses an angular body (crystalloid) arid one or more rounded bodies (globoids).
Defat thin sections containing aleurone grains and treat with the following reagents.
- Alcoholic picric acid—Ground tissue and crystalloid are stained yellow.
- Millon’s reagent—Protein is stained red on warming.
- Iodine solution—Only crystalloid and ground substances are stained yellowish-brown.
Calcium Oxalate Crystals
Calcium oxalate crystals are considered excretory products of plant metabolism. They occur and provide valuable information for the identification of crude drugs in entire and powdered forms.
- Microsphenoidal or sandy crystals—Belladonna.
- Single acicular crystals—Cinnamon, gentian,
- Prismsmatic crystals—Quassia, hyoscyamus, senna, rauwolfia, cascara.
- Rosettes crystals—Stramonium, senna, cascara, rhubarb.
- Bundles of acicular crystals—Squill, ipecacuanha.
The sections to be examined for calcium oxalate should be cleared with caustic alkali or chloral hydrate. These reagents very slowly dissolve the crystals, so the observation should be made immediately after clearing the section. The polarizing microscope is useful in the detection of small crystals.
Mount the cleared section or powder in the following reagents and observe the crystals.
- Acetic acid—Insoluble
- Caustic alkali—Insoluble
- Hydrochloric acid—Soluble
- Sulphuric acid (60% w/w)—Soluble, on standing replaced by needles of calcium sulphate.
Aggregates of crystals of calcium carbonate are called ‘cystoliths’, which appear like small bunches of grapes in the tissue. Calcium carbonate dissolves with effervescence in acetic, hydrochloric, or sulphuric acid. When treated with 60% w/w sulphuric acid, needled-shaped crystals of calcium sulfate slowly separate.
Fixed Oils and Fats
Fixed oils and fats are widely distributed in both the vegetative and reproductive parts of the plant. They are more concentrated in the seeds as reserved lipids. Fixed oils occur as small refractive oil globules, usually present in association with aleurone grains. Fixed oil and fat show certain common characteristics and respond to the following tests:
- They are generally soluble in ether and alcohol with few exceptions.
- 1% solution of osmic acid colours them brown or black.
- Dilute tincture of alkanna stains them red on standing for about 30 minutes.
- A mixture of equal parts of a strong solution of ammonia and a saturated solution of potash slowly saponifies fixed oil and fat.
Mucilages are polysaccharide complexes of sugar and uronic acids, usually formed from the cell wall. They are insoluble in alcohol but swell or dissolve in water. The following tests are useful for the detection of mucilage in cells.
- Solution of ruthenium red stains the mucilage pink. Lead acetate solution is added to prevent undue swelling or solution of the substance being tested.
- Solution of corallin soda and 25% sodium bicarbonate solution (alkaline solution of corallin) stain the mucilage pink.
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