chapter 3 and 4 (plant structure and angiosperm reproduction)
TRANSCRIPT
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Roots
A root is an organ which anchors a vascular plant, absorbs minerals and
water, and often stores organic nutrients. Most gymnosperms and eudicots have
one main vertical root that develops from an embryonic root, otherwise known asa taproot system. In angiosperms, the taproot often stores organic nutrients that
the plant requires during flowering and fruit production. The taproot also gives rise
to branch roots. Seedless vascular plants and most monocots have no main root,
but they do have a fibrous root system. The entire root system helps to anchor a
plant, but in most plants the absorption of water and minerals occurs through root
hairs near the tip of the root. There are also modified roots which come about
from different environmental adaptations.
Buttress roots are aerial roots that look
like buttresses and support the tall trunks of
some trees, such as the ceiba tree.
Strangling aerial roots gradually
wrap around their hosts. Sadly, the host
tree eventually dies of strangulation and
shading.
Pneumatophores, also known as air
roots, are produced by trees such as
mangroves that inhabit tidal swamps. By
projecting above the surface, they enable
the root system to obtain oxygen.
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Stems
This strawberry plant has a specialized
stem called a runner. A runner is a type of
stolon, horizontally growing on top of the
ground and rooting at the nodes, which aids
in reproduction.
A rhizome is a horizontal underground stem that
functions mainly in reproduction but also in storage.
This is a Euphorbia plant sending out rhizomes.
A stem is an organ made up of an alternating system of nodes, the points at
which leaves are attached, and internodes, the stem segments between nodes.
Stems have four main functions. One function is to provide support for and the
elevation of leaves, flowers and fruits. The stems keep the leaves in the light and
provide a place for the plant to keep its flowers and fruits. Another purpose of the
stem is to transport of fluids between the roots and the shoots in the xylem and
phloem. Stems also store plant nutrients. And finally stems produce new living
tissue, for the normal life span of plant cells is one to three years. Stems have cells
Tubers, such as these potatoes, areenlarged ends of rhizomes specialized for storing
food. The eyes are arranged in a spiral pattern
and are located in clusters of axillary buds that
mark nodes on the potato.
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eaves
In most vascular plants, the leaf is the main photosynthetic organ. Leaves
consist of a flattened blade and a stalk, the petriole, which joins the leaf to the
stem. There are many different variations of leaves in the plant world. Most
monocots have parallel major veins which run the length of the leaf blade,
whereas eudicots usually have a multibranched network of major veins. Leaf
morphology can be used to identify and classify angiosperms. Most leaves are
specialized for photosynthesis, but some plant species have leaves that have
become adapted to provide support, protection, storage, or even
reproduction.
The spines of a cactus are actually leaves, and
photosynthesis is carried out mainly by the green stems;
while these leaves provide protection.
The red parts of the poinsettia are
often mistaken for petals, but they are
actually modified leaves called bracts which
are brightly colored to attract pollinators.
Most succulents, such as this
ice plant, have leaves adapted for
storing water.
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The Three Tissue Systems
Dermal Tissue
The dermal tissue is the outer
protective covering. It forms the first line
of defense against physical damage
and pathogenic organisms, much like
human skin. In nonwoody plants, the
dermal tissue usually consists of the
epidermis. The epidermis protects the
plant form water loss and disease, and
has specialized characteristics in each
organ. In, woody plants, the protective
tissues are known as the periderm.
Vascular Tissue
The vascular tissue system
carries out long- distance transport of
materials between roots and shoots.
The two vascular tissues are the xylem
and the phloem. The xylem carries
water and dissolved minerals from the
roots up to the shoots. The phloem
transports organic nutrients such as
sugars form where they are made to
where they are needed. The vascular
tissue of a root or stem is all together
called the stele. The arrangement of
stele varies, depending on species
and organ.Ground Tissue
Ground tissue is basically all the
rest of the tissues of the plant which do
not fall under the dermal or vascular
tissue categories. Ground tissue that is
internal to the vascular tissue is called
pith, and ground tissue that is external
to the vascular tissue is called cortex.The ground tissue system contains
various cells specialized in functions
such as storage, photosynthesis, and
support.
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Plant cells
Parenchyma Cells
Parenchyma cells are relatively
unspecialized cells that retain the ability
to divide. They can only divide though
under special conditions. They also
perform most of the plants metabolic
functions, synthesizing and storing
various organic products. Parenchyma
cells have primary walls which are thick
and flexible, but often lack secondary
walls. The protoplast generally has a
large central vacuole. In addition, some
parenchyma cells in stems and roots
contain plastids which store starch.
Collenchyma Cells
Collenchyma cells help support
young parts of the plant shoot.
Collenchyma cells have thicker
primary walls then parenchyma cells,
though the walls are unevenly
thickened. Collenchyma cells provide
flexible support without restraining
growth because they lack secondary
walls and they also lack a hardening
agent in their primary walls. At
functional maturity, these cells are
living and flexible, elongating with the
stems and leaves they support.
Sclerenchyma CellsSclerenchyma cells, like collenchyma cells, function to support elements in
the plants, but sclerenchyma cells are rigid and contain a thick secondary wall.
There are two types of sclerenchyma cells: sclereids and fibers. These cells are
specialized entirely for support and strengthening. Sclereids, which are shorter than
fibers and vary in shape, have very thick, lignified secondary walls. Sclereids pass
on the hardness to nutshells (see picture below) and seeds and the gritty texture to
pear fruits. Fibers, which are usually arranged in threads, are long, slender, and
tapered. Some fibers are used commercially, such as hemp fibers for weaving into
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Primary Growth Secondary Growth
Primary growth lengthens roots
and shoots. Primary meristems, which
are located at the tips of roots andshoots, are responsible for increase in
length.
In roots, the apical meristem is
located near the tip, where it
regenerates the root cap. These
apical meristems give rise to three
primary meristems: ground meristems,
which develop into ground tissues,
procambium, which develop into
vascular tissues and the vascular
cambium, and protoderm, which
develops into the dermal system. In
shoots, the apical meristem is located
in the terminal bud, where it gives rise
to a repetition of internodes and leaf-
bearing nodes.
Secondary growth adds girth to
stems and roots in woody plants.
Secondary meristems, located in the
margins of the stem and root (vascular
and cork cambium), are responsible for
increase in girth.
Lateral meristems are secondary
meristems that form "tubes" within the
stem and root of the plant. There are
two lateral meristems: the vascular
cambium, located between xylem and
phloem, and the cork cambium,
located between phloem and bark.
The vascular cambium develops into a
meristematic cylinder that produces the
secondary xylem and phloem. The cork
cambium gives rise to the secondary
plant bodys protective covering, or
periderm, which consists of cork
cambium plus the layers of cork it
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Xylem Phloem
Made of Dead Cells Living cells
Cell wall thickness Thick Thin
Cell wall material Lignin (rigid) Cellulose
Permeability Impermeable Permeable
Transports Water & Minerals Food
Carried to
Leaves Growing parts & storageorgans
Direction offlow Upwards Up & down
The xylem and the phloem
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Chapter 4:Angiosperm Reproduction
Flower Structure
There are four floral organs: sepals, petals, stamens, and carpels. The four
floral organs are separated by short internodes and attached to a part of the
stem called the receptacle. Stamens and carpels are reproductive organs,
whereas sepals and petals are sterile. Sepals, which enclose and protect the
floral bud before it opens, are usually green and more leaflike in appearance
than other floral organs. In many species, the petals are more brightly colored
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The nest floral organ, the stamen, consists of a stalk called the filament and
a terminal structure called the anther. In the anther there are chambers with
pollen sacs where pollen is produced. Finally, the carpel, this organ has an ovary
at its base and a long, slender neck called the style. At the top of the style is the
stigma, a sticky structure which serves as the landing platform for pollen. In most
species, two or more carpels are fused onto a single structure. This results in an
ovary with two or more chambers, each containing one or more ovules. The term
pistil is sometimes used to describe a single carpel or a group of fused carpels.
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Double Fertilization
Double fertilization is a complex fertilization mechanism that has evolved in
angiosperms. This process involves the joining of a female gametophyte (embryo
sac) with two male gametes (sperm). It begins when a pollen grain attaches to
the stigma of the carpel. After a pollen grain has landed on an accessible
stigma, the pollen grain takes in moisture and begins to germinate, forming a
pollen tube that extends down toward the ovary through the style. The tip of the
pollen tube then enters the ovary and penetrates through the micropyle. The
micropyle is an opening in the protective layers of the ovule. The pollen tube
proceeds to release the two sperm in or near the embryo sac.
One sperm fertilizes the egg cell and the other sperm combines with the
two polar nuclei of the large central cell of the embryo sac. The sperm and
haploid egg combine to form a diploid zygote; while the other sperm and two
haploid polar nuclei form a triploid nucleus (some plants may form polyploid
nuclei). The large cell of the embryo sac will then form the endosperm, a nutrient-
rich tissue which provides nourishment to the developing embryo. The ovary,
surrounding the ovules, develops into the fruit, which is used for protection and
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Methods of pollination
This picture depicts grains of
pollen visible on the stamen of a tulip.
Pollination is the transfer of pollen from an anther to a stigma and is the
first step in a chain of events which can lead to fertilization. This step can be
accomplished in numerous ways. In some angiosperms, including grasses and
trees, wind is a pollinating agent. These plants often release enormous amounts
of pollen to compensate for the unreliability of this dispersal mechanism. At
certain times of the year the air is loaded with pollen grains, which is not fun for
those with pollen allergies (like me). Some aquatic plants rely on water to
disperse pollen. Most angiosperms rely on insects, birds, or other animals to
transfer ollen directl to other flowers.
An Andrena bee collects pollen
among the stamens of a rose. The bee's
stash of pollen is on its hind leg.
A bee covered in pollen.
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From seed tofruit
While the seeds are developing from ovules, the ovary of the flower is
developing into a fruit, which protects the enclosed seeds. Fertilization triggers
hormonal changes that cause the ovary to begin conversion into a fruit. If a
flower has not been pollinated, fruit usually does not develop.
During fruit development, the ovary wall becomes the thickened wall of
the fruit (the pericarp). The other parts of the plant whither and are shed as the
plant grows.
Fruit are classified, depending on their developmental origin, into severaltypes. Most fruits are derived from a single carpel or several fused carpels and are
called simple fruit. Some simple fruits are fleshy, such as a peach, and others are
dry, such as a pea pod. An aggregate fruit results from a single flower that has
one separate carpel, each forming a small fruit. These fruitlets are clustered
together on a single receptacle, therefore forming a raspberry. A multiple fruit
develops from a group of flowers tightly packed together (an inflorescence). An
example of this is a pineapple, for when the walls of many ovaries start to thicken