CHAPTER 31 … LECTURE NOTES

 

Plants form the foundation on which most terrestrial ecosystems are built.

·        As the primary producers in most systems (through the process of photosynthesis), plants serve as the first link in the food chain which affects all species of animals in the system.

 

Plants were first studied by early humans who had to distinguish between edible and poisonous plants.

·        These early humans later began to use plant products to make useful tools and other items.

·        Modern plant biology continues to center on how to use plants and plant products to benefit humans.

 

I. Plant biology reflects the major themes in the study of life

 

New methods and the discovery of unique, interesting experimental organisms has expanded the knowledge base in many areas of plant biology.

·        Studying such organisms (e.g. Arabidopsis) helps relate processes which occur at the molecular and cellular level to what is observed at the whole plant level.

 

Correlating the structure and functions of plants is a second focus of plant biology.

·        Structure and function of plants are the result of interactions with the environment over both evolutionary and short-term time scales.

=> On the evolutionary scale, plants adapted to the problems of a terrestrial existence as they moved from water to land (i.e. woody tissues provide support).

=> On the short-term scale, environmental stimuli cause individual plants to show structural and physiological responses (i.e. stomata close during the hottest part of the

day, thus conserving water).

 

While plants and animals faced many of the same problems (support, internal transport, etc.) they solved such problems in different ways.

·        The two groups evolved independently from unicellular ancestors which had different modes of nutrition; this alone established a different evolutionary direction.

 

II. A plant's root and shoot systems are evolutionary adaptations to living on land

 

Plant biologists study two levels of plant architecture: morphology and anatomy.

 

Plant morphology = The study of the external structure of plants (e.g. arrangement of the parts of a flower).

 

Plant anatomy = The study of the internal structure of plants ( e.g. arrangement of the cells and tissues in a leaf).

 

Angiosperms (flowering plants) are the most diverse and widespread of the plants with about 275,000 extant species.

·        Characterized by flowers and fruits which are believed to be adaptations for reproduction and seed dispersal.

·        Taxonomists divide angiosperms into two taxonomic classes: monocots and dicots which possess either one or two seed leaves, respectively, in combination with other characteristics. (See Campbell, Figure 31.3)

 

A plant can be divided into two basic systems, a subterranean root system and an aerial shoot system (stems, leaves, flowers).

·        This two system arrangement reflects the evolutionary history of plants as terrestrial organisms. (See Campbell, Figure 31.4)

·        Unlike the algal ancestors which were completely surrounded by nutrient rich water, terrestrial plants face a divided habitat:

=> Air is the source of CO2 for photosynthesis and sunlight cannot penetrate into the soil.

=> Soil provides water and dissolved minerals to the plant.

·        Each system depends on the other for survival of the whole plant.

=> Roots depend on shoots for sugar and other organic nutrients-

=> Shoots depend on roots for minerals, water and support.

·        Materials are transported through the plant by vascular tissues.

=> Xylem conveys water and dissolved minerals to the shoots.

=> Phloem conveys food from shoots to roots and other nonphotosynthetic parts, and from storage roots to actively growing shoots.

 

A. The Root System

 

Root structure is well adapted to:

·        Anchor plants.

·        Absorb and conduct water and nutrients-

·        Store food.

 

There are two major types of root systems:

 

The taproot system is seen in many dicots.

·        One large, vertical root (the taproot) produces many smaller secondary roots.

·        Provides firm anchorage.

·        Some taproots such as carrots, turnips, and sweet potatoes, are modified to store a large amount of reserve food.

 

Afibrous root system is found primarily in monocots (palms, bamboo, grasses).

·        A mat of threadlike roots spreads out below the soil surface-

·        Provides extensive exposure to soil water and minerals.

·        Roots are concentrated in the upper few centimeters of soil, preventing soil erosion.

 

Absorption of water is greatly enhanced by root hairs, which increase the surface area of the root.

·        Root hairs are normally most numerous near the root tips.

·        Water and mineral absorption are also enhanced by mycorrhizae, symbioses between roots and fungi.

 

Adventitious roots = Roots arising above ground from stems or leaves.

·        Form in addition to the normal root system.

·        Some, such as prop roots of corn, help support the plant stem.

 

B. The Shoot System

 

Shoot systems are comprised of vegetative shoots and floral shoots.

·        Vegetative shoots consist of a stem and attached leaves; may be the main shoot or a vegetative branch.

·        Floral shoots terminate in flowers.

 

1. Stems

 

Stem morphology includes: (See Campbell, Figure 31.4)

Nodes = The points where leaves are attached to stems.

 

Internodes = The stem segments between the nodes.

 

Axillay bud = An embryonic side shoot found in the angle formed by each leaf and the stem; usually dormant.

 

Terminal bud = The bud on a shoot tip; it usually has developing leaves and a compact series of nodes and internodes.

 

Growth of a shoot is usually concentrated at the apex of the shoot where the terminal bud is located.

·        The presence of a terminal bud inhibits development of axillary buds, a condition called apical dominance.

·        Apical dominance appears to be an evolutionary adaptation to increase exposure of plant parts to light by concentrating resources on increasing plant height.

 

Axillary buds begin to grow under certain conditions and after damage or removal of the terminal bud.

·        Some may develop into floral shoots.

·        Others develop into vegetative shoots with terminal buds, leaves, and axillary buds.

=> This development results in branching which also increases exposure of plant parts to light.

 

Some plants-have modified stems which are often mistaken for roots. There are several types of modified stems, each of which performs a specific function. (See Campbell, Figure 31.6)

·        Stolons are horizontal stems growing along the surface of the ground ( e.g. strawberry plant runners).

·        Rhizomes are horizontal stems growing underground ( e.g. irises).

=> Some end in enlarged tubers where food is stored (e.g. potatoes).

·        Bulbs are vertical, underground shoots with leaf bases modified for food storage

( e.g. onions).

 

2. Leaves

 

Leaves are the main photosynthetic organs of a plant.

·        A leaf usually exists in the shape of a flattened blade which is joined to the node of a stem by a petiole.

=> Most monocots lack petioles; instead, the leaf base forms a sheath surrounding the stem.

·        Monocot leaves have parallel major veins running the length of the blade.

·        Dicot leaves have a multi-branched network of major veins. Can be palmate or pinnate.

·        All leaves have numerous minor cross-veins.

 

Plant taxonomists use a variety of leaf characteristics to classify plants. (See

Campbell, Figure 31.7)

 

Classification of leaf arrangement on a stem. The arrangement may be:

·        Opposite -2 leaves at each node 180° apart.

·        Alternate -each node has one leaf and leaves at adjacent nodes point in opposite directions.

·        Whorled- a node has 3 or more leaves attached.

 

Leaves may also be classified as:

·        Simple leaf- one undivided blade.

·        Compound leaf- divided into several leaflets.

 

Classification by leaf shape. Leaves can be:

·        Lanceolate.

·        Oval.

·        Cordate (heart-shaped).

·        Triangular.

 

Classification by leaf margin, which can be:

·        Entire (smooth).

·        Undulate.

·        Serrate.

·        Lobed.

 

Some plants have leaves that have become adapted for functions other than photosynthesis. (See Campbell, Figure 31.8)

·        Tendrils are modified leaflets that cling to supports.

·        Spines of cacti function in protection.

·        Many succulents have leaves modified for storing water.

·        Some plants have brightly colored leaves that help attract pollinators to the flower.

 

III. The many types of plant cells are organized into three major tissue systems

 

A. Types of Plant Cells

 

Each type of plant cell has structural adaptations that make it possible to perform that cell's function. Some are coupled with specific characteristics of the protoplast. (See Campbell, Figure 31.10)

 

Protoplast = Contents of a plant cell exclusive of the cell wall-

 

I. Parenchyma Cells

 

Parenchyma cells are the least specialized plant cells-

·        Primary walls are thin and flexible.

·        Most lack secondary walls-

·        The protoplast usually has a large central vacuole.

·        Function in synthesizing and storing organic products.

=> Photosynthesis occurs in the chloroplasts of mesophyll cells-

·        Some in stems and roots have colorless plastids that store starch.

·        Most mature cells do not divide, but retain the ability to divide and differentiate into other cell types under special conditions ( e.g. repair and replacement after injury).

 

2. Collenchyma Cells

 

Collenchyma cells usually lack secondary walls-

·        The primary cell wall is thicker than in parenchyma cells but is of an uneven thickness.

·        They are usually grouped in strands or cylinders to support young parts of plants without restraining growth.

·        They are living cells which elongate as the stems and leaves they support grow.

 

3. Sclerenchyma Cells

 

Sclerenchyma cells function in support.

·        They have very rigid, thick secondary walls strengthened by lignin.

·        Many lack protoplasts at functional maturity, so they can not elongate and may be dead, functioning only as support.

·        There are two forms: fibers (long, slender, tapered cells occurring in bundles) and sclerids (shorter, irregularly-shaped cells).

 

4. Tracheids and Vessel Elements: Water-Conducting Cells

 

Xylem consists of two cell types, both with secondary walls and both dead at functional maturity.

·        Before the protoplast dies, secondary walls are deposited in spiral or ring patterns (which allows them to stretch) in parts of the plant that are still growing.

 

Tracheids are long, thing tapered cells having lignin-hardened secondary walls with pits (thinner regions where only primary walls are present). (See Campbell, Figure

31.11a)

·        Water flows from cell to cell through pits.

·        Also function in support.

 

Vessel elements are wider, shorter, thinner-walled, and less tapered than tracheids.

(See Campbell, Figure 31.11 b and c)

·        Vessel elements are aligned end to end.

·        The end walls are perforated, permitting the free flow of water through long chains of vessel elements called xylem vessels.

 

5. Sieve-Tube Members: Food-Conducting Cells

 

Sieve-tube members are chains of phloem cells that transport sucrose, other organic compounds, and some minerals.

·        The cells are alive at functional maturity.

·        Protoplasts lack a nucleus, ribosomes and a distinct vacuole.

 

In angiosperms, the end walls of sieve-tube members have pores and are called sieve plates.

·        The pores probably facilitate the movement of fluid between cells.

·        At least one companion cell is connected to each sieve-tube member by many plasmodesmata; the companion's nucleus and ribosomes may also serve the sieve-tube member which lacks these organelles.

=> Companion cells also help load sugar produced in the mesophyll into sieve- tubes of leaves of some plants.

 

NOTE: It would be useful to emphasize how close inspection of cell structure often reveals its function, and vice versa.

 

B. The Three Tissue Systems of a Plant

 

Plant cells are arranged into three tissue systems: dermal, vascular, and ,ground tissue systems.

 

Each of the three systems is continuous throughout the plant, although, specific characteristics and spatial relationships vary in different plant organs. (See Campbell,

Figure 31.12)

 

1. Dermal Tissue System

 

Dermal tissue system (or epidermis) = Single layer of tightly packed cells covering and protecting the young parts of the plant-

·        Functions in protection and has special characteristics consistent with the function of the organ it covers.

=> Root hairs specialized for water and mineral absorption are extensions of epidermal cells near root tips.

=> The waxy cuticle that helps the plant retain water is secreted by epidermal cells of leaves and most stems.

 

2. Vascular Tissue System

 

Vascular tissue system = The xylem and phloem that functions in transport and support; is continuous throughout the plant.

 

3. Ground Tissue System

 

Ground tissue system = Predominantly parenchyma, with some collenchyma and sclerenchyma, that fills the space between dermal and vascular tissue systems; functions in photosynthesis, storage and support.

 

IV. Meristems generate cells for new organs throughout the lifetime of a plant: an overview of plant growth

 

Plant growth begins with germination of the seed and continues for the lifespan of the plants.

 

Indeterminate growth = Continued growth as long as the plant lives.

·        In contrast, most animals cease growing after reaching a certain size (determinate growth).

·        Certain plant organs, such as flower parts, show determinate growth.

 

Plants do not live indefinitely, they do have finite life spans-

·        Most have genetically determined lifespans.

·        Some are environmentally determined-

·        Annuals complete their life cycles in one year or growing season.

·        Biennials typically have life spans of two years-

·        Perennials, such as trees and some grasses, live many years.

Indeterminate growth is made possible by meristems (perpetually embryonic tissues).

·        Meristematic cells are unspecialized and divide to generate new cells near the growing point.

·        New meristematic cells formed by division may remain in the region and produce new cells (initials) or be displaced and become incorporated and specialized into tissues (derivatives).

·        Apical meristems, located in root tips and shoot buds, supply cells for plants to grow in length.

=> Primary growth (elongation) is initiated by apical meristems and forms primary tissues organized into the 3 tissue systems.

·        Secondary growth (increased girth) is the thickening of roots and shoots which occurs in woody plants due to development of lateral meristems.

Lateral meristems = Cylinders of dividing cells extending along the lengths of roots and shoots.

=> Cell division in the lateral meristems produces secondary dermal tissues which are thicker and tougher than the epidermis it replaces.

=> Also adds new layers of vascular tissues.

 

V. Apical meristems extend roots and shoots: a closer look at primary growth

 

The primary plant body with its three tissue systems is produced by primary growth.

·        The youngest portions of woody plants and herbaceous plants are examples of primary plant bodies.

·        Apical meristems are responsible for the primary growth of roots and shoots.

 

A. Primary Growth of Roots

 

Root growth is concentrated near its tip and results in roots extending through the soil.

·        The root tip is covered by a root cap, which protects the meristem and secretes a polysaccharide coating that lubricates the soil ahead of the growing root.

·        The root tip contains 3 zones of cells in successive stages of primary growth. Although described separately, these zones blend into a continuum. (See Campbell, Figure 31.14)

 

1. Zone of Cell Division

·        Located near the tip of the root; includes the apical meristem and its derivatives, the primary meristems.

·        The apical meristem is centrally located; it produces the primary meristems and replacement cells of the root cap.

·        A quiescent center is located near the center of the apical meristem. It is composed of resistant, slowly dividing cells which may serve as reserve replacement cells in case of damage to the meristem.

• The primary meristems form as three concentric cylinders of cells (the protoderm, procambium, ground meristem) which will produce the tissue systems of the roots.

 

2. Zone of Cell Elongation

• In this region, cells elongate to at least 10 times their original length.
• The elongation of cells in this region pushes the root tip through the soil.
• Continued growth is sustained by the meristem's constant addition of new cells to the youngest end of the elongation zone.

 

3. Zone of Maturation

• Farthest from the root tip.
• Region where the new cells become specialized in structure and function and where the three tissue systems complete their differentiation.

 

The apical meristem produces three primary meristems, which in turn give rise to the three primary tissues of roots:

 

1. The protoderm is the outermost primary meristem.

• Gives rise to the epidermis over which water and minerals must cross.
• Root hairs are epidermal extensions which increase surface area, thus enhancing uptake.

 

2. The procambium forms a stele (central cylinder) where xylem and phloem develop.

• In dicots, xylem radiates from the stele's center in 2 or more spokes, with phloem in between the spokes.
• In monocots, a central pith ( core of parenchyma cells) is ringed by vascular tissue in an alternating pattern of xylem and phloem.

 

3. The ground meristem is located between the protoderm and procambium; it gives rise to the ground tissue system. The ground tissue:

• Is mostly parenchyma and fills the cortex (root area between the stele and epidermis).
• Stores food; the cell membranes are active in mineral uptake-
• Has endodermis, the single-cell thick, innermost layer of the cortex that forms the boundary between the cortex and the stele.

=> It selectively regulates passage of substances from soil to the vascular tissue of the stele.

 

Lateral roots may sprout from the outermost layer of the stele of a root.

• The pericycle, just inside the endoderm is, is a layer of cells that may become meristematic and divide to form the lateral root.
• A lateral root forms as a clump of cells in the pericycle, then elongates and pushes through the cortex until it emerges from the primary root.
• The lateral root maintains its vascular connection to the stele of the main root.

 

B. Primary Growth of Shoots

 

Shoot apical meristem is a dome-shaped mass of dividing cells at the tip of the terminal bud.

• Forms the primary meristems that differentiate into the 3 tissue systems.
• On the flanks of the apical meristem dome are leaf primordia which form leaves.
• Meristematic cells left by the apical meristem at the base of the leaf primordial develop into axillary buds.

 

Most shoot elongation actually occurs due to growth of slightly older internodes below the shoot apex.

• Growth is a result of both cell division and elongation within the internode.
• Intercalary meristems are present at the base of each internode in grasses and some other plants.

=> These tissues permit prolonged internode elongation along the length of the shoot.

 

Axillary buds may form branches later in the life of the plant-

• Branches originate at the surface of the shoot and are connected to the vascular system which lies near the surface-
• This is a direct contrast to development of lateral roots which form deep in the pericycle of the root.

 

1. Primary Tissues of Stems

 

The vascular tissue of the stem is organized into strands of vascular bundles that run the length of the stem.

• Converge at the transition zone (shoot à root) to join the root stele.
• Each bundle is surrounded by ground tissue, including pith and cortex.

 

In dicots, bundles are arranged in a ring with pith inside and cortex outside.

• Xylem faces the pith, phloem faces the cortex.
• Pith and cortex are connected by pith rays, thin layers of ground tissue.

 

In monocots, vascular bundles are scattered throughout the ground tissue of the stem.

• The stem ground tissue is mostly parenchyma.
• Ground tissue of stems is mostly parenchyma, strengthened in many plants by collenchyma located beneath the epidermis.

 

The protoderm of the terminal bud gives rise to the epidermal portion of the dermal tissue system.

 

2. Tissue Organization of Leaves

 

Leaves are cloaked by an epidermis of tightly interlocked cells-

• It protects against physical damage and pathogens.
• A waxy cuticle prevents water loss.

 

 

 

• Stomata are pores flanked by guard cells which regulate gas exchange with the surrounding air and photosynthetic cells inside the leaf.
• Stomata also allow transpiration (water loss from plant by evaporation).
• Stomata are usually more numerous on the bottom surface of the leaf. This location minimizes water loss.

 

The ground tissue of a leaf is mesophyll.

• Consists mainly of parenchyma cells equipped with chloroplasts for photosynthesis.
• Dicots usually have two distinct regions of mesophyll:

Palisade parenchyma = One or more layers of columnar cells of the upper half of a leaf.

Spongy mesophyll = Irregularly-shaped cells surrounded by air spaces through which oxygen and carbon dioxide circulate. Located in the lower half of the leaf.

 

The leaf vascular tissue is continuous with that of the stem through leaf traces which are branches from the stem vascular bundles.

• Leaf traces continue in the petiole as a vein, which branches repeatedly throughout the mesophyll of the leaf blade.
• This arrangement brings the photosynthetic tissue of the leaf into close contact with xylem and pholem.
• Functions also as a skeleton to support the shape of the leaf.

 

3. Modular Shoot Construction and Phase Changes During Development

 

Modular construction of a Shoot:

• Shoots are constructed of a series of modules produced by the serial development of nodes and internodes within the shoot apex.
• Each module consists of a stem, one or more leaves, and an axillary bud associated with each leaf.
• Elongation of the internode provides for primary growth of the plant. Modules found in the body of a plant represent various ages of tissue. The age of a module is proportional to its distance from the apical meristem.

Modules found in the body of a plant represent various ages of tissue. The age of a module is proportional to its distance from the apical meristem.

• The developmental phase of each module changes over time.

 

 

• There is a gradual change from a juvenile vegetative state to a mature vegetative state.

=> Leaf morphology usually changes during this transition.

• In some plants, the mature shoot apex may undergo a second phase change to a flower-producing (reproductive) state.

=> Primary growth of these shoot tips is terminated since the apical meristem is consumed in production of the flower.

 

VI. Lateral meristems add girth to stems and roots: a closer look at secondary growth

 

A secondary plant body results from secondary growth.

• The secondary plant body is comprised of the secondary tissues produced during growth in diameter.
• Secondary growth results from two lateral meristems: the vascular cambium and the cork cambium.

=> Vascular cambium produces secondary xylem and pholem.

=> Cork cambium produces a tough, thick covering for roots and stems that replaces the epidermis.

• Secondary growth occurs in all gymnosperms and most dicot angiosperms.

=> It is rare in monocots.

 

A. Secondary Growth of Stems

Vascular cambium forms when meristemic parenchyma cells develop between the primary xylem and primary phloem of each vascular bundle and in the rays of ground tissue between the bundles.

 

Fasicular cambium = Cambium within the vascular bundle.

 

Interfasicular cambium = Cambium in the rays between vascular bundles.

• Together, meristematic bands in the fasicular and interfasicular regions form a continuous cylinder of dividing cells around the xylem and pith of the stem.
• Ray initials (meristematic cells of the interfasicular cambium) produce radial files of parenchyma cells (= xylem and phloem rays) which permit lateral transport of water and nutrients as well as storage of starch and other reserves-
• Fusiform initials (cells of fasicular cambium) produce new vascular tissues; secondary xylem to the inside of the vascular cambium and secondary phloem to the outside.

 

Accumulated layers of secondary xylem produces wood which consists mostly of tracheids, vessel elements and fiber.

• The hardness and strength of wood results from these cells which, while dead at maturity, have thick, lignified walls.
• Forms annual growth rings due to yearly activity: cambium dormancy, spring wood production and summer wood production.

 

The secondary phloem does not accumulate extensively. The secondary phloem, and all tissues external to it, develop into bark which eventually sloughs off the tree trunk.

 

Cork cambium is a cylinder of meristematic tissue that forms protective layers of the secondary plant body.

• First forms in the outer cortex of the stem.
• Phelloderm (parenchyma cells) forms to the inside of cork cambium initials as they divide.
• Cork cells form to the outside of cork cambium initials as they divide.
• As cork cells mature, they deposit suberin (a waxy material) in their walls and die.
• These dead cork tissues protect the stem from damage and pathogens while reducing water loss.
• The combination of cork cambium, layers of cork, and phelloderm form the periderm (the protective coat of the secondary plant body that replaces primary pidermis).
• The term bark refers to all the tissues external to the vascular cambium (phloem, phelloderm, cork cambium and cork).

 

Cork cambium is a cylinder of fixed size and does not grow in diameter .

• As continued secondary growth splits this, it is replaced by new cork cambium formed deeper in the cortex.
• When no cortex is left, it develops from parenchyma cells in the secondary phloem (only the youngest secondary phloem, internal to cork cambium, functions in sugar transport).
• Lenticels are present in the bark.

 

Lenticel= Spongy region in the bark which permit gas exchange by living cells within the trunk.

 

B. Secondary Growth of Roots

Vascular cambium and cork cambium also function in secondary growth of roots.

• The vascular cambium produces secondary xylem to its inside and secondary phloem to its outside.

=> It is first located between xylem and phloem of the stele.

=> The cortex and epidermis split and are shed as the stele grows in diameter.

• Cork cambium forms from the pericycle of the stele and produces the periderm, which becomes secondary dermal tissue.

=> Periderm is impermeable to water, consequently, the roots with secondary growth function to anchor the plant and transport water and solutes between the younger roots and the shoot system.

 

Older roots become woody, and annual rings appear in the secondary xylem.

• Old roots and old stems may look very similar.