Biology 102
Fall 2001
R. Brundage

Lecture 6: Part 2

Plant Tissues




I.Overview of the Plant Body

A.Although no one species of the 260,000 species of plants can be considered
            typical, the focus here is on angiosperms.

B.Shoots and Roots

            1.Shoots consist of stems, leaves, and flowers (reproductive structures).

                         a.Water, minerals, and organic substances are transported.

                         b.Stems are frameworks for upright growth and to display flowers.

                         c.Parts of the system store food.

            2.Roots usually grow below ground.

                        a.A root system absorbs water and minerals from soil and conducts
                                    them upward.

                        b.Roots store food; they also anchor and support the plant.

C.Three Plant Tissue Systems

            1.The ground tissue system makes up the bulk of the plant body.

            2.The vascular tissue system contains two kinds of conducting tissues that
                        distribute water and solutes through the plant body.

            3.The dermal tissue system covers and protects the plant’s surfaces.

D.Meristems?Where Tissues Originate

            1.Meristems are localized regions of self-perpetuating, embryonic cells.

            2.There are two kinds of meristems:

                        a.Apical meristem at the tips of roots and stems is responsible for
                                    growth and elongation.

            1.Descendants of some of these cells will develop into the
                        specialized tissues of the elongating root and stem.

            2.Growth originating at root and shoot tips is labeled
                        primary growth.

            3.Lateral meristem tissues are responsible for the increase in diameter of
                        older roots and stems.

E, Vascular Meristems

            1.Vascular cambium and cork cambium are the two kinds
                        of lateral meristems.

            2.These are responsible for secondary growth which adds
                        to woody parts of trees for example.

II.Types of Plant Tissues

A.Simple Tissues

            1.Parenchyma makes up most of the soft, moist primary growth of plants.

                        a.Its thin-walled, pliable cells stay alive and retain the capacity to
                                    divide.

                        b.Various types participate in photosynthesis (mesophyll), storage,
                                    secretion, and other tasks.

            2.Collenchyma cells are thickened and help strengthen the plant (for
                        example, "strings" in celery).

                        a.It is commonly arranged at strands or cylinders beneath the
                                    dermal tissue of stems and stalks.

                        b.The primary cell walls of collenchyma become thickened with
                                    cellulose and pectin at maturity, often at their corners.

            3.Sclerenchyma cells provide mechanical support and protection in mature
                        plants.

                        a.The secondary walls are thick and often impregnated with lignin,
                                    which strengthens and waterproofs cell walls.

                        b.Sclerenchyma cells form fibers such as in hemp and flax; others
                                    called sclereids form strong coats around seeds as in a peach
                                    pit.

B.Complex Tissues

            1.Vascular tissues function in the distribution of substances throughout the
                        plant.

                        a.Xylem uses two kinds of cells (dead at maturity) to conduct
                                    water and minerals absorbed from the soil:

            2.Vessel members are shorter cells joined end to end to
                        form a vessel with perforation plates at the ends of each
                        member.

            3.Tracheids are long cells with tapered, overlapping
                        ends.

            4.Phloem transports sugars and other solutes throughout the plant body.

            5.Phloem contains living conducting cells called sieve
                        tube members which bear clusters of pores in the walls
                        through which the cytoplasm of adjacent cells is
                        connected.

            6.Companion cells, adjacent to the sieve tube members,
                        help to load sugars produced in leaves and unload them
                        in storage and growth regions.

C.A dermal tissue system called epidermis covers all primary plant parts.

                        a.A waxy cuticle covers the external surfaces of the plant to restrict water
                                    loss and resist microbial attack.

                        b.Stomata openings between pairs of guard cells permit water and gaseous
                                    exchange with the air.

                        c.The periderm replaces the epidermis when roots and stems increase in
                                    diameter and become woody.

III.Dicots and Monocots?Same Tissues, Different Features

            1.Dicots include common trees and shrubs (other than conifers).

            2.Monocots include grasses, lilies, irises, and palms.

            3.Monocot seeds have one cotyledon ("seed leaf"), and dicot seeds have two.

IV.Primary Structure of Shoots

A.How Stems and Leaves Form

             1.Leaves develop from leaf primordia along the apical meristems of stems.

                        a.A node is the point where a leaf or leaves attach to the stem.

                        b.An internode is the region on the stem between two nodes.

            2.Buds develop in the leaf axils (the upper angle where leaves attach to the
                        stem).

                        a.A bud is an undeveloped shoot of mostly meristematic tissue
                                    covered by modified leaves (bud scales).

                        b.Buds give rise to stems, leaves, flowers.

B.Internal Structure of Stems

            1.A vascular bundle is a multistranded cord of primary xylem and phloem
                         running lengthwise through the ground tissue of shoots.

            2.The arrangement of vascular bundles is genetically different in dicots and
                        monocots:

                        a.The stems of most dicots have vascular bundles arranged as a
                                    ring that divides the ground tissue into the outer cortex and inner
                                    pith.

                        b.In most monocots, the vascular bundles are scattered throughout
                                    the ground tissue.

V.A Closer Look at Leaves

A.Similarities and Differences Among Leaves

            1.Leaves are metabolic factories equipped with photosynthetic cells.

            2.Deciduous trees drop their leaves as winter approaches, evergreens retain
                        theirs.

            3.Leaves vary enormously in shape, size, texture, and surface features.

                        a.Monocot leaves tend to have a flat surface, like a knife blade, the
                                    base of which encircles and sheaths the stem.

                        b.Dicot leaves have a broad blade attached by a petiole to the stem;
                                    the blade may be lobed or composed of leaflets.

            4.Leaves represent a large surface area that is exposed to sunlight and
                        carbon dioxide.

B.The Fine Structure of Leaves

            1.Epidermis covers every leaf surface exposed to air.

                        a.A cuticle layer minimizes water loss.

                        b.Stomata are located mostly on the lower epidermis.

            2.Mesophyll, consisting of photosynthetic parenchyma cells, extends
                        throughout the interior of the leaf.

                        a.Air spaces, which connect to the stomata, participate in gaseous
                                    exchange.

                        b.Palisade mesophyll cells lie closer to the epidermis and are
                                    columnar in shape compared to the spongy mesophyll below
                                    them.

            3.The leaf’s veins are vascular bundles of xylem and phloem that form a
                        network for movement of water, solutes, and photosynthetic products.

VI.Primary Structure of Roots

A.Taproot and Fibrous Root Systems

            1.In most dicots, the primary root emerges from the seedling, increases in
                        diameter, and grows downward.

                        a.Lateral roots emerge sideways along its length.

                        b.The primary root plus lateral roots form the taproot system.

            2.In monocots, the taproot is replaced by adventitious roots that arise from
                        the stem.

                        a.These roots and their branchings form a fibrous root system.

                        b.Fibrous roots do not penetrate as deeply as taproots.

B.Internal Structure of Roots

             1.Cells in the apical meristem divide and then differentiate into root
                        epidermis, ground tissues, and vascular tissues behind the meristematic
                        region.

                        a.The root cap protects the apical meristem and pushes through the
                                    soil.

                        b.Cells are torn loose as the root grows.

              2.Protoderm gives rise to the root epidermis with its extensions called root
                        hairs for the outer absorptive interface with the environment.

              3.Vascular tissues form a vascular cylinder arranged as a central column.

                        a.The column is surrounded by root cortex (ground tissue), which
                                    has abundant air spaces within it.

                        b.The endodermis–the innermost layer of the cortex–surrounds the
                                    vascular cylinder and helps control water movement into it.

                        c.Just inside the endodermis is the pericycle–it is meristematic and
                                    can give rise to lateral roots.

C.Regarding the Sidewalk-Buckling, Record-Breaking Root Systems

            1.Most roots extend to a depth of 2 to 5 meters, but desert species can
                        extend to 50 meters.

            2.Roots can radiate outward for over 15 meters.

            3.Total surface area of a single rye plant can equal 600 square meters.

VII.Accumulated Secondary Growth?The Woody Plants

A.Woody and Nonwoody Plants Compared

            1.Seasonal growth cycles proceed from germination, to seed formation, to
                        death.

                        a.Annuals complete their life cycle in one season; they are
                                    nonwoody, or herbaceous plants such as corn.

                        b.Biennials, such as carrots, live two seasons: vegetative growth
                                    the first, flower and seed formation the second.

                        c.Perennials live many years and have secondary growth
                                    (examples: roses, grape vines, apple trees).

            2.Woody plants such as dicots and gymnosperms show secondary growth
                        by producing large amounts of secondary xylem.

B.Activity at the Vascular Cambium

            1.Vascular cambium is a cylinderlike lateral meristem.

                        a.It produces secondary xylem on its inner face and secondary
                                    phloem on its outer.

                        b.The secondary growth displaces the cells of the vascular
                                    cambium toward the stem surface.

            2.Secondary xylem and phloem form at the vascular cambium of the roots
                        also.

VIII.A Look at Wood and Bark

A.Formation of Bark

            1.In response to rupture of the outer cortex (by girth expansion), cork
                        cambium produces the periderm–a corky replacement of the epidermis.

                        a.Periderm and secondary phloem constitute bark.

                        b.Periderm consists of cork, secondary cortex, and the cork
                                    cambium.

                        c.Cork contain suberin; it protects, insulates, and waterproofs the
                                    stem or root surface.

            2.Lenticels are small channels through the corky surface of bark that allow
                        for exchange of gases.

B.Heartwood and Sapwood

            1.Heartwood lies at the center of older stems and roots.

                        a.It is a depository for resins, oils, gums, and tannins.

                        b.It makes the tree strong and able to defy gravity.

            2.Sapwood is secondary growth located between heartwood and the
                        vascular cambium.

                        a.It is wet, pale in color, and not as strong.

                        b.It is rich in the sugar-rich fluid of the phloem (for example,
                                    maple trees).

C.Early Wood, Late Wood, and Tree Rings

            1.In regions with cool winters or dry spells, the vascular cambium is
                        inactive during part of the year.

                        a.Early wood (start of growing season) contains xylem with large
                                    diameters and thin walls.

                        b.Late wood contains xylem with small diameters and thick walls.

                        c.Growth rings appear as alternating light bands of early wood and
                                    dark bands of late wood.

            2.Hardwood (such as oak) has vessels, tracheids, and fibers in its xylem.

            3.Softwood (such as conifers) have no vessels or fibers.

D.Limits to Secondary Growth

            1.Some trees live in habitats too harsh and remote for most invaders.

            2.Most trees use a strategy of compartmentalization to wall off invaders,
            building a fortress of thickened cell walls around wounds, or deploying
                        toxic compounds.