Ergastic Substances & Sclerenchyma Cells

Ergastic Substances or Cell Inclusions

Cell Inclusions are non-living substances present in the cells.

They are also called ergastic substances or ergastic bodies. They may be present in

soluble or insoluble state and can be organic or inorganic in nature.These are present in

components or sub-components of cell. They are raw materials or products of

metabolism.THEY may be exported or expelled from the cell.

The cell inclusions belong to three categories:-

Reserve foodExcretory or secretory productsMineral matter

Reserve food: They are of four main types-• Starch • Glycogen • Fat droplets• Aleurone grains

Starch grains occur in plant cells. The grains are found in chloroplasts and amyloplasts. As such they are insoluble. The grains may occur singly when they are called simple. They are called compound starch grains when two or more of them occur in amyloplasts.

Here SIMPLE STARCH grains of potato and wheat repectively

COMPOUND STARCH grains in avena and potato

Each starch grain has a central proteinaceous area called HILUM. Starch is deposited around it in the form of layers. Depending upon the position of hilum,a starch grain may be concentric or eccentric.

FAT DROPLETS: Fat droplets or globules occur abundantly inside the seeds either in endosperm(e.g., Castor,Coconut) or cotyledons(e.g., Groundnut,Mustard

ALEURONE GRAINS

They represent the storage proteins which are generally insoluble and occur inside special leucoplasts called aleuroplasts. Depending upon their internal structure,aleurone grains are of four types:

• Amorphouus• Protein matrix containing a crytalloid

e.g., outer layer of endosperm in wheat,maize,barley,grains.

• Protein matrix with globoid• Protein matrix having both crytalloid and globoid

inclusion, e.g, endosperm cells of castor seeds.CRYSTALLOID is crystal like protein-carbohydratebody while glboid contains lipid and phytin

EXCRETORY OR SECRETORY PRODUCTS:

They are of several types like:• Essential oils• Alkaloids• Resins• Gums• Tannins• Waxes• Latex etc.

Tanniferous bodies are widely distributed in different parts of the plant body and they are particularly abundant in leaves,vascular tissues,periderm,unripe fruits,seed coat. They are small granular or rounded particles,yellow,red or brown in color.These phenolic compounds are commercially useful in tanning industry.

Other soid and semi-solid substances,such as oils,fats and waxes also occur frequently. Oils and fats are common reserve materials of seeds,while waxes occur usually as protective covering on the epidermis.

MINERAL MATTER(CRYSTALS)Crystals of different composition,which are byproducts

of the metabolic processes of the cells,occur in different parts of the plant.

Calcium oxalate crystals are most common in plants. Their shape varies considerably and may be

elongated,needle-like,rectangular,rhomboidal,sphaeroidal or prismatic.

• The elongated crystals when occurring solitary are known as styloids and when in bundles they are called raphides. Crystals of inorganic compounds, such as silica and gypsum although uncommon are also present in some plants.

• In moneyplant,Needle-shapedRaphides appear as Scratches on slide.• In Opuntia,arrangedIn the form of spheres- “Sphaeroraphides”

• Silicon salts are often deposited in the cell walls of grasses. Cystoliths, which are internal outgrowth of cell wall occur in many plants.

e.g., FicusCalcium carbonateCrystals present inSacs-appear like bunch ofGrapes,”CYSTOLITH”

Sclerenchyma cells Mature sclerenchyma

cells are dead and have secondary cell walls thickened with cellulose and

usually impregnated with lignin. In contrast to collenchyma, which is pliable(able to

fold), sclerenchyma is elastic.Unlike the collenchyma, mature sclerenchyma is

composed of dead cells. with extremely thick cell walls (secondary walls) that make up

to 90% of the whole cell volume.  It is the hard, thick walls that make sclerenchyma cells

important strengthening and supporting elements in plant parts that have ceased

elongation

TYPES

• There are two types of sclerenchyma cells, namely sclereids and fibres.

Fibres: The cells are needle-shaped with pointed tips, thick walls and rather small lumen. Secondary cell walls, impregnated with, are formed. Simple pits are also present. Fibres are abundant in the vascular tissue of angiosperms, i.e. flowering plants.

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• Contrasting are hard fibres that are mostly found in monocots. Typical examples are the fibres of many grasses, agaves. Their cell walls contain, besides cellulose, a high proportion of lignin. Starting at the centre of the fibre are the thickening layers of the secondary wall deposited one after the other. Growth at both tips of the cell leads to simultaneous elongation.

. During development ,the layers of secondary material seem like tubes, of which the outer one is always longer and older than the next. After completion of growth the missing parts are supplemented, so that the wall is evenly thickened up to the tips of the fibres.

ECONOMIC FIBRES• The phloem fibres of dicotyledons represent bast

fibres and these are classofied as soft fibres because lignified or not,they are relatively soft and flexible. Some of the well-nown sources and usages of bst fibres are hemp(Cannabis sativa),jute(Corchorus capsularis),flaax(Linum usitatissimum),textiles(e.g., linen).

• Phloem fibres of some dicotyledons are used for making paper.

The fibres of monocotyledons-usually called leaf fibres because they are obtained from leaves-are classified as hard fibres. Examples of sources and uses of leaf fibres are Manila hemp(Musa textilis),Pineapple fibre(Ananas comosus) and Sisal(Agave sisalana).

Leaf fibres of monocotyledons(together with the xylem) serve as raw material for making paper.

• Sclereids: The cells are irregular in shape. The cell walls are thick, hard and lignified which makes the lumen very small. Simple pits (canals) are found in the thickened cell walls and link adjacent cells. Sclereids are commonly found in fruit and seeds. These have a shape of a star.

• They are small bundles of sclerenchyma tissue in plants that form durable layers, such as the cores of apples. They can be grouped into bundles, can form complete tubes located at the periphery or can occur as single cells or small groups of cells within parenchymatissues. But compared with most fibres, sclereids are relatively short

On the basis of their shape and thickness of walls sclereds have been variously classified:

Astrosclereids: They are star-shaped(stellate) cells found in the leaves of Nymphaea and stems and leaves of Thea

Brachysclereids or Stone cells: They are short roughly isodiametric sclereids,occurring in the cortex,pith and phloem of stems(Cinnamomum),pulp of fruits(Pyrus)

Macrosclereids: They are elongated columnarSclereids distributed in Seeds and fruits in theForm of a layer(leguminous seeds)

Macrosclereids

Osteosclereids: They are bone or barrel shaped and dilated at ends.

They occur in the leaves and seed coat of many cot many monocotyledons.

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