Essentials of BiologySylvia S. Mader

Chapter 20Lecture Outline

Prepared by: Dr. Stephen EbbsSouthern Illinois University Carbondale

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

20.1 Plant Organs

• Flowering plants have two major components to their structure.– A root system– A shoot system composed of the stem,

leaves, and reproductive organs.

• At the end of the root and shoot system is a terminal bud from which vertical growth, called primary growth, occurs.

20.1 The Body’s Organization (cont.)

Leaves

• Recall that photosynthesis, the process by which plants make carbohydrates, occurs in the leaves.

• To conduct photosynthesis, leaves need solar energy, water, and carbon dioxide.

• Photosynthetic leaves share similar structural components.– The blade, the wide part of the leaf– The petiole, the stalk connecting leaf to stem.

Leaves (cont.)

• There is tremendous diversity in leaf structure between plant species.

• In some plant species, leaves may serve additional functions, such as storage.

• Some plants are deciduous, meaning that they drop their leaves during certain seasons.

Leaves (cont.)

Stems

• The stem is the main axis of the plant.

• Stems can produce side (lateral) branches from lateral (axillary) buds.

• Nodes are the points where leaves attach to stems.

• An internode is the region between nodes.

Stems (cont.)

• The stem also contains the vascular tissue that transports water and nutrients to leaves to support photosynthesis.

• In some plant species, stems may also carry out photosynthesis or serve as a storage organ.

Roots

• Roots anchor plants to the soil.

• Roots also absorb water and nutrients from the soil.

• The surface area of roots is greatly increased by the production of root hairs.

Roots (cont.)

• There are different types of root systems.– Some plants have a single taproot.– Grasses have fibrous root systems.– Some plants have prop roots for support.

• For perennial plants, the roots act as a storage order that allows the shoot system to regrow each year.

Roots (cont.)

Monocot Versus Eudicot Plants

• Flowering plants are divided into two major groups based upon the difference in embryonic leaves (cotyledons).– Plants that produce a single cotyledon are

called monocots.– Plants that produce two cotyledons are called

eudicots.

• Cotyledons provide developing plants with nutrients and serve other roles.

Monocot Versus Eudicot Plants (cont.)

• The arrangement of the vascular tissue differs between monocots and eudicots.

• Plants have two types of vascular tissue.– The xylem transports water and minerals.– The phloem transports organic nutrients.

• The vascular tissues serve as a type of circulatory system for plants.

Monocot Versus Eudicot Plants (cont.)

• The pattern of venation in the leaves of monocots and eudicots differs.– Monocots have parallel venation.– Eudicots have a net-like pattern.

• The number of species also differs between monocots and eudicots. – Eudicots include a large number of species.– There are fewer monocots species.

Monocot Versus Eudicot Plants (cont.)

20.2 Plant Tissues and Cells

• Plant growth occurs continually from dividing cells called the meristem.

• The apical meristems are located at the tip of the root and shoot.

• Cellular division of the apical meristems increase the length of the root and shoot.

20.2 Plant Tissues and Cells (cont.)

• The outer cell layer of plant tissues is the epidermis.

• The root epidermis can also have epidermal root hairs to increase surface area.

20.2 Plant Tissues and Cells (cont.)

• The leaf epidermis is covered with a waxy cuticle, providing a barrier to water loss.

• The leaf epidermis also have stomata which regulate gas and water exchange.

20.2 Plant Tissues and Cells (cont.)

• In tree trunks, epidermis is replaced by cork, produced by the cork cambium.

• Cork is waterproof because of a chemical called suberin.

20.2 Plant Tissues and Cells (cont.)

• The interior of the plant leaves, stems, and roots is composed of ground tissue.

• There is also a meristematic vascular tissue called the vascular cambium, which produces new vascular tissue.

• There are three cell types in ground tissue.– Parenchyma– Collenchyma– Sclerenchyma

20.2 Plant Tissues and Cells (cont.)

• Parenchyma cells are the least specialized cell type.

• Parenchyma cells are photosynthetic cells found throughout the plant.

20.2 Plant Tissues and Cells (cont.)

• Collenchyma cells have thick cell walls.

• Collenchyma cells are arranged in bundles to provide flexible support below the epidermis.

20.2 Plant Tissues and Cells (cont.)

• Sclerenchyma cells have cell walls reinforced with lignin.

• Sclerenchyma cells are often dead cells.

• Sclerenchyma cells provide support in mature tissues.

20.2 Plant Tissues and Cells (cont.)

• The xylem is the vascular tissue that transport water and minerals from roots.

• Vessel elements are one type of xylem with large, perforated cell walls.

• Tracheids are smaller xylem cells whose walls have numerous pits.

20.2 Plant Tissues and Cells (cont.)

20.2 Plant Tissues and Cells (cont.)

• The phloem of the vascular system is composed of sieve-tube members.

• The sieve-tube members have perforated plates on each end of the cell.

• Each sieve-tube member has a companion cell which controls the activity of the enucleated sieve-tube member.

20.2 Plant Tissues and Cells (cont.)

20.3 Organization of Leaves

• Leaf structure varies from plant species to plant species.

• There may be a single blade of the leaf or multiple blades, forming a compound leaf.

20.3 Organization of Leaves (cont.)

20.3 Organization of Leaves (cont.)

• The top and bottom of a typical eudicot leaf is composed of epidermis– The epidermis often has hairs or glands.– Stomata are located on the lower epidermis.

• The interior of the leaf is composed of photosynthetic mesophyll cells.– The spongy mesophyll is arranged randomly to

increase surface area for gas exchange.– The palisade mesophyll is comprised of elongated,

vertically-oriented cells.

20.3 Organization of Leaves (cont.)

20.4 Organization of Stems

• Primary growth, driven by cell division in the apical meristem, contributes to the growth of stems.

• The organization of the terminal bud protects the apical meristem.

Nonwoody Stems

• Plant stems that do not contain wood are called herbaceous stems.

• The vascular bundles of herbaceous eudicot stems are arranged in a ring under the epidermal layer.

• The vascular bundles of herbaceous monocot stems are randomly distributed.

Nonwoody Stems (cont.)

Nonwoody Stems (cont.)

Woody Stems (cont.)

• Woody stems undergo both primary and secondary growth.

• Secondary growth is an increase in girth.

• The vascular cambium of woody plants is meristematic and produces new xylem and phloem cells each year.

Woody Stems (cont.)

• Woody stems have three distinct regions.– Bark

– Wood

– Pith

• The vascular cambium occurs between the bark and the wood.

Woody Stems (cont.)

Bark

• Tree bark contains several cell types.– Cork– Cork cambium– Cortex– Phloem

• The cork cells have several functions.– Cork cells protect the stem– Specialized cork cells form lenticels to

facilitate gas exchange.

Wood

• Wood is composed of the secondary xylem produced each year by the stem.

• Spring wood has wide xylem vessels with thin walls, due to transport of large amounts of water.

• When water is scarce later in the summer, the xylem vessels of summer wood become narrower with thicker walls.

• Spring and summer wood together make an annual ring.

20.5 Organization of Roots

• Within the eudicot root, there are longitudinal zones where cells are in different stages of differentiation. – The apical meristem is composed of dividing

cells protected by a root cap.– Cells in the next zone are elongating

vertically.– In the last zone, the cells mature before

completing their development.

20.5 Organization of Roots (cont.)

Tissues of the Eudicot Root

• The eudicot root has several tissue types.– The epidermis is the outermost layer.– The cortex in the center of the root is

comprised parenchyma cells.– The endodermis is an internal cell layer that

regulates the movement of water and nutrients into the vascular tissue.

– The pericycle is an inner ring of dividing cells that can produce lateral roots.

– The vascular tissue in the center of the root contains xylem and phloem for transport.

Tissues of the Eudicot Root (cont.)

Organization of Monocot Roots

• Monocots have the same growth zones as eudicot roots but do not undergo secondary growth.

• The center of monocot roots is composed of ground tissue called pith.

• The pith is surrounded by a vascular ring with alternating bundles of xylem and phloem.

Comparison With Stems

• Roots and stems are both produced by primary growth from apical meristems.

• However, the branching of roots and stems occur differently.– Stems branch from buds on the stem.– Roots branch from the internal pericycle.

• The vascular cambium of eudicot stems and roots produces secondary growth.

20.6 Plant Nutrition

• Plants are unique in that they require only inorganic nutrients to survive.

• Plants convert these inorganic nutrients to the organic compounds needed for life.

• Some inorganic elements are essential, meaning that plants have an absolute requirement for those elements.

20.6 Plant Nutrition (cont.)

• The essential nutrients are divided into two categories based upon their relative concentrations in plant tissues.– Macronutrients are elements that are required

in large amounts.

– Micronutrients are required in small amounts for specialized functions.

20.6 Plant Nutrition (cont.)

• There are nine macronutrients.– Carbon– Hydrogen– Oxygen– Phosphorus– Potassium– Nitrogen– Sulfur– Calcium– Magnesium

20.6 Plant Nutrition (cont.)

• There are seven micronutrients, which serve primarily as enzyme cofactors.– Iron

– Boron

– Manganese

– Copper

– Zinc

– Chloride

– Molybdenum

20.6 Plant Nutrition (cont.)

• Deficiencies in one or more of these nutrients can stunt plant growth.

Adaptations of Roots for Mineral Uptake

• Mineral nutrients enter plants through the root system.

• Roots have several modifications that enhance their ability to acquire nutrients.

• Some of those modifications involve specific symbiotic relationships.

Adaptations of Roots for Mineral Uptake (cont.)

• In plants such as legumes, specialized bacteria reside in root nodules.

• These bacteria are capable of converting atmospheric nitrogen gas into a form useable by the plants.

• The plant roots provide carbohydrates to the bacteria to support their growth.

Adaptations of Roots for Mineral Uptake (cont.)

• Most plants have a symbiotic relationship with mycorrhizal fungi.

• The fungal hyphae increases the surface area available for water and nutrient uptake.

• The plant roots provide the fungi with carbohydrates and amino acids.

Adaptations of Roots for Mineral Uptake (cont.)

20.7 Transport of Nutrients

• The water and nutrients taken up by roots and root hairs are transported to leaves via the interconnected vessel elements of the xylem.

• This movement is provided in part by root pressure, a positive pressure created when water enters the root by osmosis.

20.7 Transport of Nutrients (cont.)

• The cohesion-tension model explains how water travels up the xylem to leaves.

• Recall that leaves have numerous openings called stomata.

• When these stomata are open, water evaporates from the interior of the leaf to the outside air, a process called transpiration.

20.7 Transport of Nutrients (cont.)

• As plant leaves transpire water, a tension is created that pulls water from roots to leaves.

• This tension is maintained because water molecules display an attraction to one another called cohesion.

• Water also adheres to the xylem elements in a process called adhesion.

20.7 Transport of Nutrients (cont.)

Opening and Closing of Stomata

• The opening and closing of the leaf stomata is controlled by turgor pressure within the guard cells.

• As water enters the guard cells, these cells swell, opening the stomate.

• As water exits the guard cells, the loss of turgor causes the stomate to close.

Opening and Closing of Stomata (cont.)

Organic Nutrients in the Phloem

• The phloem transport carbohydrates from photosynthesizing leaves to roots, young leaves, and other tissues that require carbohydrates.

• The transport of carbohydrates through the phloem occurs by a mechanism called the pressure-flow model.

Organic Nutrients in the Phloem (cont.)

• Phloem transport is considered source to sink transport.

• As mature leaves photosynthesize, they become a source of sugar.

• The carbohydrates in the phloem are transported to tissues that require sugars, called sink tissues.

Organic Nutrients in the Phloem (cont.)