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Plant Skeleton††† Cell Wall††† Sclerenchyma Cells††† Fibers†††† Protoplasm††† Reserve Food††† Carbohydrates††† Sugar††† Glucose††† Fructose††† Sucrose††† Starch††† Cellulose††† Reserve Cellulose††† Hemicellulose††† Pectins††† Gums††† Mucilages††† Fats††† Proteins††† Aleurone Grains††† Essential Oils††† Secretions and Excretions††† Tannins††† Pigments††† Resins††† Latex††† Waxes††† Alkaloids††† Glucosides††† Organic Acids††† Enzymes††† Vitamins††† Hormones
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††††††††† In 1952 A. F. Hill of Harvard University outlined the various parts of plants that are essential to an understanding of their nature and uses for humans and animals.† It was noted that a protecting encloses the vast majority of plant cells and limiting makeup called the Cell Wall.† The wall affords strength and rigidity to the plant and serves as a kind of skeleton.† These walls are always composed of Cellulose that occurs either alone or with other substances.† Cellulose is a nonliving substance that is made by the plant from grape sugar.† It is a† very complex carbohydrate chemically with the formula (C6H10O5)n†† Cells walls are variable in size and appearance.† Some have walls that are heavily thickened, which are called Sclerenchyma Cells.† These are designed to support the plant.† As the plant body increases in size, more support is required and various sclerenchyma tissues are formed that are made up almost entirely of Fibers.† Fibers are long pointed cells with very thick walls and small cavities.† They have a tendency to interlace and can be stretched and contracted.† Some fibers have cell walls that are almost pure cellulose, such as cotton.† In others some lignin is also present as in the bast fibers found in plant barks.† Lignin greatly increases the strength of a wall without diminishing its ability to conduct water. †When a protective covering is necessary the cellulose walls may be infiltrated with manufactured waterproofing materials such as suberin, cutin or mucilage.† In several cases inorganic materials such as silica may be present in cell walls.
††††††††† Properties that make cell walls useful to the plant are often responsible for the economic value to humans.† The lignified walls of wood has many uses wherever a rigid but easily worked material is required.† The more elastic fibers are the foundation of the textile industry and along with wood they constitute the main raw material of the paper industry.† Cell walls with suberin provide cork.† Walls that are almost pure cellulose are used to make synthetic fibers, cellophane, explosives and other industrial products.† Because cellulose and its derivatives are combustible, all types of cell walls can be used as fuel.† Coal is after all the walls of plants that flourished during the Carboniferous Period and which have gradually lost their gaseous elements.† A gradual succession of fuels that show a progressive loss of hydrogen and oxygen can be tracked from cellulose to lignin, peat, soft coal and hard coal.
††††††††† A large amount of the sugar that is manufactured during photosynthesis is used in the formation of new protoplasm, to replace that which has broken down and to provide for growth of the plant.† Plant protoplasm is a highly complex substance and its chemical nature is not entirely understood even though common elements are included in its makeup.† it contains simple sugars and more highly manufactured carbohydrates; fats in various stages of synthesis; a large amount of protein material that is derived partly from grape sugar and partly from nitrates absorbed from the soil; salts of different inorganic elements, such as phosphorus, iron, magnesium, sulfur, potassium and calcium; and vitamins, enzymes and other secretions.† When food is cooked it greatly alters the original nature of plant protoplasm.† It is generally agreed that fresh, uncooked plant food may have greater health benefits due to the presence of vitamins and other protoplasmic constituents in an unimpaired condition.
††††††††† In most cases plants elaborate much more food than can be used immediately for plant growth, or as a source of energy.† The surplus is stored in highly modified cells in different locations as a reserve supply to be used for growth and other activities at a later time.† Underground stems, roots, buds and seeds are the principal storage organs of plants.† The three main types of food materials that are manufactured by plants are carbohydrates, fats and proteins.
††††††††† These are the simplest of plant foods.† They consist of carbon, hydrogen and oxygen in the proportion of two parts of hydrogen to one of oxygen.† The main carbohydrates are sugar, starch and cellulose.
††††††††† Sugar. --Grape sugar that is manufactured by the plant in photosynthesis is most often present in plant cells.† This basic material of metabolism, known as Glucose, has the formula C6H12O6.† It is at times stored in large amounts such as is found in the stems of maize.† Fruit sugar, or Fructose, another product of photosynthesis, has the same formula, but it possesses slightly different properties.† It is most commonly found only in fruits.
††††††††† The higher and more complex sugars are formed from these simple sugars.† The most important of the higher sugars is cane sugar, or Sucrose with the formula C12H22O11.†† It accumulates in large quantities in sugar beets and sugar cane and to a lesser degree in many other plants.† All the sugars are soluble in water and thus are readily available for use by the plant.† They are highly nutritious and serve as valuable food for animals and humans.† We utilize these sugars not only as they occur in plant tissues but by extracting and purifying them.
††††††††† Starch..-- Starches are insoluble compounds with a complex nature and formula (C6H10O6)n.† They are derived from grape sugar and constitute the first visible product of photosynthesis.† Starch is the most common type of reserve food in green plants and is of the highest importance in their metabolism.† However, due to its insoluble nature starch must be digested, i.e., made soluble, before it can be used.† This is done through the aid of enzymes that are present in the cells.† Starch is stored in large thin-walled cells in the form of distinctive grains.† Humans are very dependent on starch that constitutes a most important plant food and is vital in the industrial world as well.
††††††††† Cellulose.-- This is the highest kind of carbohydrate.† Besides its presence in cells walls, it has little, if any, function as a reserve food even though there is evidence that certain bacteria make use of it.
††††††††† Reserve Cellulose.-- These resemble cellulose physically but they differ in their chemical properties.† They include the hemicelluloses, pectins, gums and mucilages.† Some of these compounds have a dual role.† They aid in the support of cells walls and serve as reserve food.† Hemicelluloses may gradually change into pectins and then into gums.
††††††††† Hemicellulose.-- These are often found as extra layers of cells walls, particularly in seeds of tropical plants such as the date and ivory-nut palm.† They are easily digested by plants but only slightly so by humans, and thus are not suited for human food.† However, they have application in some industries.
††††††††† Pectins.-- These are the fruit jellies that occur in most plant cells, especially in fruits and vegetables.† They are very soluble in water and can be used as food by both plants and animals.† Pectins also increase water retention in cells.† The middle lamella, the cementing material that holds cell walls together, consists of pectin compounds.† Pectins solidify after they have been removed from the plant and humans take advantage of this in the preparation of jams and jellies.
††††††††† Gums.-- the breaking down of cellulose or other carbohydrate compounds derives these.† They consist of an organic acid in combination with inorganic salts.† They may be secreted naturally in the tissues or may arise as the result of wounding.† Gums aid in keeping water in the plant and also serve as a reserve food.† They are used in industries, medicine and as food.
††††††††† Mucilages.-- These are closely related to gums.† When wet with water they do not dissolve but form slimy masses.† They are secreted in sacs, canals or hairs.† They have a varied function and may serve as reserve food, as an aid in controlling the loss of water or too rapid diffusion, as a mechanism for water storage, and as a means for easing seed dispersal.† Mucilage is often found in association with cellulose in cell walls.† They have been used successfully in medicine.
††††††††† Fats are compounds of carbon, hydrogen and oxygen like carbohydrates, but they have much less oxygen.† Because of this they are frequently called hydrocarbons.† The formula for a typical fat Triolein shows their chemical nature:† C57H104O4..† Fats are made from carbohydrates by two processes, (1) the production of fatty acids and (2) the formation of glycerin.† These two products unite to form the fats that are either liquid or solid.† In the liquid state fats are called oils, or fatty oils, and occur in the form of small globules.† Fats are present in small amounts in all living protoplasm, but are stored up as reserve food mainly in the seeds and fruits.† They are insoluble and have to be digested before use.† They have a high energy content and are valuable food for both plants and animals.† Fats play an important role in medicine and industry.
††††††††† Proteins are also derived partly from carbohydrates through the formation of amino acids.† These latter simple compounds are then combined with nitrates from the soil and other substances to form the highly complex protein molecule.† The main characteristic of proteins is their high nitrogen content.† Sulfur is also present, and frequently phosphorus.† Gliadin is a typical protein that occurs in wheat and has the formula:† C736H1161N184O208S3.† Even though proteins are the main constituent of protoplasm, they are stored mostly only in seeds, where they occur as solid granules called Aleurone Grains.† Hundreds of proteins are known to occur in plant tissues.† Once proteins have been changed to a soluble form they constitute an important food for both plants and animals.† They are especially valuable as muscle and nerve builders rather than as sources of energy, and are an essential part of the animal diet.† Proteins are rarely extracted from plant tissues for food purposes, the exception being the uses put to soybeans.† Proteins have very few industrial uses.
††††††††† Plants manufacture different types of substances in the form of secretions and excretions.† These are diverse in chemical composition and function.† Some are secreted in special cells or tissues for a definite purpose, while others have no apparent use and are thought to be by-products of metabolism.† Sometimes these materials of great commercial value and include the essential oils, pigments, resins, tannins, latex, waxes, alkaloids, glucosides, organic acids, enzymes, vitamins and hormones.
††††††††† Often called volatile oils, these differ from fatty oils by being highly aromatic and volatile.† They are formed in glands or special cells.† Their function seems to be primarily to attract insects that are involved in pollination or to repel hostile insects and animals by their acrid taste.† They could have some antiseptic and bactericidal action in plants.† These aromatic oils are used in the preparation of perfumes and soap and in other industries, as well as in medicine and as food flavorings.
††††††††† The plant manufactures all of the coloring materials found within its main body.† These are chemically and functionally diverse.† The most important is chlorophyll, an especially complex substance.† It contains the pigments xanthophyll and carotin and is one of the essential factors in photosynthesis.† Other colors are of value only as a means of attracting insects and other animals for pollination and dispersal, while some are only incidental byproducts of the plantís activity.† When the pigments are stable they can be extracted and used as dyes.
††††††††† These are bitter, astringent materials secreted in the bark, wood or other parts of many plants.† Their function may be to aid in the healing of wounds and in the prevention of decay and may also play a part in the formation of cork and pigments.† They also serve as a protection against natural enemies.† Tannins have peculiar properties that render them invaluable in certain industries.† They can react with proteins, such as the gelatin in animal skins, to produce a hard, firm substance.† Thus they are used in the tanning of leather.† They are also able to react with iron salts to produce a black color.† This makes them valuable in the dye industry and the manufacture of inks.† Tannins have application in medicine due to their astringent properties.
††††††††† These are complex materials that are probably derived from carbohydrates.† They are secreted in glands or canals and often occur in combination with essential oils and gums.† They are formed either naturally or from injury to the tissues.† Resins are water insoluble and thus render any surface impervious to moisture.† They are thus important in the manufacture of paints and varnishes.† For the plant resins may serve to retain moisture or resist decay through their antiseptic action.† Some resins have been used in medicine.
††††††††† Plants often secrete a milky or colored fluid that is called latex.† It is a mixture of resins, gums, hydrocarbons, food and other substances formed in special called or vessels usually in the bark or leaves.† Its use by the plant is not clear but may be involved in protection.† Valuable industrial products such as rubber and chewing gum are made from latex.
††††††††† There is frequently a covering of the leaves and fruits that is secreted by the plant to protect it against excessive water loss.† This wax is similar to fat in composition.† Waxes have been harvested and used to some extent in commerce, e.g., car waxes..
††††††††† These are vegetable bases that contain nitrogen and are believed to be decomposition products of proteins.† They are secreted in special cells or tubes.† They may afford protection against natural enemies because of their bitter taste.† Alkaloids are odorless compounds that have a marked physiological effect on animals.† Thus they are of importance in medicine and have constituted some of the most valuable drugs.† They also include powerful plant poisons and narcotics.† Such substances as caffeine and theobromine that are really closely related purine bases, are frequently classified as alkaloids.
††††††††† Although similar to alkaloids in their properties, glucosides are derived from carbohydrates rather than proteins.† They are thought to give a protection function as they usually occur in the bark.† However, they may serve to regulate the acidity and alkalinity of plant cells.† These substances have been useful as drugs.
††††††††† These are widely distributed among the plants, especially in fruits and vegetables.† They may occur in a free state, as calcium, potassium or sodium salts or in combinations with alcohols.† Fruit acids are thought to show attraction to animals and thus aid in dispersal of fruits and seeds.† They are also involved in metabolism and growth.
††††††††† Enzymes are present in all living organisms.† There are many different kinds, but they usually are present in very low amounts.† They act as catalysts in chemical reactions.† They cause all the chemical changes that occur in living matter without actually entering into the reaction themselves.† One of their most important functions is in digestion, the process by which insoluble materials are broken down into soluble ones and thereby are made available for transportation to all parts of the organism for ultimate use.† Enzymes are colloidal and protein in nature.† They are specific in their actions.† They are concerned not only with oxidation and other destructive phases of metabolism but with the constructive phases also.† They participate in photosynthesis and in the formation of proteins and fats and are present in every living cell of the plant.
††††††††† These are substances that seem to be essential for the well-being of both plants and animals.† They are formed by plants and although animals may store them they are incapable of producing them.† Vitamins occur in extremely minute amounts and thus are difficult to study.† They are necessary for normal metabolism, growth, development and reproduction.† They appear to control most of the constructive phases of metabolism.† Vitamins are also indispensable for the prevention of some human diseases, such as scurvy.† Green vegetables, fruits and seeds are important sources of vitamins.† Seaweeds are especially valuable for they contain many different kinds of vitamins.
††††††††† Hormones are produced in one part of an organism and then transferred to other parts where they may influence some specific physiological process.† Plant hormones function to regulate various growth phenomena such as tropisms, cell enlargement and cell elongation.† They also play a role in the production of roots and flowers and in the formation of fruit.