Transcript Chapter 23
Chapter 23 Roots, Stems, and Leaves Roots 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. 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 systems1. 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 The structure of Roots Root hairs Xylem and phloem are located in the center of the root. Xylem The arrangement of xylem and phloem tissues accounts Phloem for one of the major Pericycle differences between Endodermis monocots and dicots. The tip of each root is covered by a protective layer of parenchyma cells called Apical meristem the root cap. Root cap Root Growth 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. Stems 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. Stem Adaptations Some stems are adapted to storing food. Stems that act as food-storage organs include: 1. Corms 2. Tubers 3. rhizomes. Corm A corm is a short, thickened, underground stem surrounded by leaf scales. Tuber & Rhizome 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 Internal Structure 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. Vascular Bundle 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 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 How old is my tree? Annual growth rings These annual growth rings can be used to estimate the age of the plant. Woody stems are composed primarily of dead xylem cells. Cork Phloem Vascular cambium Xylem Woody Stems 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. Stems transport water Water lost through leaves 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 Xylem Water Stems transport sugar 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. Stem Transport 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 Source of sugars Sugar Phloem Sink Sieve plate Companion cell 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 The arrangement of leaves on a stem can vary. 1. Opposite: Directly opposite each other on a stem 2. Alternate: on both sides but not opposite each other 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 structure Cuticle Upper epidermis Palisade mesophyll Vascular bundle Xylem Phloem Lower epidermis Spongy mesophyll Stomata Guard cell 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 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. Guard cells 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 Transpiration Water 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 Thickened walls Guard cell Epidermal cells 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 Stigma Petals Style Ovary Anther Stamen Filament Sepal Peduncle Pistil 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 daylightdarkness 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 daylight hours. These plants are called dayneutral 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 4. Cling onto animals fur 5. May float away if found near the water