Chapter 38 The Plant Body

Introduction

· · The oldest known plant is a bristlecone pine. It is more than 4,900 years old.

· · Plants can cope very successfully with their environments.

· · Like animals, plants must acquire essential compounds to live, including water, carbon dioxide, and minerals.

· · Unlike most animals, most plants cannot move. By growing, plants accomplish some of the same things that animals achieve through motion.

· · Plants can respond to their environment via chemically mediated signals.

Vegetative Organs of the Flowering Plant Body

· · Flowering plants possess three kinds of vegetative (nonreproductive) organs: roots, stems, and leaves.

· · Most flowering plants belong to one of two major lineages.

· · Monocots are generally narrow-leaved flowering plants such as grasses.

· Eudicots are broad-leaved flowering plants such as roses.

· · Monocots and eudicots account for 97% of the species of flowering plants.

· · Most of the remaining species (including water lilies and magnolias) are structurally similar to eudicots, but they are not included with the eudicots because the resulting lineage would not be monophyletic. ·

· · The shoot system of a plant consists of the stems and the leaves, as well as flowers.

· · Leaves are the main sites of photosynthesis in plants.

· · Stems hold and display the leaves to the sun and provide connections for the transport of materials between roots and leaves.

· · A node is the point where a leaf attaches to a stem.

· · Regions of stem between nodes are known as internodes.

· · The root system provides support and nutrition.

Roots anchor the plant and take up water and minerals

· · There are two main types of root system: taproot and fibrous root.

· · Many eudicots have a taproot system: a single, large, deep-growing primary root with smaller lateral roots.

· · Monocots and some eudicots have a fibrous root system composed of numerous thin roots roughly equal in diameter.

· · A fibrous root system holds soil in place very effectively.

· · Some plants have adventitious roots. These roots arise from points along the stem where roots would not usually occur.

Stems bear buds, leaves, and flowers

· · A bud is an embryonic shoot.

· · A stem bears leaves at its nodes, and where each leaf meets the stem, there is a lateral bud.

· · At the tip of each stem or branch, there is an apical bud, which produces the cells for the growth and development of that stem or branch.

· · Some stems are highly modified.

· · A potato is a portion of the plant's stem, and its "eyes" contain lateral buds.

· · The runners of strawberries are horizontal stems.

Leaves are the primary sites of photosynthesis

· · Leaves are well adapted for gathering light.

· · The blade of a leaf is a thin, flat structure attached to the stem by a stalk called a petiole.

· · The petiole holds the leaf at an angle almost perpendicular to the sun.

· · Leaves at different sites on a single plant can be shaped differently.

· · Most species of plant bear leaves of a particular broadly defined type.

· · A simple leaf has a single blade.

· · A compound leaf has multiple blades (or leaflets) arranged along an axis or radiating from a central point.

· · Some plant species have highly modified leaves, such as the thorns of a cactus.

Plant Cells

· · Plant cells have all the organelles common to eukaryotes.

· · Some plant cells have additional distinguishing features including chloroplasts (or other plastids) and vacuoles.

· · Every plant cell is surrounded by a cellulose-containing cell wall.

Cell walls may be complex in structure

· · Cell wall formation is the final step when plant cells divide.

· · The daughter cells secrete a glue that constitutes the middle lamella, which forms a layer between them.

· · Next, each daughter cell secretes cellulose and other polysaccharides to form a primary wall.

· · Finally, once cell expansion stops, some plant cells may deposit more polysaccharides, sometimes impregnated with material such as lignin (in wood) or suberin (a complex lipid in cork).

· · Additional polysaccharides added after cell expansion are referred to as the secondary wall.

· · Plasmodesmata are pore-like structures that pass through primary cell walls, allowing substances to move freely from cell to cell without crossing the plasma membrane.

· · A plasmodesma is wide enough that portions of the endoplasmic reticulum extend between cells.

· · Pits allow water and nutrients to pass between cells that have a secondary wall.

Parenchyma cells are alive when they perform their functions

· · Parenchyma cells are the most numerous type of cell in young plants.

· · Parenchyma cells usually have thin walls and large central vacuoles.

· · The photosynthetic cells in leaves are parenchyma cells filled with chloroplasts.

· · Some parenchyma cells store lipids or starch.

· · Other parenchyma cells serve as "packing material" and play a vital role in supporting the stem.

Collenchyma cells provide flexible support while alive

· · Collenchyma cells are supporting cells that lay down primary cell walls that are thick in the corners.

· · Collenchyma provides support to leaf petioles, nonwoody stems, and growing organs.

Sclerenchyma cells provide rigid support after they die

· · Sclerenchyma cells are the main supporting cells of a plant.

· · There are two types of sclerenchyma cells: elongated fibers and variously shaped sclereids.

· · Fibers often organize into bundles

· · Sclereids may pack together very densely.

Xylem transports water from roots to stems and leaves

· · The xylem conducts water from roots to aboveground plant parts.

· · Conducting cells called tracheary elements are the part of xylem that transports water and minerals.

· · Tracheids are evolutionarily more ancient tracheary elements found in gymnosperms.

· · Both tracheary elements and tracheids undergo programmed cell death and do their jobs as empty cells (only the cell walls remain).

· · Vessel elements are the water "pipeline" system in flowering plants.

· · The cells that form vessels are dead and empty like tracheary elements.

· · Vessel elements are generally larger in diameter than tracheids and are laid down end-to-end to form hollow tubes.

· · During the course of angiosperm evolution, vessel elements have become shorter, and their end walls have become less obliquely oriented and obstructed.

Phloem translocates carbohydrates and other nutrients

· · Cells of the phloem are alive when they do their job, unlike those of the xylem.

· · The characteristic cell of the phloem is the sieve tube member.

· · Like vessel elements, the cells of the phloem are arranged end-to-end and form long sieve tubes, which transport carbohydrates and other materials.

· · The plasmodesmata in sieve tube members enlarge as they mature, resulting in end walls that look like sieves.

· · At functional maturity, a sieve tube is filled with sieve tube sap (water, sugars, and other solutes).

· · The sieve tube members have adjacent companion cells.

· · Companion cells retain all their organelles and may, through activities of their nuclei, regulate the performance of the sieve tube members.

Plant Tissues and Tissue Systems

· · A tissue is an organization of cells that work together as a functional unit.

· · Parenchyma cells make up parenchyma tissue, which is a simple tissue (made of one cell type).

· · Xylem and phloem are complex tissues; they are composed of a number of different cell types.

· · Tissues are grouped into tissue systems that extend throughout the body of the plant from organ to organ.

· · There are three plant tissue systems: vascular, dermal, and ground.

· · The vascular tissue system includes the xylem and phloem; it is the conductive or "plumbing" system of the plant.

· · The phloem transports carbohydrates from sites of production (sources such as leaves) to sites of utilization (sinks) elsewhere in the plant.

· · The xylem distributes water and mineral ions taken up by the roots to the stem and leaves.

· · The dermal tissue system is the outer covering of the plant.

· · All parts of the young plant body are covered by an epidermis, which is a single or multiple layer of cells.

· · The epidermis contains epidermal cells and other specialized cells such as guard cells.

· · The shoot epidermis secretes a layer of wax-covered cutin, the cuticle, which helps retard water loss from stems and leaves.

· · The ground tissue system makes up the rest of a plant and consists primarily of parenchyma tissue.

· · Ground tissue functions primarily in storage, support, photosynthesis, and the production of defensive and attractive substances.

Forming the Plant Body

· · The plant establishes its basic body plan in early embryonic stages

· · Two patterns contribute to the plant body plan.

· · The apical–basal pattern is the arrangement of cell and tissues along the main axis from root to shoot.

· · The radial pattern is the concentric arrangement of tissue systems.

Plants and animals develop differently

· · The growing stem of a plant consists of modules or units, laid down one after another.

· · Each unit consists of a node with its attached leaf or leaves, the internode below that node, and the later bud or buds at the base of that internode.

· · New units are formed as long as the stem continues to grow.

· · Each branch of a plant may be thought of as a unit that is in some ways independent of the other branches.

· · Leaves are units of another sort, produced in fresh batches to take over the daily function of gathering energy for the plant.

· · Root systems are also branching structures.

· · All parts of the animal body grow as an individual develops from embryonic stages but cease to grow once adulthood is reached (determinate growth).

· · In plants, the growth of roots and stems is indeterminate and is generated from specific regions of active cell division.

· · The localized regions of cell division in plants are called meristems.

· · The cells of meristematic tissues are analogous to the stems cells found in animals.

A hierarchy of meristems generates a plant's body

· · There are two types of meristems.

· · Apical meristems give rise to the primary plant body, which is the entire body of many plants.

· · Lateral meristems give rise to the secondary plant body.

· · The stems and roots of some plants form wood and become thick; it is the lateral meristems that give rise to the tissues responsible for this thickening.

· · Apical meristems:

· · Apical meristems are located at the tips of roots and stems and in buds.

· · Shoot apical meristems supply the cells that extend stems and branches.

· · Root apical meristems supply the cells that extend roots.

· · Shoot and root apical meristems give rise to three types of cylindrical primary meristems that produce the primary tissues of the plant body: protoderm, ground meristem, and procambium.

· · Apical meristems are responsible for primary growth, which leads to elongation and organ formation.

· · Lateral meristems:

· · Some roots and stems develop a secondary body (commonly referred to as wood and bark).

· · Secondary body tissues are derived from two lateral meristems: vascular cambium and cork cambium.

· · Vascular cambium is a cylindrical tissue consisting of vertically elongated cells that divide frequently.

· · The cork cambium produces protective cells that protect the outermost layers of the stem from water loss and microorganisms.

· · The layer of growth of the cork cambium is called the periderm.

· · Growth in the diameter of the stems and roots, produced by vascular and cork cambia, is called secondary growth.

· · Wood is secondary xylem.

· · Bark is everything external to the vascular cambium.

The root apical meristem gives rise to the root cap and the primary meristems

· · The root apical meristem produces all the cells that contribute to growth in the length of the root.

· · The root cap protects the delicate growing area of the root as it pushes through the soil.

· · The root cap also detects the pull of gravity and controls the downward growth of roots.

· · Tissues of the root are divided into three zones: cell division, cell elongation, and cell differentiation.

The products of the root's primary meristems become root tissues

· · Figure 34.16 depicts the tissue products of the three primary meristems of the root.

· · Root hairs are long, flattened epidermal cells that increase the root’s surface area.

· · The cortex is directly internal to the root epidermis and often functions in food storage.

· · Endodermis is the next root structure moving in from the cortex. It contains suberin, which makes the cells waterproof.

· · The stele is the vascular component of the root, which includes xylem, phloem, and pericycle tissues.

· · The pericycle consists of one or more layers of undifferentiated cells.

· · In monocots, a region of parenchyma cells called the pith lies in the center of the root.

The products of the stem's primary meristems become stem tissues

· · The shoot apical meristem forms three primary meristems, which in turn give rise to three tissue systems.

· · Leaves form from lateral buds.

· · Vascular tissue in the stem is arranged in vascular bundles.

· · The eudicot stem also contains pith and cortex storage tissues.

Many stems and roots undergo secondary growth

· · Secondary growth increases the diameter of stems and roots.

· · Secondary growth results from the activity of vascular and cork cambia.

· · Figure 34.19 shows how vascular cambium thickens stems and roots.

· · Vascular rays connect storage parenchyma to the sieve tubes of the phloem.

· · Only eudicots have a vascular cambium and a cork cambium and thus undergo secondary growth.

· · Wood:

· · Cross sections of most tree trunks in temperate zone forests have annual rings.

· · Annual rings form due to differential rates of growth in spring (when water is plentiful) and summer.

· · Wood that is no longer conducting water is known as heartwood.

· · Sapwood is wood that is actively conducting water and minerals in the tree.

· · Bark:

· · As secondary growth continues, expanding vascular tissue stretches and breaks the epidermis and cortex, which ultimately break away and are lost.

· · Cork is produced before the epidermis and cortex break and provides a protective function to the underlying tissue.

· · Phelloderm is cork produced on the inside and outside of the stem or root.

· · Lenticels allow gas exchange when bark forms on a stem or root.

Leaf Anatomy Supports Photosynthesis

· · Leaf anatomy is adapted to carry out photosynthesis, limit evaporative water loss, and transport the products of photosynthesis to the rest of the plant

· · The two zones in leaf parenchyma that photosynthesize are called mesophyll.

· · Veins supply mesophyll cells with water and minerals, and they transport the products of photosynthesis to the rest of the plant.

· · The epidermis of the leaf is the outermost cell layer, which is covered by a waxy cuticle. The epidermis functions to keep water and photosynthetic products in the leaf.

· · Guard cells allow controlled gas exchange through pores in the leaf called stomata.

· · Leaves receive water and mineral nutrients from the roots by way of the stem and veins. In return, the leaves export products of photosynthesis (providing energy) to the rest of the plant.