Notes on Cell Inclusions (With Diagram)

Ergastic substances or cell inclusions are the products of cell metabolism, appearing and disappearing at various stages of cell’s life-cycle.

In majority of cases they are waste products of simple chemical nature compared to protoplasmic components which are more complex.

These ergastic substances may be present in the cell walls or vacuoles or in the organelles of protoplasm. They may be present in soluble or insoluble state and may be organic or inorganic in nature.

These substances belong to three categories:

1. Reserve food,

2. Inorganic Materials (Mineral matter)

3. Secretory products

4. Excretory products.

1. Reserve food:

They occur in the form of starch, glycogen, fat droplets and aleurone grains.

(i) Starch grains (Fig. 3.48):

Starch is the most important storage food. It is insoluble in water. Starch grains are found in all parts of the plant although in storage organs, e.g., seeds, fruits, rhizome etc., these are found in larger amount.

It is of two types:

(a) Temporary starch and

(b) Permanent starch.

The temporary starch, which is also known as assimilatory starch, is formed in the process of photosynthetic during day and converted to sugar during night. The permanent starch, which is also known as reserve starch, is found mostly in rhizome, seeds and fruits. Sugar above a certain level is converted to permanent starch.

Starch grains contain a polymer of C₆H₁₀O₅. Starch grains are of different shapes and form. Each starch grain has a central proteinaceous area called helium. Starch is deposited around it in the form of eccentric or concentric layers. The starch grains are oval eccentric in potato; oval and concentric in gram or pea; rounded, flat and concentric in wheat and polyhedral with radiating lines in maize.

Glycogen Granules:

They are minute granules of storage carbohydrates which occur in animal cells. The granules are flattened, circular or oval bodies which may form rosette-shaped aggregates.

(ii) Fat droplets:

They occur in both plant and animal cells. In plants fat droplets or globules occur abundantly inside the seeds either in endosperm (e.g., castor, coconut) or cotyledons [e.g., groundnut, mustard].

(iv) Aleurone Grains (Fig. 3.49):

They are insoluble storage proteins occur inside special leucoplasts called aleuroplasts. They occur in the outer endosperm cells of cereals, such as wheat, rice, maize grains.

2. Inorganic Materials (Mineral Matter) (Fig. 3.50):

The accumulation of inorganic materials within the plant and their cells mostly takes place in the form of calcium salts or anhydrous silicate salts. One very important type of deposit is that of calcium oxalate which is common in plants of many families. Their crystalline particles are of various shapes such as prismatic, needle-shaped, rhomboidal (diamond-shaped) etc. Very often the crystals occur as compound aggregates called druses, sphaerites etc. Elongated crystals are called styloids and raphides. Raphides occur in the form of bundles. Some crystals occur in special type of cells such as in case of idioblast cells.

Very often, crystals of calcium carbonate are also found in some plants. One very well known example is the cystolith found in some plants [e.g., Ficus leaves), which are found on an outgrowth of cell wall towards the interior of cell and this outgrowth bears CaC0₃ depositions. Leaves of Ficus species have cystoliths in their epidermal cells (Fig. 3.50-c).

3. Secretory Products:

Many substances secreted by special glands and organs, are found in plants, such as:

(i) Colouring matter:

Plants possess green colouring matter because of the presence of chlorophyll a and chlorophyll b. They also contain orange and yellow pigments, carotene and xanthophyll. The flowers and fruits become differently coloured because of the presence of carotene and xanthophyll. Blue, purple and pink colours are due to anthocyanin pigments which are found in vacuolar sap of fruits and petals of flowers and young leaves of some plants.

(ii) Enzymes:

Enzymatic proteins occur in colloidal state in the protoplasm. These enzymes convert complex organic food into simple compounds. For instance, enzyme diastase converts starch into glucose.

(iii) Nectar:

Nectar, secreted by nectaries in plants, attracts insects for pollination because it is sweet and contains sucrose, glucose and fructose.

4. Excretory Products:

Several chemical substances which are of no use to plants are produced during metabolic reactions. These waste products are called excretory products, but the plants do not have any special mechanism to remove these substances. However, some of these are thrown away by way of dropping of old leaves, bark and flowers. These excretory products are found as cell inclusions.

Some of the excretory products are:

(i) Alkaloids:

These are nitrogenous compounds, made up of carbon, hydrogen, oxygen and nitrogen. They are found in storage organs of plants such as seeds, bark and leaves. They are insoluble in water but soluble in alcohol. They have sour taste and some are poisonous. However, a large number of alkaloids, such as quinine, reserpine, nicotine, caffeine, thein, strychnine, morphine, atropine, are used as medicines.

(ii) Glucosides:

These are degradation products of carbohydrates. Some, such as digitoxin used in heart diseases, are used as medicine.

(iii) Tannins:

They are sour in taste and related to glucosides. They occur ^{n vacuolar sap, cell wall, bark and leaves of some plants. They are found mostly in unripe fruits. They are used on a large scale for hardening of leather, a process called tanning of leather.

(iv) Latex:

It is a milky substance secreted by latex glands. Robber secreted by the rubber tree Hevea brasiliensis is an important example.

(v) Essential oils:

These are volatile oils produced by special glint’s and cells. Aromain flowers, leaves and bark are due to essential oils.

(vi) Resins:

Produced by the oxidation of essential oils. These are found in some special glands or canals either alone or in combination with essential oils. These are insoluble in water but soluble in ether and alcohol. These are used in the manufacture of paints and varnishes.

(vi) Gums:

Produced by the disintegration of cellulose cell wall. They are soluble in water. Used for sticking purposes, and also as medicine,

(vii) Organic Acids:

These are found in leaves and fruits. Tartaric acid is found in fruits of Tamarindus, oxalic acid in Oxalis and citric acid in Citrus fruits.