Chapter 23

Roots, Stems, and Leaves

Roots are plant organs that anchor a plant, usually absorb water and dissolved minerals, and contain vascular tissues that transport materials to and from the stem.

Roots

Types of Roots

The surface area of a plant’s roots can be as much as 50 times greater than the surface area of its leaves

Most roots grow in soil but some do not There are two main types of root systems-

1. taproots

2. fibrous roots

Taproots

Carrots and beets are taproots, which are single, thick structures with smaller branching roots

Taproots accumulate and store food

Fibrous root

Fibrous roots systems have many, small branching roots that grow from a central point.

Some plants, such as corn have a type of root called prop roots, which originate above ground and help support a plant

Other types of Roots

Many climbing plants have aerial roots that cling to objects such as walls and provide support for climbing stems

Other types of roots When bald cypress trees grow in swampy soils, they

produce modified roots called pneumatophores, which are referred to as “knees.”

The knees grow upward from the mud, and eventually, out of the water.Knees help supply oxygen to the roots

Xylem and phloem are located in the center of the root.The arrangement of xylem and phloem tissues accounts for one of the major differences between monocots and dicots.The tip of each root is covered by a protective layer of parenchyma cells called the root cap.

Root hairs

Xylem

Phloem

Pericycle

Endodermis

Apical meristem

Root cap

The structure of Roots

There are two areas of rapidly dividing cells in roots where the production of new cells initiates growth.

The root apical meristem produces cells that cause a root to increase in length.

In dicots, the vascular cambium develops between the xylem and phloem and contributes to a root’s growth by adding cells that increase its diameter.

Root Growth

As the root grows through the soil, the cells of the root cap wear away.

Replacement cells are produced by the root apical meristem so the root tip is never without its protective coverings.

Root Growth

Stems

Stems usually are the aboveground parts of plants that support leaves and flowers. They have vascular tissues that transport water, dissolved minerals, and sugars to and from roots and leaves.

Green, herbaceous stems are soft and flexible and usually carry out some photosynthesis.

Stems

Trees, shrubs, and some other perennials have woody stems.

Woody stems are hard and rigid and have cork and vascular cambriums.

Stems

Some stems are adapted to storing food.

Stems that act as food-storage organs include:

1. Corms

2. Tubers

3. rhizomes.

Stem Adaptations

Corm

A corm is a short, thickened, underground stem surrounded by leaf scales.

A tuber is a swollen, underground stem that has buds from which new plants can grow.

Rhizomes also are underground stems that store food.

Tuber & Rhizome

Tuber

The vascular tissues in stems are arranged differently from that of roots.

Stems have a bundled arrangement or circular arrangement of vascular tissues within a surrounding mass of parenchyma tissue.

Internal Structure

In most dicots, xylem and phloem are in a circle of vascular bundles that form a ring in the cortex.

The vascular bundles of most monocots are scattered throughout the stem.

Vascular bundles

Vascular bundle

Vascular Bundle

Woody Stem

As the stems of woody plants grow in height, they also grow in thickness

This added thickness, called secondary growth, results from cell divisions in the vascular cambium of the stem

The xylem tissue produced by secondary growth is also called wood

In temperate regions, a tree’s annual growth rings are the layers of vascular tissue produced each year by secondary growth

These annual growth rings can be used to estimate the age of the plant.

Woody stems are composed primarily of dead xylem cells.

Annual growth rings

Xylem

Vascular cambium

PhloemCork

How old is my tree?

As secondary growth continues, the outer portion of a wood stem develops bark.

Bark is composed of phloem cells and the cork cambium.

Bark is a tough, corky tissue that protects the stem from damage by burrowing insects and browsing herbivores.

Woody Stems

Water lost through leaves

Water

Xylem

Stems transport water

Water, sugars, and other compounds are transported within the stem

As water moves up through the xylem, it also carries dissolved minerals to all living plant cells

The contents of phloem are primarily dissolved sugars but phloem also can transport hormones, viruses, and other substances.

The sugars originate in photosynthetic tissues that are usually in leaves.

Stems transport sugar

Any portion of the plant that stores these sugars is called a sink, such as the parenchyma cells that make up the cortex in the root.

The movement of sugars in the phloem is called translocation

Sink

Source of sugars

Sugar

Phloem

Sieve plateCompanion cell

Stem Transport

Leaves

Function of leaves

The primary function of the leaves is photosynthesis

Most leaves have a relatively large surface area that receives sunlight

Sunlight passes through the transparent cuticle into the photosynthetic tissues just beneath the leaf surface

Parts of the Leaf

When you think of a leaf, you probably think only of a flat, broad, green structure

Sizes, shapes, and types of leaves vary enormously.– Blade: entire leaf unit

– Margin: edge of the leaf

– Base: Part of the leaf closest to the stem

– Apex: tip of the leaf

– Petiole: connects the leaf blade to the stem

– Stipule: tiny leaf like structures that may or may not be present close to the stem

Simple and Compound Leaf

A simple leaf is one with a blade that is not divided

When the blade is divided into leaflets, it is called a compound leaf

Leaf arrangement on a stem

1. Opposite: Directly opposite each other on a stem

2. Alternate: on both sides but not opposite each other

The arrangement of leaves on a stem can vary.

Leaf arrangement on a stem

3. Whorled: Three or more leaves growing around a stem at the same position

The lines that appear on the surface of a leaf and look like blood vessels are called Veins

1. Pinnate 2. Palmate

Leaf structureLeaf structure

StomataGuard cell

Spongy mesophyll

Lower epidermis

Cuticle

Upper epidermis

Palisade mesophyll

Vascular bundle

Xylem

Phloem

Leaf Structure

Most photosynthesis takes place in the palisade mesophyll

Below the palisade mesophyll is the spongy mesophyll, which is composed of loosely packed, irregularly shaped cells

These cells usually are surrounded by many air spaces that allow carbon dioxide, oxygen, and water vapor to freely flow around the cells

Gases can also move in and out of a leaf through the stomata

Stomata (STOH mah tuh) (singular, stoma) are openings in leaf tissue that control the exchange of gases.

Stomata are found on green stems and on the surfaces of leaves.

Stomata

Cells called guard cells control the opening and closing of stomata.

The opening and closing of stomata regulates the flow of water vapor from leaf tissues.

The loss of water through the stomata is called transpiration

Guard cells

Water

Guard cell

Epidermal cells

Thickened walls

Transpiration

The guard cells have flexible cell walls

When water enters the guard cells, the pressure causes them to bow out, opening the stoma

As water leaves the guard cells, the pressure is released and the cells come together, closing the stoma

Leaf Modifications

Many plants have leaves with structural adaptations for functions besides photosynthesis

1. Some plant leave have epidermal growths that release irritants when broken or crushed

Leaf Modifications

Cactus spines are modified leaves that help reduce water loss from the plant and provide protection from predators

Leaf Modifications

Carnivorous plants have leaves with adaptations that can trap insects or other small animals

Pitcher Plant

Chapter 24

Plant Reproduction

Reproduction in plants

The process of sexual reproduction in flowering plants takes place in a flower

A flower’s structure is genetically determined and usually made up of four kinds of organs:

1. sepals 2. petals 3. Stamens 4. pistils

Petals

Stigma

Style

Ovary

Pistil

Peduncle

Sepal

Filament

AntherStamen

Complete Flower

A flower that has all four organs—sepals, petals, stamens, and pistils—is called a complete flower

Incomplete Flower

A flower that lacks one or more organs is called an incomplete flower

For example the flowers of plants such as sweet corn, and grasses, have no petals and are adapted for pollination by wind rather than by animals.

Photoperiodism

The relative lengths of daylight and darkness each day have a significant effect on the rate of growth and the timing of flower production in many species of flowering plants

The response of flowering plants to daily daylight-darkness conditions is called photoperiodism– Plant biologists originally thought that the length of

daylight controlled flowering, but they now know that it is the length of darkness that controls flowering, and that the darkness must be uninterrupted

Types of Photoperiodism

Plants are

1. short-day plants

2. long-day plants

3.day-neutral plants

4. intermediate day plants

Short Day Plants

A short-day plant flowers when the number of daylight hours is shorter than that of its critical period

Short-day plants usually flower sometime during late summer, fall, winter, or spring

Long Day Plants

A long-day plant flowers when the number of daylight hours is longer than that of its critical period

Long-day plants usually flower in summer, but also will flower if lighted continually

Day Neutral Plants

Some plants will flower over a range in the number of day-light hours. These plants are called day-neutral plants

Includes many plants Flowering in cucumbers,

tomatoes, and corn are not influenced by dark period

Intermediate Day Plants

An intermediate-day plant will not flower if days are shorter or longer than its critical period

Several grasses and sugarcane are in this category

Pollination

The process of transferring pollen grains from the anther to the stigma

1. Wind

Wind is random It scatters pollen

randomly Pollen can land places

besides the stigma

2. Animals

Ensure pollen gets in the right place

Use beetles, butterflies, moths, bees, flies, hummingbirds, and bats

These are the most successful plant groups on Earth

3. Nectar

Produced by flowers Attracts & serves as

food for animal pollinators

Liquid is made up of proteins & sugars

Usually collects in a cup like area that the base of the petals

Animals as pollinators

The animals position on the petals & brush against the anthers

Pollen grains stick to the body & brush off onto the stigma of another plant

Results in pollination

Nectar feeding pollinators are attracted by color and scent

Butterflies: attracted to bright colors, with platforms or cluster petals in the daytime– Daisies, phlox & rhododendrons

Nectar feeding pollinators are attracted by color and scent

Moths: attract to plants that stay open all night, with pale colors but has a strong sweet scent

Do not need a landing pad, moths hover– Tobacco, night-blooming cereus, &

honeysuckle

Nectar feeding pollinators are attracted by color and scent

Bees: collect pollen & nectar Attracted to yellow/blue flowers with a

sweet scent– Peas, mint, primrose, irises…

Seed Dispersal

Dispersal of seeds is important because it reduces competition

1. Animals such as raccoons, deer, bears, and birds help distribute many seeds by eating fruits.

Seed Dispersal

2. Seeds that are eaten usually pass through the digestive system undamaged and are deposited in the animal’s wastes

3. The ripened fruits of many plants split open to release seeds with structural adaptations for dispersal by wind or by clinging to animal fur.

Seed Dispersal

Cling onto animals fur May float away if found

near the water