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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 plant’s 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. |