Plant Cells and Tissues

Plant Cells and Tissues

• Histology studies the microscopic structure of tissues.
• Meristems are embryonic tissue regions that actively produce new cells indefinitely.
• Meristems have thin cell walls, undifferentiated protoplastids, dispersed small vacuoles, and a small amount of endoplasmic reticulum.
• Meristems can be distinguished by the presence of large nuclei.
• Apical meristem produces cells at the tips of main and lateral shoots and roots (vertically).
• Lateral meristem adds cells inward or outward of the plant body (horizontally).

Cells based on morphology

• Parenchyma cells have thin, uniformly thick primary cell walls and are alive.
• Parenchyma cells are most common in ground tissue.
• Aerenchyma is a subtype of parenchyma and consists of irregularly shaped cells bound by large airspaces, such as those in a canna shoot.
• Chlorenchyma is a subtype of parenchyma that possesses chloroplasts and functions in photosynthesis, such as in a coleus shoot.
• Secretory parenchyma is a subtype of parenchyma that lines canals and possesses large vesicles containing various substances, like in a pine old stem.
• Storage parenchyma is a subtype of parenchyma that has leucoplasts filled with starch (amyloplasts), proteins (proteinoplast), or oil (elaioplasts), such as those from a potato.
• Collenchyma cells have thick, uneven primary cell walls, are alive, and flexible.
• Collenchyma cells provide flexible structural support in younger regions and have thickened cell walls.
• Angular collenchyma is a subtype found in dumbcane (dieffenbachia sp.).
• Sclerenchyma cells/tissues have a thick secondary cell wall and are dead.
• Sclerenchyma cells are present in ground and vascular tissue.
• Sclerenchyma cells provide structural support and can function in water transport.
• Fibers are a subtype of sclerenchyma cells that are elongated and flexible, such as cotton stained with 18% alcoholic phloroglucinol-sulfuric acid.
• Sclereids are a subtype of sclerenchyma cells that are short and rigid, such as a coconut shell stained with 18% alcoholic phloroglucinol-sulfuric acid.
• Tracheids are tracheary elements that are long, narrow, and tapered at the ends.
• Vessels are tracheary elements that are short, wide, and open at the ends.

Meristems

• Found at the tip of the shoots (shoot apical meristems) and roots (root apical meristem).
• Lateral meristems are found among the mature plant tissues.

Structures and Zones of Root Apical Meristem

• The zone of division contains the root apical meristem (excluding the root cap) and the region between the root apical meristem and procambium.
• Tissues converge toward the center in the zone of division, showing densely stained nuclei and thin cytoplasm due to constant cell division.
• Above the root apical meristem is the zone of elongation, which includes the protoderm, ground meristem, and procambium.
• The protoderm, the outermost layer of cells, becomes the dermal tissue system.
• The ground meristem consists of larger cells and becomes the ground tissue system.
• The procambium is inner to the ground meristem, with thin, elongated cells having large nuclei, and it becomes the vascular tissue system.
• The root cap protects the root apical meristem and aids in soil penetration.

Shoot Apical Meristem

• Structures include protoderm, procambium, and ground meristem.
• These extend toward developing stem and leaves.
• The leaf primordium gives rise to leaves.
• Bud primordium/Axillary bud develops into new shoots.

Dermal Tissue System

• Can be uniseriate (single layered) or multiseriate (multiple layers).
• Uniseriate dermal tissue:
  • Found in corn root.
  • Epidermis has root hairs.
  • Exodermis is a layer below the epidermis.
• Multiseriate dermal tissue:
  • Found in orchid root.
  • Epidermis is velamen (multiple layers).
  • Functions of velamen include water absorption and storage, prevention of water loss, gas exchange, and absorption of nutrients and light protection.
  • Exodermis borders between the velamen and cortex.
  • Originate from the protoderm of apical meristems.

Epidermal Derivatives

• Root hairs (Zea mays, root) increase surface area for water absorption, projecting from epidermal cells.
• Types of trichomes include:
  • Simple trichomes (Theobroma cacao).
  • Branching trichome (Vervascum thapsus).
  • Glandular trichome (Muntingia calabura).
  • Scale trichome (Elaeagnus triflora).
  • Stellate trichome (Urena lobata).
  • Stinging trichome (Dendrocnide meyeniana).
• Stomata are found on the boat of moses (adaxial).

Components of Stomatal Complexes

• Pavement cells are regular cells with prominent pigmented vacuoles.
• Stomatal pore is the center of the stomatal complex, opening for air.
• Guard cells surround the stomatal pore and are bean-shaped.
• Subsidiary cells surround guard cells and are also known as accessory cells, such as those in the boat of moses (Tradescantia sp).

Ground Tissue System

• Originates from the ground meristem and makes up the bulk of plant organs.
• In root piths, ground tissue originates from the procambium.
• Typically found in the pith and cortex; in leaves, it consists of mesophyll (ex. Zea mays).
• Cortex contains multiple layers and is bounded by the exodermis and endodermis.
• Exodermis is immediately beneath the epidermis but is not present in some roots.
• Endodermis separates the cortex from vascular tissues and the pith, having prominent cell wall thickening.
• Pith is the innermost region and is not present in some roots.

Vascular Tissue System

• Phloem in Monocot & Eudicot Angiosperms: distinguishable by a pattern of parenchyma cells.
• Sieve tube elements are wider and enucleated.
• Companion cells show nuclei and denser cytoplasm.
• Some specimens showcase phloem fibers and phloem parenchyma.
• Phloem in Gymnosperms cannot be distinguished by size.
  • Sieve cells
  • Albuminous cells
• Xylem is distinguishable as a bundle of sclerenchymatous, water-conducting cells (tracheary elements).
• Tracheary elements include:
  • Tracheids which are long, narrow, and tapered
  • Vessels which are short, wide, and open
• Xylem parenchyma originates from the procambium and is composed of more than one cell type.
• Gymnosperms contain tracheids only, and vessels are absent.

Patterns of Cell Wall Thickening

• Annular
• Helical
• Scalariform
• Reticulate
• Simple pitted
• Bordered pitted

The Root

• The root system is the underground part of the plant.
• The two basic functions are the absorption of water and minerals (nutrients) and anchorage.
• Some roots are structurally modified to perform specialized functions.

Root Systems

• Taproot system has one main axis (taproot) and branching lateral roots (Eudicots)
  • Root hair consist of fuzzy projections from the taproot.
• Fibrous root systems consist of adventitious roots that may directly arise from the stem and may also have lateral roots (Monocots)
  • Taproot is lost early in its development.

Modified Roots

• Buttress roots are adventitious and enlarged for structural support.
• Climbing roots are adventitious, arise from nodes/internodes, and provide clinging and structural support.
• Pneumatophores are lateral roots that grow upward from the ground for gas exchange.
• Prop roots are adventitious and grow downward from branches for structural support.
• Root nodules are lateral roots with swollen nodules fixed on the root, housing nitrogen-fixing bacteria.
• Stilt roots
• Strangling roots are an entire root system where aerial roots grow downward from branches for structural support
• Storage roots are tap roots that are enlarged/swollen for storage.

Primary Root Tissues

• Epidermis is the outermost layer of parenchyma and the primary root and can have root hairs.
• Ground tissue consists of:
  • Cortex, which is below the epidermis and sometimes beneath the exodermis.
  • Endodermis, which has thicker cell walls and separates the cortex from the pith and vascular tissues.
• Stele (vascular tissues):
  • Pericycle, the outermost layer of the stele from which lateral roots arise.
  • Primary xylem & primary pholem are arranged in a radially alternating manner
    • Protoxylem is smaller, consists of tracheids, and produces secondary cell walls.
    • Metaxylem is larger, made of either tracheids or vessels and grow larger before depositing secondary cell walls.
• Exarch (roots) - protoxylem is found outer to the metaxylem.
• Endarch (stems) - metaxylem is found outer to the protoxylem
• Protoxylem poles indicate the number of bundles of xylem.
  • Eudicot: diarch, triarch, tetrarch
  • Monocot: polyarch
• Pith is the ground tissue that makes up the core of the root.

Secondary Tissues of the Root

• Secondary growth results in the increase in circumference and cross-sectional area of the plant organ and lateral growth.
• Growth is brought about by the lateral meristem
  • Vascular cambium originates from procambium and pericycle and gives rise tosecondary xylem.
  • The cambium separates secondary xylem and phloem.
  • Secondary phloem.
• Cortex
• Cork cambium originates from the epidermis, cortex, endodermis, or pericycle and is outer to the cortex, with
  • Periderm
    • Phellogen (cork cambium)
    • Phelloderm (cork parenchyma)
    • Phellem (cork)

The Stem

• The stem is a plant organ used to support appendages and to translocate water and nutrients.
• Some stems are structurally modified to perform specialized functions.

Stem Morphology

• The 4 stem regions are:
  • Node: the point where a leaf is or was attached.
  • Internode: the region between two consecutive nodes.
  • Terminal region: the shoot's tip.
  • Leaf axil: the region between the stem and leaf.
• The leaf is an expanded photosynthetic organ.
  • Eudicot: attached to a node via petiole (stalk-like).
  • Monocot: attached via leaf sheath (surrounds entire circumference of the stem).
• Buds are underdeveloped shoots found at the terminal and axil regions.
• Leaf scars mark the position of a previously attached leaf.
• Bundle scars mark the position of vascular tissue.
• Lenticels are pores where gas exchange occurs and appear as white or brown circular protrusions or cracks across the stem.

Modified Stems

• Bulbs are flattened stems for roots.
• Cladode are green, fleshy stems for photosynthesis and take on the role of leaves.
• Corms are underground food storage.
• Rhizomes are underground, swollen, and horizontally growing for storage and lateral spreading.
• Tendrils provide structural support.
• Thorns are for defense.
• Tubers are swollen stems for storage.

Stem Anatomy

• Monocots have primary tissues of stems:
  • Epidermis is a single, outermost layer of the stem.
  • Ground tissue makes up the bulk of the stem.
  • Vascular bundles are scattered.
    • Bundle sheath cells are made up of parenchyma and sclerenchyma cells and covers the vascular bundle.
    • Phloem is the outer half with larger sieve tube elements and smaller companion cells.
    • Phloem fibers (older stems) are between the phloem and bundle sheath.
    • Xylem is the inner half with larger metaxylem located at the sides and smaller protoxylem centrally located.
    • Lacuna is a collapsed protoxylem and xylem parenchyma.
• Eudicots structure is similar to monocot stems.
  • Ground tissue can be further distinguished into the cortex and the pith.
  • Vascular bundles are fewer and arranged in a ring-like manner with no bundle sheath.
  • Vascular cambium is between the phloem and xylem.
  • Protoxylem and metaxylem are not definite, with no lacunae.

Secondary Tissues of the Stem

• Monocots do not undergo secondary growth.
• Eudicots and pine stems experience secondary growth in dermal and vascular tissues via the cork and vascular cambium.
• Eudicots contain:
  • Pith, the innermost layer of parenchyma.
  • Secondary xylem, the size of which varies due to presence of tracheids and vessels.
  • Vascular cambium, which is 2-3 cell layers thick.
  • Secondary phloem.
  • Cortex.
  • Xylem rays, which traverse radially and connect the pith with the cortex.
  • Periderm, which is thicker in old stems compared to old roots and has three sublayers:
    • Phelloderm (cork parenchyma) is compact and outer to the cortex (innermost).
    • Phellogen (cork cambium) is outer to the phelloderm and similar to vascular cambium.
    • Phellem (cork) is outermost, with suberized cells that may contain lenticels.
    • Lenticels are tissue breakages that function in gas exchange.
• Pine stems structure is similar to eudicot.
  • Xylem rays are thinner.
  • Resin ducts are present.

The Leaf

• Leaves function for photosynthesis and are a major site of transpiration.
• Some leaves are structurally modified to perform specialized functions.
• Blade (lamina) is flattened.
• Midrib (central) supports the blade.

Phyllotaxy

• Phyllotaxy is the arrangement of leaves along the stem, which is the number of leaves attached to a single node
  • Alternate arrangements have one leaf
  • Opposite arrangements have two leaves
  • Whorled arrangements have more than two leaves
• Leaf attachment can be petiolate or sessile
  • Petiolate leaves have a blade attached by a petiole.
  • Sessile leaves have a blade attached to the node directly.
• Complexity can be simple or compound
  • Simple leaves have a single, undivided lamina attached to a single node
  • Compound leaves a separated into discrete leaflets attached to a petiolule
    • Trifoliate leaves have three leaflets
    • Pinnately compound leaves are attached to a central axis (rachis - continuous to the petiole; petiolules are attached to the rachis)
      • Second order pinnately compound leaves - leaflets/their petiolules are attached to another axis (rachilla) several rachillae are attached to the rachis
    • Palmately compound leaves - leaflets/their petiolules are attached to a central point at the end of a petiole
• Stipules are nodal outgrowths which may resemble buds, leaves or thorns
  • occur perpendicular to the petiole's axis or across the petiole itself
  • present in every node
• Venation is the arrangement of vascular tissue
  • in both types, the midrib runs along the center parallel to the leaf axis
    • reticulate is typical in eudicots
      • secondary veins branch off the midrib; split into lower order veins and last order veins; end blindly into the ground tissue
    • parallel is typical in monocots
      • lower order veins are indistinguishable from one another and run parallel to the midrib

Outline

• General shape of the lamina
  • Linear - almost like a straight line (?)
  • Elliptic - perfectly oval
  • Lanceolate - thin triangle
  • Oblanceolate - upside down thin triangle
  • Ovate - thick triangle
  • Obovate - upside down thick triangle
• Margin is the edges of the lamina
  • Entire - smoothly shaped
  • Undulate - wavy; curving in and out of the leaf plane
  • Crenate - rounded incisions
  • Serrate - sharp incisions; parallel to midrib
  • Dentate - sharp incisions perpendicular to the midrib

Base

• Point of the petiole's or sheath's attachment
  • Cuneate is <45° from the midrib
  • Attenuate - 45°-90° from midrib
  • Peltate - attached to the bottom middle of the blade as opposed to the edge
  • Cordate - heart shaped (?)
  • Oblique - uneven blade edge where petiole is attached
  • Auriculate - irregularly shaped edge where petiole is attached; frog ass
  • Rounded - purely circular; no angle formed
  • Hastate - diamonds
• Apex is lamina's tip
  • Acuminate - tapers into a sharp point
  • Rounded - oval like
  • Truncate - rounded rectangle
  • Acute - triangular
  • Emarginate - Heart shaped

Modified Leaves

• Bracts are brightly colored leaves used to attract pollinators and are typically associated with plants that have smaller flowers
• Carnivorous leaves are used for consuming and supplying additional nutrients to the plant
• Meristematic leaves are leaf on leaf which aids in reproduction
• Phyllodes are leaf extensions
• Pseudostem is really just a bunch of leaves pretending to be the stem while the actual stem is hidden underground (banana)
• Spines are modified for protection where the stem takes on the role of photosynthesis (cactus)
• Storage are fleshy leaves that contain nutrients, food or water (aloe vera, onion bulb)

Leaf Anatomy

• Monocot (specimen: corn)
  • Midrib is thicker than the blade
    • Blade
      • Adaxial - upper surface
      • Abaxial - lower surface
      • Stoma (epidermis)
      • Epidermis - outermost layer
        • Pavement cells - typical epidermal cells
        • Cuticle - thick, non-cellular layer composed of cutin that provides external protection from the sun
        • Stomata are perpendicularly oriented to the epidermis and aid in gas exchange and transpiration (typically more abundant in the abaxial side)
          • Airspace is present beneath stomata
        • Bulliform cells facilitate leaf rolling to prevent desiccation and occur across the blade
          • These are in groups and four times larger than regular cells
• Mesophyll is the ground tissue
  • Chlorenchyma cells are inner to the epidermis of the blade
    • Bundle sheath covering vascular bundles or veins which are across the blade and midrib
• Eudicot anatomy is similar to monocots
  • (specimen: santan (?))
    • Midrib - rounder in eudicot and has a prominent midvein
      • The midvein is the main vascular center of the leaf and connects to the vascular tissue of the stem
        • bundle sheath covering also
    • Bundle sheath are covering vascular bundles
      • phloem
      • xylem
• No bulliform cells
• Mesophyll
  • Palisade mesophyll - uni- or multiseriate tissue comprised of elongated cells perpendicular to the plane of the leaf
    • This is tightly packed and elongated
  • Spongy mesophyll is loosely arranged round or irregularly shaped cells with prominent airspaces in between
    • irregularly shaped, loosely arranged

Gymnosperms (Pine needle)

• Cannot be demarcated into a midrib and blade (consistent, needle-like shape)
• Epidermis is the outermost layer consisting of:
  • Sunken stomates which are not on the same level as pavement cells which reduce water loss
  • Pavement cells are regular epidermal cells
  • Thick cuticle covers the epidermis which reduces water loss
• Hypodermis is directly beneath the epidermis and provides support from environmental stresses
• Mesophyll
  • Lobed mesophyll contains resin ducts
  • No vascular bundles
• Endodermis surrounds transfusion tissue which also surrounds xylem and phloem

Water Dynamic in Plants

• Transpiration is the exit of water in the form of vapor
  • May occur as stomatal, cuticular, or lenticular depending on the exit structure involved
  • Factors that affect transpiration rate: light intensity, humidity, wind velocity, and temperature
• Stomata are the primary site of transpiration and gas exchange in leaves
• Regulation of stomatal pore is vital in balancing water conservation with gas exchange
  • Depends on the balance in turgor pressure; light intensity
• Some plants are able to exhibit rapid movements in response to stimuli, different from tropism
  • Pulvinus exhibit rapid changes in turgor pressure and facilitate movement

Regulation of Transpiration

• Potometer is an enclosed water system (burette, pipette, and plant)
  • Used to measure transpiration rate
  • The water level should decrease within the burette as water moves towards the shoot due to the existing potential gradient between the plant and air space
  • Transpiration rate - change in water level over time (mL/hr)
• Set-ups
  • Normal light, no wind - control
  • Dim light, no wind - relatively smaller transpiration rate
  • Normal light, windy - fastest transpiration rate
  • plugged stomates - petroleum jelly on surface of leaves

Regulation of Stomatal Pores

• Opening and closing of stoma is determined by the movement of water between the guard cells and the accessory cells
  • Turgid guard cells - open stoma
  • Flaccid guard cells - closed stoma
  • Solute concentration -
    • Light - triggers opening of stomates