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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