Chapter 3 and 4 (Plant structure and Angiosperm reproduction)

8/9/2019 Chapter 3 and 4 (Plant structure and Angiosperm reproduction)

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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

Chapter 3 and 4 (Plant structure and Angiosperm reproduction)

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