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INTRODUCTION
What is botany?
Botany is the branch of biology concerned with the scientific study of
plants. Traditionally, botanists studied all organisms that were not
generally regarded as animal. However, advances in our knowledge about
the myriad forms of life, especially microbes (viruses and bacteria), have
led to spinning off from Botany the specialized field called
Microbiology. Still, the microbes are usually covered in introductory
Botany courses, although their status as neither animal nor plant is firmly
established.
• Importance of plants
• Photosynthesis
• Produces food. Photosynthesis feeds the plant but also feeds
heterotrophic organisms (like us) that eat the plant. About
200,000,000,000 tons of carbon per year are “fixed” by plants.
That’s 6,000 tons per second.
• Produces oxygen. Plants also produce oxygen needed by aerobic
species (like us).
• Plant products people use
• Food
• Spices
• Wood
• Textiles
• Drugs
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• Fossil fuels
• Aesthetics
• Research. Many basic principles of biology have been discovered
through study of plants.
• Plants are more like animals than they are different
• There bodies are composed of cells
• They carry out most of the same biochemical reactions. such as
glycolysis and the Krebs cycle, protein synthesis, etc., etc.
• Their genes are composed of DNA
• Their cells divide by mitosis and meiosis
• Most reproduce by sexual reproduction
• They evolve by natural selection
• Plants are simple in structure
• They are much simpler than higher animals. Plants have only 7
basic types of tissues and only three main organs.
• Plants are chemically complex
• Chemically, plants are more complex than animals. Plants can
produce all of the thousands of organic compounds needed for life
from water, carbon dioxide, and a few minerals.
• The branches of botany
 Some branches based on general properties of plants
• Taxonomy
• Morphology
• Anatomy
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• Physiology
• Ecology
• Genetics
• Cell biology
 Some branches based on kind of plant
• Phycology-algae
• Mycology-fungi
• Bryology-mosses
• Pterology-ferns
What is a plant?
• Two-kingdom system.
• Originally, all living organisms were classified in two kingdoms
• Plant Kingdom - How do we define “plant?”
• Autotrophic
• Nonmotile
• Cells have cell walls
• Animal Kingdom - How do we define “animal?”
• Heterotrophic
• Motile
• Cells lack walls
• Problems.
• As new organisms were described (especially microscopic ones)
it was recognized that many species did not easily fit into “plant”
or “animal.”
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• Examples of “problem” organisms
• Euglena. Motile (animal?), no cell wall (animal?), but
photosynthetic (plant?)
• Slime molds. Motile (animal?), heterotrophic (animal?), spores
have walls (plant?)
• Mushrooms. Cell walls (plant?), nonmotile (plant?), but
heterotrophic (animal?)
• Five-kingdom system.
• This is one solution to the “problem” organisms
• Kingdom Monera. Cells are prokaryotic. Includes bacteria and
cyanobacteria.
• Kingdom Protista. A complex group of “simple” eukaryotic
organisms. The plant-like protists are the algae. They are
photosynthetic but much simpler than higher plants.
• Kingdom Myceteae (Kingdom Fungi). These are eukaryotic
species with cell walls but lacking photosynthesis. Includes yeasts,
molds, mushrooms.
• Kingdom Plantae. The so-called “land plants.” Most of these fit the
usual definition of plant. They are photosynthetic like algae but
more complex in structure.
• Kingdom Animalia. Higher animals (we will let the zoology class
cover these).
by Dr. John Tiftickjian [[email protected]]
Sciences).
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. Delta State University (Biological
Plant cell structure
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Plants are unique among the eukaryotes, organisms whose cells have
membrane-enclosed nuclei and organelles, because they can manufacture
their own food. Chlorophyll, which gives plants their green color, enables
them to use sunlight to convert water and carbon dioxide into sugars and
carbohydrates, chemicals the cell uses for fuel.
Cell wall
A thick, rigid membrane that surrounds a plant cell. This layer of
cellulose fiber gives the cell most of its support and structure. The cell
wall also bonds with other cell walls to form the structure of the plant.
Cell membrane
The thin layer of protein and fat that surrounds the cell, but is inside the
cell wall. The cell membrane is semipermeable, allowing some
substances to pass into the cell and blocking others.
Plasmodesmata
Plasmodesmata are small tubes that connect plant cells to each other,
providing living bridges between cells.
Chloroplasts
The most important characteristic of plants is their ability to
photosynthesize, in effect, to make their own food by converting light
energy into chemical energy. This process is carried out in specialized
organelles
called
chloroplasts.
Endoplasmic reticulum
The endoplasmic reticulum is a network of sacs that manufactures,
processes, and transports chemical compounds for use inside and outside
of the cell.. In plants, the endoplasmic reticulum also connects between
cells via the plasmodesmata.
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Rough endoplasmic reticulum - (rough ER) a vast system of
interconnected, membranous, infolded and convoluted sacks that are
located in the cell's cytoplasm (the ER is continuous with the outer
nuclear membrane). Rough ER is covered with ribosomes that give it a
rough appearance. Rough ER transport materials through the cell and
produces proteins in sacks called cisternae (which are sent to the Golgi
body,
or
inserted
into
the
cell
membrane).
Smooth endoplasmic reticulum - (smooth ER) a vast system of
interconnected, membranous, infolded and convoluted tubes that are
located in the cell's cytoplasm (the ER is continuous with the outer
nuclear membrane). The space within the ER is called the ER lumen.
Smooth ER transport materials through the cell. It contains enzymes and
produces and digests lipids (fats) and membrane proteins; smooth ER
buds off from rough ER, moving the newly-made proteins and lipids to
the Golgi body and membranes.
Golgi body
Also called the golgi apparatus or golgi complex) a flattened, layered,
sac-like organelle that looks like a stack of pancakes and is located near
the nucleus. The golgi body packages proteins and carbohydrates into
membrane-bound vesicles for "export" from the cell.
Mitochondria
Spherical to rod-shaped organelles with a double membrane. The inner
membrane is infolded many times, forming a series of projections (called
cristae). The mitochondrion converts the energy stored in glucose into
ATP (adenosine triphosphate) for the cell.
Nucleus
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Spherical body containing many organelles, including the nucleolus and
surrounded by nuclear membrane. The nucleus is a highly specialized
organelle that serves as the information processing and administrative
center of the cell. This organelle has two major functions: it stores the
cell's hereditary material, or DNA, and it coordinates the cell's activities,
which include growth, intermediary metabolism, protein synthesis, and
reproduction (cell division).
Nucleolus
An organelle within the nucleus - it is where ribosomal RNA is produced.
Nuclear membrane
The membrane that surrounds the nucleus.
Vacuole
Each plant cell has a large, single vacuole that stores compounds, helps in
plant growth, and plays an important structural role for the plant.
Ribosomes
Small organelles composed of RNA-rich cytoplasmic granules that are
sites of protein synthesis.
Cytoplasm
The jellylike material outside the cell nucleus in which the organelles are
located.
Microtubules
These straight, hollow cylinders are found throughout the cytoplasm of
all eukaryotic cells (prokaryotes don't have them) and carry out a variety
of
functions,
ranging
from
transport
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to
structural
support.
Plant Tissues
Plants are composed of three major organ groups: roots, stems and leaves.
As we know from other areas of biology, these organs are comprised of
tissues working together for a common goal (function). In turn, tissues
are made of a number of cells which are made of elements and atoms on
the most fundamental level. In this section, we will look at the various
types of plant tissue and their place and purpose within a plant. It is
important to realize that there may be slight variations and modifications
to the basic tissue types in special plants.
Plant tissues are characterized and classified according to their structure
and function. The organs that they form will be organized into patterns
within a plant which will aid in further classifying the plant. A good
example of this is the three basic tissue patterns found in roots and stems
which serve to delineate between woody dicot, herbaceous dicot and
monocot plants. We will look at these classifications later on in the
tutorial.
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Classification of Plant tissues
Plant tissues
Meristematic
tissues
Primary
Permanent
tissues
Secondary
Simple
Complex
Dermal
Parenchyma
Collenchyma
Sclerenchyma
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Vascular
A- Meristematic Tissues
Tissues where cells are constantly dividing are called meristems or
meristematic tissues. These regions produce new cells. These new cells
are generally small, six-sided boxlike structures with a number of tiny
vacuoles and a large nucleus, by comparison. Sometimes there are no
vacuoles at all. As the cells mature the vacuoles will grow to many
different shapes and sizes, depending on the needs of the cell. It is
possible that the vacuole may fill 95% or more of the cell’s total volume.
There are 3 types
of
meristem
Apical meristem
Lateral meristem
Intercalary
meristem
-- Apical meristems are located at or near the tips of roots and shoots. As
new cells form in the meristems, the roots and shoots will increase in
length. This vertical growth is also known as primary growth. A good
example would be the growth of a tree in height. Each apical meristem
will produce embryo leaves and buds as well as three types of primary
meristems: protoderm, ground meristems, and procambium. These
primary meristems will produce the cells that will form the primary
tissues.
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Root apex
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Soot apex
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-- Lateral meristems account for secondary growth in plants. Secondary
growth is generally horizontal growth. A good example would be the
growth of a tree trunk in girth. There are two types of lateral meristems to
be aware of in the study of plants.
1- The vascular cambium, the first type of lateral meristem, is
sometimes just called the cambium. The cambium is a thin, branching
cylinder that, except for the tips where the apical meristems are located,
runs the length of the roots and stems of most perennial plants and many
herbaceous annuals. The cambium is responsible for the production of
cells and tissues that increase the thickness, or girth, of the plant.
2- The cork cambium, the second type of lateral meristem, is much like
the vascular cambium in that it is also a thin cylinder that runs the length
of roots and stems. The difference is that it is only found in woody plants,
as it will produce the outer bark.
Both the vascular cambium and the cork cambium, if present, will begin
to produce cells and tissues only after the primary tissues produced by the
apical meristems have begun to mature.
-- Intercalary meristems are found in grasses and related plants that do
not have a vascular cambium or a cork cambium, as they do not increase
in girth. These plants do have apical meristems and in areas of leaf
attachment, called nodes, they have the third type of meristematic tissue.
This meristem will also actively produce new cells and is responsibly for
increases in length. The intercalary meristem is responsible for the
regrowth of cut grass.
B- Permanent tissues
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Tissues in plants that do not actively produce new cells. These tissues are
called nonmeristematic tissues (permanent tissues). Nonmeristematic
tissues are made of cells that are produced by the meristems and are
formed to various shapes and sizes depending on their intended function
in the plant. Sometimes the tissues are composed of the same type of cells
throughout, or sometimes they are mixed. There are simple tissues and
complex tissues.
1- Simple tissues
There are three basic types, named for the type of cell that makes up their
composition.

Parenchyma (cells with thin primary walls that retain their
protoplasm)
Parenchyma is the most common and versatile ground tissue. It forms, for
example, the cortex and pith of stems, the cortex of roots, the mesophyll
of leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma
cells are living cells and may remain meristematic at maturity, meaning
that they are capable of cell division. They have thin but flexible cellulose
cell walls, and are generally polygonal when close-packed, but
approximately spherical when isolated from their neighbours. They have
large central vacuoles, which allows the cells to store and regulate ions,
waste products and water.
Parenchyma cells have a variety of functions:

In leaves, they form the mesophyll and are responsible for
photosynthesis and the exchange of gases[1], parenchyma cells in
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the mesophyll of leaves are a specialized parenchymatous tissue
known as chlorenchyma (parenchyma with chloroplasts).

Storage of starch, protein, fats and oils and water in roots, tubers
(e.g. potato), seed endosperm (e.g. cereals) and cotyledons (e.g.
pulses and groundnut)

Secretion (e.g. hydathodes, nectaries and cells lining the inside of
resin ducts)

Wound repair and the potential for renewed meristematic activity

Other specialized functions such as aeration (aerenchyma) and
support.
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
Collenchyma (cells with thick primary walls that retain their
protoplasm)
cells form collenchyma tissue. These cells have a living protoplasm, like
parenchyma cells, and may also stay alive for a long period of time. Their
main distinguishing difference from parenchyma cells is the increased
thickness of their walls. In cross section, the walls looks uneven.
Collenchyma cells are found just beneath the epidermis and generally
they are elongated and their walls are pliable in addition to being strong.
As a plant grows these cells and the tissues they form, provide flexible
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support for organs such as leaves and flower parts. Collenchyma tissue
have the following features.

Collenchyma tissue provide flexible support while alive.

Collenchyma cells are supporting cells.

Their primary walls are characteristically thick at the corners of the
cells.

Collenchyma cells are generally elongated.

In collenchyma cells, the primary wall thickens, but no secondary
wall forms.

Collenchyma provides support to leaf petioles, nonwoody stems,
and growing organs.

Tissue made of collenchyma cells is flexible, permitting stems and
petioles to sway in the wind without snapping.
There are three principal types of collenchyma:

Angular collenchyma (thickened at intercellular contact points)

Tangential collenchyma (cells arranged into ordered rows and
thickened at the tangential face of the cell wall)

Lacunar collenchyma (have intercellular space and thickening
proximal to the intercellular space)
Cross section of collenchyma cells
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Diagram of Collenchyma Tissue
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 Sclerenchyma (cells with lignified secondary walls that have lost
their protoplasm at maturity, i.e. are 'dead')
Sclerenchyma is a supporting tissue in plants. Two groups of
sclerenchyma cells exist: fibres and sclereids. Their walls consist of
cellulose, hemicellulose and lignin. Sclerenchyma cells are the principal
supporting cells in plant tissues that have ceased elongation.
Sclerenchyma fibres are of great economical importance, since they
constitute the source material for many fabrics
Unlike the collenchyma, mature sclerenchyma is composed of dead cells
with extremely thick cell walls (secondary walls) that make up to 90% of
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the whole cell volume. It is the hard, thick walls that make sclerenchyma
cells important strengthening and supporting elements in plant parts that
have ceased elongation. The difference between fibres and sclereids is not
always clear. Transitions do exist, sometimes even within one and the
same plant.
1- Fibers
Fibers are sometimes found in association with a wide variety of tissues
in roots, stems, leaves and fruits. Usually fiber cells are much longer than
they are wide and have a very tiny cavity in the center of the cell.
Currently, fibers from over 40 different plant families are used in the
manufacture of textiles, ropes, string.
Fibres usually originate from meristematic tissues. Cambium and
procambium are their main centers of production. They are usually
associated with the xylem and phloem of the vascular bundles. The fibres
of the xylem are always lignified, while those of the phloem are
cellulosic. Reliable evidence for the fibre cells' evolutionary origin from
tracheids exists. During evolution the strength of the tracheid cell walls
was enhanced, the ability to conduct water was lost and the size of the
pits reduced. Fibres that do not belong to the xylem are bast (outside the
ring of cambium) and such fibres that are arranged in characteristic
patterns at different sites of the shoot.
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Cross section of sclerenchyma fibers
2- Sclereids
Sclereids are sclerenchyma cells that are randomly distributed throughout
other tissues. Sometimes they are grouped within other tissues in specific
zones or regions. They are generally as long as they are wide. An
example, would be the gritty texture in some types of pears. The grittiness
is due to groups of sclereid cells. Sclereids are sometimes called stone
cells.
Fresh mount of a sclereid
Moore, Randy; Clark, W. Dennis; and Vodopich, Darrell S. (1998). Botany (3rd
ed.).
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2- Epidermal tissue system
The epidermal tissue includes several differentiated cell types: epidermal
cells, guard cells, and epidermal hairs (trichomes). The epidermal cells
are the most numerous, largest, and least specialized. These are typically
more elongated in the leaves of monocots than in those of dicots.
--- Epidermis
The epidermis is a single-layered group of cells that covers plants' leaves,
flowers, roots and stems. It forms a boundary between the plant and the
external world. The epidermis serves several functions, it protects against
water loss, regulates gas exchange, secretes metabolic compounds, and
(especially in roots) absorbs water and mineral nutrients. The epidermis
of most leaves shows dorsoventral anatomy: the upper (adaxial) and
lower (abaxial) surfaces have somewhat different construction and may
serve different functions. The walls of the epidermal cells of the above
ground parts of plants contain cutin, and are covered with a cuticle. The
cuticle reduces water loss to the atmosphere, it is sometimes covered with
wax in smooth sheets or long filaments. Woody stems and some other
stem structures produce a secondary covering called the periderm that
replaces the epidermis as the protective covering.
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--- Trichomes
Trichomes or hairs grow out from the epidermis in many species. In root
epidermis, epidermal hairs, termed root hairs are common and are
specialized for absorption of water and mineral nutrients.
Plant hairs may be unicellular or multicellular , branched or unbranched.
Multicellular hairs may have one or several layers of cells.
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Ascensão, L., N. Marques and M.S. Pais. 1995. Glandular trichomes on
vegetative and reproductive organs of Leonotis leonurus (Lamiaceae).
Annals of Botany 75: 619-626.
--- Stomata
An important feature of leaf epidermis is the presence of stomata which
occur either on both sides of the leaf or only on one side of the leaf.
When stomata occur on both sides, the leaf is known as amphistomatic,
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when they are confined to the upper side, the leaf is known as epistomatic
and when to the lower side, the leaf is called hypostomatic.
Each stomata consists of an opening (pore) bounded by two specialized,
usually kidney shaped, epidermal cells known as guard cells. The guard
cells have unevenly thickened walls. The inner wall facing the aperture is
highly thickened while the one away from the aperture is thin and
extensible. The guard cells are also covered with cuticle which extends to
the inner wall forming the boundary of the pore and the sub-stomatal
chamber.
Guard cells are surrounded by a variable number of epidermal cells which
are called subsidiary or accessory cells. These cells may be
morphologically similar to the other epidermal cells or very different
from them.
Raven, Peter H.; Evert, Ray F.; Curtis, Helena (1981), Biology of plants,
New York, N.Y.: Worth Publishers, pp. 427–28
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3- Vascular tissues
Vascular tissue is a complex conducting tissue, formed of more than one
cell type, found in vascular plants. The primary components of vascular
tissue are the xylem and phloem. These two tissues transport fluid and
nutrients internally. There are also two meristems associated with
vascular tissue: the vascular cambium and the cork cambium. All the
vascular tissues within a particular plant together constitute the vascular
tissue system of that plant.
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Xylem
It is a complex permanent tissue, which is specialized for the conduction
of water and mineral substances in the plant body. Xylem is a
heterogenous tissue made up of four different types of cellular elements.
They are:

Xylem tracheids

Xylem tracheae

Xylem fibers and

Xylem parenchyma
1- Xylem Tracheids
They are found abundantly in pteridophytes, gymnosperms and primitive
angiosperms. In these groups of plants, the tracheids represent the most
active water conducting elements. In advanced angiosperms, the tracheids
are found restricted to leaf margin and leaf tip.
The tracheids are elongated, dead cells, with tapering ends. They are
characterised by the presence of a thick cell wall consisting of primary
wall and a secondary wall. The primary wall is composed of cellulose
where as the secondary wall is made up of lignin. There is a spacious
lumen that extends throughout the length of the tracheid. In some cases,
due to the deposition of lignin, the primary wall develops numerous
concave depressions called pits. When pits are present, the tracheid is
described as pitted and when pits are absent, it is described as simple.
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Tracheids
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2- Xylem Tracheae
They are commonly known as xylem vessels. They are the most active
water conducting elements in all higher angiosperms. The tracheae are
found arranged parallel to each other, extending from one end of the plant
body to another.
The tracheae are long cylindrical dead cells. They are characterised by a
thick cell wall consisting of a primary wall and a secondary wall. The
primary wall is made up of cellulose where as the secondary wall is made
up of lignin. There is a spacious lumen that extends throughout the length
of the trachea. The deposition of lignin in the secondary wall is not
always uniform. As a result, the xylem vessels exhibit different types of
secondary thickenings. On this basis, xylem vessels can be distinguished
into five types:

Annular vessels in which the secondary thickening is in the form
of rings placed more or less at equal distance from each other.

Spiral vessels in which the secondary thickenings are present in
the form of a helix or coil.

Scalariform vessels in which the secondary thickenings appear in
the form of cross bands resembling the steps of a ladder.

Reticulate vessels in which the secondary thickenings are irregular
and appear in the form of a network.

Pitted vessels in which the secondary thickenings result in the
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B-G: trachea in LS showing different types of thickenings
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3- Xylem Fibres
They are represented by the dead sclerenchyma fibers that are found in
between the vessels and the tracheids. They are meant for providing
mechanical support to the essential elements.
4- Xylem Parenchyma
This is the only living component in the xylem tissue. It is represented by
groups of parenchyma cells that are found in between the vessels and the
fibers. They are meant for storage of reserve food.
Types of Xylem
Xylem can be distinguished into two types namely

Primary xylem and

Secondary xylem
Primary Xylem
Primary xylem is the xylem that is formed during normal growth. It is a
derivative of primary meristem. It occurs in both monocots and dicots. In
the primary xylem, two types of xylem vessels can be distinguished,
namely protoxylem and metaxylem.
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Secondary Xylem
Secondary xylem is the xylem that is formed during secondary growth. It
is derivative of secondary meristem. It is a characteristic feature of only
dicots. Secondary xylem is commonly known as wood. It is of
commercial importance since it is extensively used in the manufacturing
of doors, windows and furniture.
Wilson, K. & D.J.B. White (1986). The Anatomy of Wood: its Diversity
and variability. Stobart & Son Ltd, London
Peter A. Raven, Ray F. Evert, Susan E. Eichhorn (1999). Biology of
Plants. W.H. Freeman and Company. pp. 576-577.
Kenrick, Paul; Crane, Peter R. (1997). The Origin and Early
Diversification of Land Plants: A Cladistic Study. Washington, D. C.:
Smithsonian Institution Press. ISBN 1-56098-730-8
Campbell, Neil A.; Jane B. Reece (2002). Biology (6th ed.). Benjamin
Cummings. ISBN 978-0805366242.
Melvin T. Tyree; Martin H. Zimmermann (2003). Xylem Structure and
the Ascent of Sap (2nd ed.). Springer. ISBN 3-540-43354-6. recent
update of the classic book on xylem transport by the late Martin
Zimmermann.
Phleom
Phloem is a complex permanent tissue, which is specialized for the
conduction of food and other organic substances. Phloem is also a
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heterogenous tissue, made up of four different types of cellular elements,
namely,

Sieve tubes

Companion cells

Phloem parenchyma and

Phloem fibres
Sieve Tubes
They represent the most active food conducting elements in the phloem
tissue. The sieve tubes are found arranged parallel to one another from
one end of the plant body to another. Each sieve tube is formed by a
series of hollow, cylindrical cells called sieve tube cells arranged one
above the other. The sieve cells are separated from each other by
horizontal perforated plates called sieve plates. The sieve cells
communicate with each other through the sieve plates.
Each sieve cell has a thin cell wall, which is composed of only cellulose.
The cell has a central mass of dense cytoplasm. The granular cytoplasm
forms a number of projections called cytoplasmic strands extending
towards the sieve plate. The nucleus is absent.
Companion Cells
They are more or less spindle shaped cells associated with the sieve tube
cells. Each companion cell is found attached to any one lateral surface of
a sieve cell. The companion cell and the neighbouring sieve cell together
represent a pair of sister cells. The companion cell has a granular
cytoplasm, prominent nucleus and one or two small vacuoles. The
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nucleus of the companion cell is said to be capable of exerting its
influence on the adjacent sieve cells.
Phloem Structure
Phloem Parenchyma
Phloem parenchyma is represented by a group of living parenchyma cells
that are found in-between the sieve tubes. They are meant only for
storage of organic food
Phloem Fibres
Phloem fibres are represented by the dead sclerenchyma fibres that are
found in between the sieve tubes. They are meant only for providing
mechanical support.
Types of Phloem
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Phloem can be distinguished into two types namely

Primary phloem

Secondary phloem
Primary Phloem
Primary phloem is the phloem that is formed during normal growth in the
plant body. It is a derivative of primary meristem. It is found in both
monocots and dicots. The primary phloem is further composed of
protophloem and metaphloem. The sieve tubes and the companion cells,
which appear earlier during normal growth, represent protophloem, while
metaphloem is represented by the sieve tubes and companion cells that
appear later. However, there is no significant morphological difference
between protophloem and metaphloem.
Secondary Phloem
Secondary phloem is the phloem that is formed during secondary growth.
It is a derivative of secondary meristem. Secondary phloem is
characteristic feature of only dicots. It is also known as bast. It is also of
commercial importance since it yields bast fibers.
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The following table summarise the characteristics of and difference
between xylem and phloem:
Characteristics
Xylem
Phloem
Dead, complex permanent
Living, complex
tissue
permanent tissue
Xylem tracheids and
Sieve tubes and
tracheae
companion cells
Associated
Xylem fibres and xylem
Phloem fibres and
elements
parenchyma
phloem parenchyma
Xylem tracheids
Only Phloem fibres
Definition
Essential
elements
Non-living
components
Living
components
Function
Sieve tubes companion
Only xylem parenchyma
cells and phloem
parenchyma
Conduction of water
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Conduction of food and
other organic substances
Types
1. Primary xylem
1.Primary phloem
(a) protoxylem
(a) protopholoem
(b) metaxylem
(b) metaphloem
2. Secondary xylem (or
2. Secondary pholem (or
wood)
bast)
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Secondary growth
The normal process of growth that occurs in every plant body is known as
primary growth. It is the result of the activity of primary meristem. The
process of primary growth results in the formation of primary permanent
tissues such as primary xylem, primary phloem and primary cortex.
However in the dicot plants, there is a process of growth that begins after
a known period of primary growth. Such a growth is known as secondary
growth. It is the result of the activity of secondary meristem. It results in
the formation of secondary permanent tissues such as secondary xylem,
secondary phloem and secondary cortex. As a result, secondary growth
brings about an increase in the girth of the plant body.
Some Important Definitions:
Primary tissues: Tissues generated from the growth of an apical
meristem.
Cambium: A lateral meristem constituting a sheet of cells. Growth of
these cells increases the girdth of the plant organ involved.
Secondary tissues: Tissues generated from the growth of a cambium.
Vascular Cambium: A cambium that gives rise to secondary xylem to
the inside, and to secondary phloem to the outside.
Two kinds of meristematic cells, called initials, are recognizable in the
cambium: fusiform and ray initials. The fusiform initials are elongated
vertically in the stem and have tapering ends. They divide to produce the
conducting cells of both the xylem and the phloem (xylem toward the
inside of the stem, phloem toward the outside).
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Considerably more xylem cells than phloem cells always are produced.
The ray initials are smaller, more cuboidal and produce parenchyma in
rows radiating out from the center of the stem. The bands of parenchyma,
called rays (vascular rays), conduct water and dissolved materials
laterally in the stem.
Periderm: A structure that consists of a cork cambium (phellogen), with
cork tissue (phellem) to the outside, and in some cases a layer of cells
derived from and to the inside of the cork cambium called phelloderm.
Functions to limit dehydration and block pathogens after the epidermis is
disrupted by the onset of secondary growth.
Cork: (phellem) you need know only the term "cork": Tissue dead at
maturity generated from a cork cambium. The cell walls of the tissue are
impregnated with suberin. This water-proofs the tissue.
Cork Cambium: A cambial layer that functions to produce cork, and in
some cases, phelloderm. In roots is derived initially from pericyle. In
stems from the cortex. Unlike the vascuar cambium these cambial layers
do not persist for the duration of the life of the plant organ. Over time one
cork cambium will be supplanted by another generated from parenchyma
cells further inside.
Phelloderm: In some periderms a layer of living secondary tissue is
generated by the cork cambium to the inside. We will not consider thie
phelloderm in the following exercise.
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Characteristics of Diagram of young dicot stem
1- Ground tissue consists of cortex and pith
2- Cortex is characterized into collenchyma and parenchyma
3- The vascular bundles arranged in ring
4- The vascular bundle is open
5- The phloem is irregular
6- Metaxylem and protoxylem arranged in rows
7- The vascular bundle is lateral
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Changes accompanied to secondary growth in dicot stem
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After significant activity in the vascular cambium, a stem exhibiting
secondary growth might look like the previous diagram. The primary
xylem is in the center of the stem, while the primary phloem is pushed
outward by the new cells that arise from the vascular cambium.
Eventually, the primary phloem is crushed into the cortex. The secondary
xylem differentiates from the cells that divide off the vascular cambium
towards the inside of the stem, while the secondary phloem differentiates
from the cells that divide towards the outside of the stem.
Thompson, N.P. and Heimsch, C. 1964. Stem anatomy and aspects of
development in tomato. American Journal of Botany 51: 7-19.
Esau, K. and Cheadle, V.I. 1969. Secondary growth in bougainvillea.
Annals of Botany 33: 807-819.
Ewers, F.W. 1982. Secondary growth in needle leaves of Pinus longaeva
(bristlecone pine) and other conifers: Quantitative data. American Journal
of Botany 69: 1552-1559.
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Secondary growth in roots
Secondary growth in roots begins with the formation of the vascular
cambium. This concentric, undifferentiated cell layer originates from the
pericycle and from the procambium, which is located between the
primary xylem and primary phloem tissues. The vascular cambium
continues to divide and differentiate to produce secondary vascular tissue
toward the center of the root and secondary phloem tissue toward the
outside of the root. The pericycle also gives rise to the cork cambium,
which produces the periderm to the outside of the root. Primary and
secondary xylem in alternate position. Primary and secondary phleom
next to each other.
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Photosynthesis
Photosynthesis is the process of converting light energy to chemical
energy and storing it in the bonds of sugar. This process occurs in plants
and some algae. Plants need only light energy, CO 2, and H2O to make
sugar. The process of photosynthesis takes place in the chloroplasts,
specifically using chlorophyll, the green pigment involved in
photosynthesis.
Photosynthesis takes place primarily in plant leaves, and little to none
occurs in stems, etc. The parts of a typical leaf include the upper and
lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the
stomates. The upper and lower epidermal cells do not have chloroplasts,
thus photosynthesis does not occur there. They serve primarily as
protection for the rest of the leaf. The stomates are holes which occur
primarily in the lower epidermis and are for air exchange: they let CO 2 in
and O2 out. The vascular bundles or veins in a leaf are part of the plant's
transportation system, moving water and nutrients around the plant as
needed. The mesophyll cells have chloroplasts and this is where
photosynthesis occurs.
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Photosynthesis occurs in chloroplasts
Chloroplasts
Chloroplasts are probably the most important among the plastids since
they are directly involved in photosynthesis. They are usually situated
near the surface of the cell and occur in those parts that receive sufficient
light, e.g. the palisade cells of leaves. The green colour of chloroplasts is
caused by the green pigment chlorophyll.
Structure
Chloroplasts are usually disc-shaped and surrounded by a double
membrane. Inside the inner membrane there is a watery protein-rich
ground substance or stroma in which is embedded a continuous
membrane system, the granal network. This network forms a threedimensional arrangement of membrane-bound vesicles called thylakoids.
The thylakoids usually lie in stacks called grana and contain the
photosynthetic pigments - green chlorophyll a and b and the yellow to red
carotenoids. The grana are interconnected by tubular membranes called
the intergranal frets or lamellae.
Functions
o
chloroplasts are the sites for photosynthesis;
o
they contain enzymes and co-enzymes necessary for the process of
photosynthesis.
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Stages of
photosynthesis
Light reactions
Dark reactions
(Calvin cycle)
Light reactions
The light-dependent reactions require light.
These reactions occur in the thylakoid membrane.
They produce Energy storage molecules ATP and NADPH, which are
needed to produce glucose in the light-independent reactions
Oxygen gas is made as a waste product.
2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
12 H2O + Energy  6 O2 + 24 H+ + 24e-
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Dark reactions
Dark reactions (light-independent) occur in the stroma.
Carbon dioxide is “fixed” into the sugar glucose.
ATP and NADPH molecules created during the light reactions power the
production of this glucose.
3 CO2+ 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6
NADP+ + 3 H2O
6 CO2 + 24 H+ + 24 e- ------> C6H12O6 + 6 H2O
2 Sets of Reactions
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Factors affecting the rate of photosynthesis
Different factors affect the rate of photosynthesis such as External
(sulight, carbondioxide, temperature, water and oxygen) and internal (age
of the leaf, number of stomata, chlorophyll etc).
External Factors Affecting Photosynthesis:
1- Light Wavelength
o
Red and blue light are most effective for driving photosynthesis.
Green (500nm) is least effective.
2. Light Intensity
o
Photosynthesis goes faster in more intense light until limited by some
other factor. Once the reactions are going as fast as they can, more
intense light has no effect.
3. Temperature
o
Up to a point, the rate of photosynthesis increases with temperature.
Above the optimum temperature for photosynthetic enzyme function,
photosynthesis is inhibited or shut down completely.
4. Humidity
o
If the humidity surrounding a plant is low, the stomata will close to
reduce water loss through transpiration. Closed stomata limit gas
exchange, and photosynthesis is slowed by a reduction in carbon dioxide
availability.
5. Carbon Dioxide Concentration
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o
More carbon dioxide in the air allows more photosynthetic conversion
into sugar, until limited by another factor. This is why some growers add
carbon dioxide to the air in a growing environment.
6. Water
o
Compared to the amount of water needed to sustain a plant, the
amount needed for photosynthesis is small. However, a dehydrated plant
will suffer physical damage and will also close stomata, thus limiting
photosynthesis.
Internal factors of photosynthesis
1. Chlorophyll concentration
--The concentration of chlorophyll affects the rate of reaction as they
absorb the light
--Lack or deficiency of chlorophyll results in chlorosis or yellowing of
leaves
--Lack of iron, magnesium, nitrogen and light affect the formation of
chlorophyll
2. Age
--Rate of photosynthesis increases with age of the leaf till maturation.
Afterwards it begins to decrease.
3. Hormones
--Cytokinin and gibberellins enhance the rate of photosynthesis while
ABA decreases.
4. Carotenoids
--Essential to prevent photo-oxidation which reduces photosynthesis
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5. Translocation
--The energy rich carbon compounds formed during photosynthesiscalled photosynthates or photoassimilates are transported to all the organs
and tissues of the plant body. This long distance transport of
photosynthates through the phloem is called as translocation.
--The photosynthates provide energy to the non-photosynthetic tissues
through respiration. In storage organs, photosynthates are stored in the
form of starch or as other carbohydrates.
Bidlack JE; Stern KR, Jansky S (2003). Introductory plant biology. New
York: McGraw-Hill.
Reece, J, Campbell, N (2005). Biology. San Francisco: Pearson,
Benjamin Cummings.
Govindjee Beatty JT,Gest H, Allen JF (2006). Discoveries in
Photosynthesis. Advances in Photosynthesis and Respiration. 20. Berlin:
Springer.
Blankenship RE (2008). Molecular Mechanisms of Photosynthesis (2nd
ed.). John Wiley & Sons Inc.
Plant physiology, (4th edition ) M.Devlin, Francis H. Withan 1983.
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Transpiration
Transpiration is the evaporation of water into the atmosphere from the
leaves and stems of plants. Plants absorb soilwater through their roots and
this water can originate from deep in the soil. Plants pump the water up
from the soil to deliver nutrients to their leaves. This pumping is driven
by the evaporation of water through small pores called "stomates", which
are found on the undersides of leaves. Transpiration accounts for
approximately 10% of all evaporating water.
Importance

supply photosynthesis (1%-2% of the total)

bring minerals from the roots for biosynthesis within the leaf

cool the leaf
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Environmental factors that affect the rate of transpiration
1. Light
Plants transpire more rapidly in the light than in the dark. This is largely
because light stimulates the opening of the stomata (mechanism). Light
also speeds up transpiration by warming the leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because water
evaporates more rapidly as the temperature rises. At 30°C, a leaf may
transpire three times as fast as it does at 20°C.
3. Humidity
The rate of diffusion of any substance increases as the difference in
concentration of the substances in the two regions increases.When the
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surrounding air is dry, diffusion of water out of the leaf goes on more
rapidly.
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly
humid thus reducing the rate of transpiration. When a breeze is present,
the humid air is carried away and replaced by drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made
up by replacement from the soil. When absorption of water by the roots
fails to keep up with the rate of transpiration, loss of turgor occurs, and
the stomata close. This immediately reduces the rate of transpiration (as
well as of photosynthesis). If the loss of turgor extends to the rest of the
leaf and stem, the plant wilts.
Benjamin Cummins (2007), Biological Science (3 ed.), Freeman, Scott,
p. 215
Debbie Swarthout and C.Michael Hogan. 2010. Stomata. Encyclopedia of
Earth
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Absorption
Absorption of Water by Plants
Plants absorb water through the entire surface - roots, stems and leaves.
However, mainly the water is absorbed by roots. The area of young roots
where most absorption takes place is the root hair zone. The root hairs are
delicate structures which get continuously replaced by new ones at an
average rate of 100 millions per day. The root hairs lack cuticle and
provide a large surface area. They are extensions of the epidermal cells.
They have sticky walls by which they adhere tightly to soil particles. As
the root hairs are extremely thin and large in number, they provide
enormous surface area for absorption. They take in water from the
intervening spaces mainly by osmosis.
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Absorption of Water & Minerals in Plants
The absorption of both water and minerals is important for the health,
growth and fruit production of plants. Water and minerals are absorbed
by the plant's roots, a process which is often aided by specialized fungi
that form symbiotic relationships with the roots that are known as
mycorrhizae.
Roots
The outermost cell layer of a root is known as the epidermis. It is these cells
that absorb water from the environment as well as minerals that are dissolved
in the water. Just past the growing tip of the root, some of the epidermal cells
are elongated. These cells function to increase the surface area of the root
and thereby improve its capacity to absorb water and minerals. Root hairs
are easily torn off or desiccated by the sun when plants are transplanted,
thereby temporarily reducing the capacity of the plant to absorb water and
minerals.
Mycorrhizae
Where mycorrhizal relationships exist, the fungus puts out thin, thready
hyphae into the surrounding soil. These absorptive hyphae increase the area
of the root system and therefore its access to water and nutrients. The water
and nutriets absorbed by the hyphae are exchanged for sugars produced by
the plant. Mycorrhizal plants usually perform and survive better than those
without this symbiotic relationship. Mycorrhizae have a vital function in the
absorption of phosphorous and the prevention of iron and manganese
deficiencies in alkaline soils.
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Mineral Nutrition of Plants
There are 14 essential mineral nutrients of plants, which are classified
according to the amount required. Those for which plants have large
requirements, or macronutrients, are nitrogen (N), potassium (K),
phosphorus (P), calcium (Ca), sulfur (S) and magnesium (Mg). It is more
common to see deficiencies of nitrogen, potassium and phosphorous than the
other three minerals. Those minerals for which plants have smaller needs, or
micronutrients are boron (B), chlorine (Cl), iron (Fe), copper (Cu),
manganese (Mn), nickel (Ni), molybdenum (Mo) and zinc (Zn). The amount
of minerals taken up by the plant depends both on the root's ability to absorb
them and the concentration of nutrients around the root surface.
Soil pH and Mineral Absorption
Soil pH is a measure of the relative amounts of acid and alkaline ions in the
soil. Equal amounts of each type of ion give a neutral soil with pH 7. The
lower the pH, the more acid the soil, and the higher, the more alkaline.
Phosphorous and various micronutrients are much more difficult for plants to
absorb with a high-alkaline pH, and woody plants such as pin oak may show
obvious symptoms of iron and manganese deficiency such as thin and pale
leaves. Plants can generally tolerate a pH range of 5.5 to 8.3 if in a welldrained soil, but levels of 6.0.to 6.5 are optimal for most.
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What is soil?
Soil is a thin layer of material on the Earth's surface in which plants have
their roots. It is made up of many things, such as weathered rock and
decayed plant and animal matter. Soil is formed over a long period of
time.
Soil Formation takes place when many things interact, such as air, water,
plant life, animal life, rocks, and chemicals.
Soil formation
Soil is formed from the weathering of rocks and minerals. The surface
rocks break down into smaller pieces through a process of weathering and
is then mixed with moss and organic matter. Over time this creates a thin
layer of soil. Plants help the development of the soil. How? The plants
attract animals, and when the animals die, their bodies decay. Decaying
matter makes the soil thick and rich. This continues until the soil is fully
formed. The soil then supports many different plants.
Weathering:
Weathering is the process of the breaking down rocks. There are two
different types of weathering. Physical weathering and chemical
weathering.
In physical weathering it breaks down the rocks, but what it's made of
stays the same. In chemical weathering it still breaks down the rocks, but
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it may change what it's made of. For instance, a hard material may change
to a soft material after chemical weathering.
Stages in the Formation of Soil
stage
1
stage
2
Soil composition
Soils are a mixture of different things; rocks, minerals, and dead,
decaying plants and animals. Soil can be very different from one location
to another, but generally consists of organic and inorganic materials,
water and air. The inorganic materials are the rocks that have been broken
down into smaller pieces. The size of the pieces varies. It may appear as
pebbles, gravel, or as small as particles of sand or clay. The organic
material is decaying living matter. This could be plants or animals that
have died and decay until they become part of the soil. The amount of
water in the soil is closely linked with the climate and other
characteristics of the region. The amount of water in the soil is one thing
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that can affect the amount of air. Very wet soil like you would find in a
wetland probably has very little air. The composition of the soil affects
the plants and therefore the animals that can live there.
Soil profile
Soil Profile refers to the layers of soil; horizon A, B, and C. If you're
wondering what horizon A is, here's your answer: horizon A refers to the
upper layer of soil, nearest the surface. It is commonly known as topsoil.
In the woods or other areas that have not been plowed or tilled, this layer
would probably include organic litter, such as fallen leaves and twigs .
The litter helps prevent erosion, holds moisture, and decays to form a
very rich soil known as humus. Horizon A provides plants with nutrients
they need for a great life.
Soil conservation
Soil erosion, caused by wind and rain, can change land by wearing down
mountains, creating valleys, making rivers appear and disappear. It is a
slow and gradual process that takes thousands, even millions of years.
But erosion may be speeded up greatly by human activities such as
farming and mining. Soil develops very slowly over a long period of time
but can be lost too quickly. The clearing of land for farming, residential,
and commercial use can quickly destroy soil. It speeds up the process of
erosion by leaving soil exposed and also prevents development of new
soil by removing the plants and animals that help build humus.
Today's farmers try to farm in a way that reduces the amount of erosion
and soil loss. They may plant cover crops or use a no-till method of
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farming. Soil is an important resource that we all must protect. Without
soil there is no life.
Search Engine:
http://www.yahooligans.com
Soils, National Science Resource Center, 1996
Soil Types
Sand, silt, and clay are the basic types of soil. Most soils are made up of a
combination of the three. The texture of the soil, how it looks and feels,
depends upon the amount of each one in that particular soil. The type of
soil varies from place to place on our planet and can even vary from one
place to another in your own backyard.
1- Sandy Soil
Sand is the largest particle in the soil. The individual grains are large
sized, thus added for increased aeration and easy draining. Nevertheless,
sandy soil alone is not good for planting purpose, as it does not retain
water, nutrient and fertilizers.
2- Silty Soil
Silt is a soil particle whose size is between sand and clay. Silt feels
smooth and powdery. When wet it feels smooth but not sticky. Similar to
clay, silty soil turns sticky after saturation with water.
3- Clay Soil
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Clay is the smallest of particles. Clay is smooth when dry and sticky
when wet. Soils high in clay content are called heavy soils, sticky and the
rate of saturation is high. . Clay also can hold a lot of nutrients, but
doesn't let air and water through it well. This type of soil can be used
after mixing with peat and sand particles.
4- Loamy Soil
Loam is one of the ideal soil types for plant growing purposes. Generally,
loam soil is fertile (unlike sandy soil) and has no water drainage problems
like clayey soil and silty soil. In short, it is fertile and well-drained soil,
excellent for cultivation.
5- Peat Soil
Peat soil is loaded with organic materials (decaying remains of plants and
animals). In comparison to other soil types, peat soil has the highest
organic matter. Considering this, it is understandable that this is an acidic
soil type (low pH range). Peat soil with moderately high pH and good
water draining ability is good for plantation.
6- Shalky Soil
In contrast to peat soil, the chalky soil type is alkaline in nature (high pH
range) and prone to dryness. Also known as basic soil, it holds very less
moisture, enabling water to drain off very easily. Despite the fact that
chalky soil contains essential plant nutrients, they are not available to
plants due to increased alkalinity.
Ningthoujam Sandhyarani By
Published: 15/10/2010
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