††††††††† 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.