In this article we will discuss about the ergastic substances present in the cells of a plant:- 1. Reserve Materials 2. Secretory Materials 3. Excretory Materials or Waste Products.
1. Reserve Materials:
Reserve materials are those non-living inclusions which are directly concerned with the nutrition of plants, i.e. they serve as plant food matters.
They are again of three types, viz.:
(B) Nitrogenous matters, and
(C) Fats and Oils.
These are composed of carbon, hydrogen and oxygen, where the latter two occur in the same proportion as in water. On heating carbohydrates get charred, leaving the black mass, carbon.
The following are the main carbohydrate food matters of plants:
Sugars are the simplest soluble carbohydrates of plants. Glucose or grape sugar (C6 H12O6) is manufactured by chloroplasts in the presence of sunlight. Fructose or fruit sugar with the same formula occurs in many fruits. Sucrose or cane sugar (C12H22O11) is abundantly present in sugar-canes and beets. This is our table sugar.
Inulin, (C6H10O5)n, is a soluble carbohydrate present in root tubers of Dahlia and of some plants of that family. When the root tubers are steeped in alcohol or glycerine for a few days, the soluble inulin precipitates out in the form of beautiful fan- shaped crystals (Fig. 124).
3. Starch Grains (Fig. 125):
Starch grains are the complex plant food and are universally found in all plant groups with the exception of fungi and bacteria. Starch grains formed by the chloroplasts are called assimilatory starch, which are converted into sugar soon. Reserve starch grains are produced by the amyloplasts out of simple sugar.
When we speak about starch, we usually mean the reserve starch grains. They are abundantly present in the cotyledons, endosperm, roots, underground stems, etc. Starch grains are of varying shapes. Every grain has a shiny point, called the hilum, which is the centre of formation.
Around the helium starchy matters are deposited layer after layer, giving the grain a stratified appearance. The layers are known as lines of stratifications. In the starch grains of potato tuber the hilum is located at one end of the grain due to unequal deposition.
They are called eccentric grains; whereas those in the cotyledons of pea are known as concentric, as they have lines of stratifications around the hilum. Starch grains are simple, when they have one hilum with lines of stratifications.
More than one simple grain may be adpressed together to form a compound grain. An intermediate form is noticed where the grain has two hila, their own stratified lines, but they are ultimately surrounded by common lines of stratifications. Such grains are called semi-compound or half-compound.
As the plants cannot take solid food, starch grains are converted into sugar before assimilation. Starch grains have a very characteristic test, viz. they turn blue when treated with iodine solution. The formula of starch grain is (C6H10O5)n, where value of ‘n’ is not known.
Glycogen, (C6H10O5)n, is another insoluble carbohydrate like starch present usually in fungi. As glycogen occurs abundantly in animal bodies, it is also called animal starch.
(B) Nitrogenous Reserve Materials:
These are very complex chemically. They have nitrogen, usually sulphur and often phosphorus in addition to carbon, hydrogen and oxygen. They are mainly of two types, viz. complex insoluble proteids and their simple soluble forms, amino acids.
Proteids may be present in many parts of the plants. They are usually insoluble in water but dissolve readily in strong acids and alkalies. A common form of aleurone grains proteid, called aleurone grain, is found in the endosperm of castor- oil seed. Each aleurone grain is more or less round in shape which encloses a large crystalline body called crystalloid and a small rounded one celled globoid (Fig. 126).
Of the two, the crystalloid only is the nitrogenous matter, the globoid chemically being a double phosphate of calcium and magnesium. The occurrence of crystalloid and globoid is not always constant in aleurone grains.
Proteid grains are converted into simple soluble amino-acids for assimilation, and as such they travel to the different parts of the plants. That proteid grains are very complex chemically, is evident from the formula of a common proteid, known as gliadin, present in wheat. It is C685H1068N198O211S6.
(C) Fats and Oils:
These are also important energy-giving reserve materials of plants. Like carbohydrates they are composed of carbon, hydrogen and oxygen, but the latter two do not occur in the same proportion as in water, there being more carbon and hydrogen and less oxygen. Fats, which are liquids at ordinary temperature, are called oils.
They are abundantly present in many seeds. The formula of a common vegetable fat, palmitin, is C51H98O6. Fats are always lighter than water and leave greasy spots on paper. They are insoluble in water but dissolve in ether and chloroform.
The fat of castor-oil seed is sparingly soluble in alcohol. When treated with caustic soda or caustic potash fats saponify, i.e. they form glycerine and soap, the latter is nothing but a sodium or potassium salt of a fatty acid. Fats are formed from fatty acids and glycerine. In plants they usually occur as oils, and are extracted by pressure.
2. Secretory Materials:
These are not food matters but are indirectly useful to the plants. Colouring matters or pigments like chlorophyll, xanthophyll, carotin, etc., are certainly not plant food matters, but chlorophyll, as we know, is indispensable for manufacture of food. Similarly other pigments also have positive utility.
A. Colouring Matters:
The green pigment chlorophyll is certainly indispensable for manufacture of food but it itself is not a food. Other pigments are also indirectly useful for the plants.
These are the digestive agents secreted by protoplasm. They have the wonderful power of converting complex insoluble food matters into their simple soluble forms, but they themselves remain unaffected during the process. Thus enzymes are the organic catalysts. They convert starch into sugar, proteids into amino-compounds, and fats into fatty acids and glycerin.
Enzymes are specific in action, thus the enzymes responsible for the breaking down of starch into sugar have no action on proteids or fats. Enzyme re-actions are sometimes reversible i.e. the enzymes having help in .converting starch into sugar are also instrumental in the formation of starch out of sugar.
3. Excretory Materials or Waste Products:
These are absolutely useless for the plants. Plants have no distinct excretory systems as animals have, but they too have devices for getting rid of waste products. There are a fairly good number of excretory products of which just a few will be discussed here.
These are complex nitrogenous matters which occur in plants. They are bitter in taste and some of them are extremely poisonous. Alkaloids are the active principles of many herbal medicines. They are usually extracted by dissolving them in alcohol. Quinine present in the bark of Cinchona, morphine in poppy, caffeine in coffee, nicotine in tobacco are some common alkaloids.
These are bitter substances present in the cell sap of many plants and also in the cell wall of dead tissues like the bark of the trees. The fruits of myrobalans (B. Hartaki, Amlaki, Bahera) contain enough of tannins. In banana, guava, mango, etc., tannins disappear with the ripening of the fruit. Tannins turn black with iron salts, and are so used for manufacture of ink. They are also used for tanning leather.
It is the milky fluid (often watery, as in banana) found in many plants. Latex is an emulsion of various substances in a watery matrix. It is the chief commercial source of rubber. Latex of papaw contains the enzymes—papain, which is helpful in the digestion of protein food.
D. Essential Oils:
These are the volatile oils which occur in many plants. The fragrant odour of flowers like rose, jasmine, lotus, is due to presence of essential oils in special glands. These oils, unlike fixed oils, are extracted by distillation.
E. Mineral Crystals:
Crystals of various forms are present in the cells. They may occur singly or a large number of them may remain conglomerated together attaining peculiar shapes. Calcium oxalate crystals are abundant in the plants, particularly in the underground organs.
Solitary crystals of calcium oxalate may be rod-like, cubical, prismatic, octahedral, etc. They are common in the dry scales of o n i o n (Fig. 127). Raphides are the crystal-bundles which look like a bunch of needles inside a sac. Sphaeraphides are beautiful crystal- aggregates which have star-like appearance. Raphides and sphaeraphides are present in Pistia (Fig. 128), arum etc.
Calcium carbonate crystals are often aggregated together on the epidermis of leaves of banyan, India-rubber. Here cells of innermost layer of epidermis often enlarge to accommodate crystals of calcium carbonate deposited on a peg-like projection of the cell wall. This crystal-aggregate, called cystolith, looks like a bunch of grapes (Fig. 129). Cystoliths of irregular shape are present in the leaves of Momordica (B. Uchche).
F. Organic Acids:
They are present in the cell sap of many plants, particularly the unripe fruits which taste sour. Common organic acids are tartaric acid in tamarind, oxalic acid in Oxalis, citric acid in citrus fruits (lemons).