Bio 127 Lec Lesson 4.5 (Specialized Plant Tissues)

External and Internal Secretory Structures

Secretory Structures

Specialized plant tissues or cells that synthesize and release substances, often as part of defense, attraction, or storage.
Types:

1. External (e.g., glandular trichomes)
2. Internal (e.g., laticifers, oil cells, idioblasts)

External Secretory Structures

Structures located on the plant surface that release substances such as oils, resins, mucilage, or salts.
Key Types:
Glandular trichomes/hairs, Salt glands, and Hydathodes

Internal Secretory Structures

Structures located within a plant’s tissues (like the xylem, phloem, parenchyma, and cytoplasm) that produce and store various substances.
Often plays roles in defense or attraction.
Key Types:
Idioblasts, oil cells, litocysts/cystoliths, oil glands, resin ducts, and lactificers

Glandular Trichomes (Glandular Hairs)

Multicellular or unicellular outgrowths from the epidermis with secretory functions.
Functions:Secretion of essential oils, terpenoids, mucilage, digestive enzymes
Provide protection against herbivores and pathogens
Aid in pollinator attraction

Structure of Glandular Trichomes

Consist of:
Basal cells (anchor to epidermis)
Stalk cells (support structure)
Head cells (secretory cells)
Substances are stored in subcuticular spaces and released through cuticle rupture or pores.

Terpenoids

An ecologically prevalent lipophilic compound synthesized and secreted by trichomes that includes essential oils and resins. Ecological Roles:
Deterring herbivores
Attracting pollinators
Assisting in fruit dispersal

Capitate Glandular Trichomes

Trichomes with a short stalk and a small secretory head (typically 1–4 cells).
Function: Secrete small amounts of compounds; seen in mint family (Lamiaceae) and other herbs.

Peltate Glandular Trichomes

Sessile or short-stalked trichomes with a broad, shield-like secretory head made of multiple cells.
Function: Produce and accumulate large volumes of essential oils or resins in a subcuticular space.
Common in: Lamiaceae (e.g., oregano, thyme)

Function of External Secretions

Defense: Deter herbivores/pathogens (e.g., sticky or toxic secretions)
Attraction: Lure pollinators via aroma and nectar
Protection: Limit water loss, UV damage, or predation
Digestion: In carnivorous plants, secretions help break down prey

Salt Glands

Multicellular epidermal structures that secrete excess salt from plant tissues onto the surface.
Function:Maintain ionic balance under saline conditions
Allow growth in mangroves and halophytes (salt-tolerant plants)

Salt Gland Structure

Composed of:
Cap cells (outermost)
Collecting cells (middle layer)
Secretory cells (inner layer)
Process: Ions are actively transported and expelled through cap cells onto the leaf surface as salt crystals.

Avicennia officinalis - Salt Gland Plant

A mangrove species with well-developed salt glands on its leaves.
Adaptation: Allows active salt excretion, visible as white salt crystals on the leaf surface.

Stinging Hairs

Specialized glandular trichomes with needle-like tips that inject toxic and irritating chemicals when broken.
Function:Defense against herbivores
Delivers substances like histamine, formic acid, or acetylcholine

Stinging Hairs Structure

Composed of:
Bulbous base embedded in the epidermis
Brittle, pointed tip that breaks easily
Process: Acts like a hypodermic needle to deliver chemicals into animal skin

Urtica dioica (Stinging Nettle) - Stinging Hair Plant

A classic plant with prominent stinging hairs on leaves and stems.
Effect: Causes a stinging or burning sensation upon contact due to its chemical payload.

Colleters

Multicellular glandular structures usually found on young leaves, stipules, or floral organs.
Function:Secrete mucilage, resins, or lipids
Protect developing tissues from desiccation, pathogens, and mechanical damage

Colleter Structure

Usually located on the adaxial (upper) side of leaf bases or floral parts
Made up of a secretory head, stalk, and basal cells
Common in Rubiaceae, Apocynaceae, and other dicot families

Nectaries

Specialized multicellular glands that secrete nectar, a sugary solution that attracts pollinators and mutualistic insects (e.g., ants).
Composition: Parenchyma with dense cytoplasm and abundant mitochondria; highly secretory.

Floral Nectaries

Nectar-secreting structures located within the flower, typically near reproductive organs (e.g., base of stamens, ovary, or petal bases).
Functions:Attract pollinators (e.g., bees, butterflies, birds) with sugary nectar
Enhance pollination efficiency and reproductive success
Anatomy: Often composed of parenchyma cells rich in mitochondria and secretory vesicles; may have associated vascular tissue for sugar transport.

Extrafloral Nectaries

Nectar-secreting glands located outside the flowers, typically on leaves, petioles, stipules, or stem surfaces.
Functions:Attract beneficial insects (e.g., ants) that deter herbivores
Act as indirect plant defense systems
Features:Not involved in pollination
Activity can be continuous or inducible depending on stress or herbivory

Function of Nectaries

Pollinator attraction through floral nectar
Indirect defense via extrafloral nectar that attracts ants and other insects
Secretion is often regulated by environmental cues and developmental stage

Hydathodes

Pore-bearing epidermal structures responsible for guttation—the release of liquid water from leaf margins or tips.
Common in: Herbaceous plants like grasses, tomato, Colocasia

Hydathode Structure

Terminal tracheids (Providing water from the xylem)
Water pores (epidermal openings serving as a water exit)
Underlain by epithem tissue (loosely arranged parenchyma that functions in temporary storage and water conduction from vascular tissue to water pores)
Sheath (modified bundle sheath cells extending from vascular bundle to epidermis)
Unlike stomata: Hydathodes remain open and are non-regulated

Function of Hydathodes

Guttation: Release of water droplets, especially at night or early morning when transpiration is low but root pressure is high
Helps relieve root pressure buildup
May excrete minerals or salts, visible as white crust on leaf margins

Idioblasts

Specialized isolated cells within plant tissues that differ in content and function from neighboring cells. Classified under internal secretory structures due to their unique storage capabilities.

Function of Idioblasts

Store and sequester defensive or metabolic compounds (e.g., oils, tannins, crystals)
Accumulate crystals or toxic compounds
Involved in detoxification, regulation, and defense

Idioblasts Structure

Usually isolated single cells scattered within ground tissues (like parenchyma)
No standard morphology; their identity is based on distinct contents
Can be found in leaves, stems, or roots, depending on the compound they store

Oil Cells

A subclass of idioblasts specialized for storing essential oils and other lipophilic secretions. Often referred to as osmophores when involved in fragrance release.

Function of Oil Cells

Release aromatic compounds to attract pollinators
Some have antibacterial or insect-repelling functions
Contribute to the plant’s chemical defense

Oil Cells Structure

Large cells filled with oil droplets
Commonly located in cortex and mesophyll, or forming oil ducts/glands
Prominent in Rutaceae (e.g., Citrus), Lamiaceae (Mentha),

Crystal Idioblasts

Idioblasts that accumulate calcium oxalate crystals in various geometric forms. These are among the most common types of internal secretory cells.

Function of Crystal Idioblasts

Act as defensive structures against herbivory (mechanical irritation)
Involved in calcium regulation, storage, and detoxification

Crystal Idioblasts Structure

Typically enlarged cells with central crystal bodies
Located in parenchyma tissues of leaves, stems, roots, and even floral parts
Have varying crystal shapes

Druse Crystals

Isodiametric (spherical or somewhat round) crystals that have many pointed facets.

Raphide Crystals

Acicular (needle-shaped) crystals that are found in bundles that can contain up to several dozen crystals, often found in biforine cells that eject them.

Styloid Crystals

Acicular (needle-shaped) crystals that occur as solitary crystals.

Prismatic Crystals

Large and often pointy-ended crystals, typically occurring as one or several per cell.

Crystal Sands

Appear as clusters of small, individual cube-like particles scattered throughout the cell cytoplasm.

Lithocysts & Cystoliths

Lithocysts (literally “rock box”) are specialized epidermal idioblasts that house cystoliths, which are calcium carbonate deposits forming stone-like bodies.

Function of Lithocysts & Cystoliths

Provide mechanical support and structural reinforcement
Function as a physical defense against herbivores or pathogens

Lithocysts & Cystoliths Structure

They are large, enlarged epidermal cells containing irregularly shaped calcium carbonate bodies inside it
Typically found in the epidermis of leaves
Prominent in families like Urticaceae, Acanthaceae, Moraceae

Tannin Idioblasts

A type of secretory idioblast (distinct from surrounding tissues) that accumulates a large amount of tannins, which are phenolic compounds derived from the shikimic acid pathway.

Function of Tannin Idioblasts

Act as anti-herbivory agents (bitter, astringent taste)
Provide antimicrobial protection
Aid in allelopathy and UV protection

Tannin Idioblasts Structure

Large, dark-staining parenchyma cells
Located in mesophyll, phloem, and cortex
Also found in bark, leaves, and seeds of many dicots (e.g., Camellia sinensis)

Resin Ducts or Canals

Tubular intercellular secretory canals that contain and transport resins, surrounded by epithelial (secretory) cells which surround the canal where the resin accumulates.

Function of Resin Ducts or Canals

Resins act as antimicrobial barriers, herbivore deterrents, and wound sealants
Have aromatic, sticky, or hardening properties
Used in traditional medicine and industrial applications (e.g., varnish, incense)

Resin Ducts or Canals Structure

Long intercellular spaces lined with epithelial secretory cells
Located in the cortex, secondary xylem, primary or secondary phloem, pith, and leaves
Common in gymnosperms (e.g., Pinus), and dicots like mango

Oil Cavities/Oil Glands

Secretory structures that contain essential oils, commonly found in members of the Rutaceae and Myrtaceae families.

Function of Oil Cavities/Oil Glands

Secrete volatile or ethereal oils which are called essential oils upon extraction
Serve as a source of aroma and flavor
Have medicinal and economic value (e.g., in herbal preparations and commercial oils)

Oil Cavities/Oil Glands Structure

Cavity-like spaces in tissues that store oil secretions
Seen in the pericarp of citrus fruits (Rutaceae) and leaves of Myrtaceae
Typically lined with a secretory epithelial cells

Laticifers

Specialized secretory cells that produce and store latex, a white or colored fluid composed of sugars, alkaloids, enzymes, and gums.
Divided into: Articulated and Nonarticulated Laticifers

Articulated Laticifers

Also called jointed laticifers or laticiferous vessels
Have long, unconnected cell chains and articulated anastomosing (forming lateral connections and a net-like structure) laticifers
Present in: Asteraceae, Campanulaceae, Caricaceae, Papaveracea

Nonarticulated Laticifers

Also called simple laticifers
Are single, elongated cells that grows intrusively into the surrounding tissue that do not form anastomosing networks.
Present in: Euphorbiaceae, Asclepiadaceae, Apocynaceae, Moraceae

Function of Laticifers

Contain latex, which may play a role in plant defense
Latex may also function in wound healing and protection

Laticifers Structure

Occur as elongated cells or networks within plant tissues
Found in families like Euphorbiaceae, Apocynaceae, and Papaveraceae
Often present in phloem or cortex regions