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Chapter 3

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The Importance of Plants With out plants, life on earth could not

exist. Directly or indirectly plants are the primary

source of food for humans and animals. Plants play another essential role by

producing oxygen. All plants life, from the smallest plankton

in the ocean to the giant redwood tree works to produce oxygen.

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The Importance of Plants

In addition to supplying food and oxygen, plants help to keep us cool, renew the air, slow down the wind, hold soil in place, provide a home for wildlife, beautify our surroundings, perfume the air, and furnish building materials and fuel.

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Parts of the Plant Most plants are made up of four basic

parts: Leaves, stems, roots, and flowers, which later

become fruit or seeds. Leaves

Leaves are the food factory of the plant, producing all food that is used by the plant and stored for later use by the plant or by animals.

Leaves vary a great deal in shape and size. Most leaves are flat.

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Parts of the Plant Some, such as the leaves of pine trees,

are needlelike, while others such are onion leaves are cylindrical.

The shape and size of leaves help to identify the plant.

The arrangement of leaves on plants also differs.

Some plants have leaves which alternate on the stem.

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Parts of the Plant Others are whorled arranged in a circle

around the stem. External Leaf Structure

Leaves consist of the petiole, or leaf stalk, and the blade, a larger, usually flat part of the leaf.

Notice that the leaf blade has veins and a midrib.

The midrib is the large center vein from which all other veins extend.

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Parts of the Plant

The veins of the leaf form its structural framework.

Most leaves have one of the forms shown in figure 3-2 on page 23.

Different leaf margins assists in plant identification.

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Parts of the Plant

Internal Leaf Structure Leaves have specialized cells which

perform very important tasks. The skin of the leaf, called the epidermis,

is a single layer of cells. Its main function is to protect the leaf

from loss of too much moisture. There are special cells in the leaf skin

known as guard cells.

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Parts of the Plant These cells open and close a small space or

pore on the underside of the leaf called a stoma.

This allows the leaf to breathe and transpire, or give off moisture and exchange gases such as oxygen and carbon dioxide.

The guard cells are crescent shaped and the inner walls are thick.

In the center of the leaf are food making cells which contain chloroplasts.

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Parts of the Plant The green color of the chloroplasts, is

what gives the green leaves their color. These cells, through photosynthesis

manufacture food. Photosynthesis is the process by which

carbon dioxide and water in the presence of light are converted to sugar and oxygen.

This is the begin of the food chain for all living things.

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Parts of Plants

Stems They have two main functions. 1. The movement of materials, such as

the movement of water and minerals, from roots upward to the leaves, and the leaves down to the roots.

2. To support the leaves and the reproductive structures.

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Parts of Plants

They are also used for food storage, as in the Irish potato.

For reproduction methods which involve stem cutting or grafting.

Green stems manufacture food just like leaves do.

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Parts of Plants

External Stem Structure Lenticels

Breathing pores. Bud Scale Scars

Indicate where a terminal bud has been located.

The distance between two scars represents one years growth.

Leaf Scars Show where leaves were attached.

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Parts of Plants

Internal Stem Structure In all stems, water and minerals travel up

the xylem and manufactured food travels down the phloem.

In dicots , plants having 2 seed leaves, xylem and phloem form two layers separated by the cambium which produces all new cells.

Some plants such as grasses have a different stem structure.

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Parts of Plants These are referred to as monocots because

they have only one cotyledon (seed leave). Corn is an example of a monocot. Monocots have vascular bundles which

contain both phloem and xylem tissue in each small bundle.

The stems of some plants, such as the Irish potato and asparagus, are used as food. Others are used as building materials, such as the lumber from tree trunks.

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Roots

Roots are usually underground and therefore are not easily visible.

Roots function to: Anchor the plant and hold it upright Absorb water and minerals from the soil

and conduct them to the stem. Store large quantities of plant food. Propagate or reproduce some plants

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Roots The first 3 functions are essential to all

plants. Structure

The internal structure of a root is much like that of a stem.

Older roots of shrubs and trees have phloem on the outside, a cambium layer and xylem inside. The phloem carries manufactured food down to the root for food and storage and the xylem carries water and minerals up to the stem.

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Roots

The external structure of the root is very different form that of the stem.

The root initiates new growth from the root cap.

Root hairs absorb moisture and minerals which are conducted to the larger roots and to the stem of the plant.

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Types of Root Systems

Plants with fibrous root sytems are much easier to transplant than plants which have a tap root system.

The tap root has fewer roots than the fibrous system.

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Flowers, Fruits, and Seeds

Parts of the Flower Seed is the most common way plants

reproduce in nature. Seeds are produced by a sexual process

with a male and female parent involved. A complete flower has both male and

female parts and only one parent is needed if the plant is self-fruitful (pollinate itself).

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Flowers, Fruits, and Seeds The description of a complete flower is a

flower that contains four main parts. The sepals, petals, stamens, and pistil.

The sepals are the green leaf-like parts of the flower that cover and protect the flower bud before it opens.

The petals are actually leafs, but are generally known as the most striking part of the flower. The bright color are what attracts insects.

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Flowers, Fruits, and Seeds The stamen makes up the male

reproductive part of the flower. Each stamen consists of a short stalk called the filament on top of that is the anther. The anther contains the pollen.

The pistil is the female part of the flower.