Bio 127 Lec Lesson 4.3 (Plant Tissues)

Types of Plant Tissues (Meristems, Xylem, and Phloem)

Plant Tissues

Collections of similar cells that perform specific functions. Two main types:
Meristematic Tissues: Actively dividing, undifferentiated cells.
Permanent Tissues: Fully differentiated and no longer dividing.Simple Permanent Tissue: (Parenchyma, Collenchyma, Sclerenchyma)
Complex Permanent Tissue: (Xylem, Phloem)

Meristematic Tissues

Plant tissues composed of undifferentiated, actively dividing cells (meristematic cells).
Function: Responsible for continuous growth and formation of new organs in plants.
They have not yet committed to a specific differentiation pathway.

Apical Meristem

Meristem located at the tips of roots and shoots, responsible for primary growth (lengthening of the plant).
Contains multiple layers: L1, L2, and L3

L1 Layer (Apical Meristem)

The outermost layer of the apical meristem.
Function: Forms the epidermis (protective outer tissue of stems and leaves).
Division Type: Primarily anticlinal (perpendicular to surface), allowing expansion while maintaining surface area.

L2 Layer (Apical Meristem)

The subepidermal layer in the apical meristem.
Function: Gives rise to portions of the cortex, leaf mesophyll, and reproductive cells (in flowers).
Division Type: Both anticlinal and limited periclinal.

L3 Layer (Apical Meristem)

The innermost layer of the apical dome.
Function: Contributes to internal tissues, such as vascular tissue, pith, and inner cortex.
Division Type: Anticlinal and periclinal, enabling multidirectional growth.

Shoot Apical Meristem (SAM)

Found at the tip of the shoot, initiates leaves, branches, and reproductive structures.
Has three zones: Central Zone (CZ), Peripheral Zone (PZ), and Rib Zone (RZ).

Central Zone (CZ) – SAM

Located at the center of the shoot apical meristem (SAM).
Function: Serves as a stem cell reservoir; cells here divide slowly and maintain pluripotency.
Cells: Large and highly vacuolated, providing long-term growth potential.

Peripheral Zone (PZ) – SAM

The region surrounding the central zone in the SAM.
Function: Area of active cell division; responsible for initiating leaf primordia and contributing to lateral organ formation.
Cells: Smaller and more actively dividing than CZ.

Rib Zone (RZ) – SAM

Located beneath the central zone in the SAM.
Function: Forms the internal tissues of the stem, including vascular tissues and pith.
Also contributes to mechanical support and conduction.

Root Apical Meristem (RAM)

Located behind the root cap; generates new cells for root elongation and tissue development.
Key Zones/Regions in RAM: Quiescent Center (QC), Proximal Meristem (PM), Distal Meristem (DM), and Root Cap.

Quiescent Center (QC) – RAM

A group of slowly dividing cells at the center of the RAM.
Function: Maintains stem cell activity, controls the balance between cell division and differentiation, and can replenish damaged cells.

Proximal Meristem (PM) – RAM

The region above the quiescent center.
Function: Produces cells that elongate and differentiate into primary tissues, including epidermis, cortex, and vascular tissues.

Distal Meristem (DM) – RAM

The region below the quiescent center, contributing to the root cap.
Function: Generates columella cells, which are gravity-sensing, and lateral root cap cells for protection and mucilage secretion.

Root Cap – RAM

A dome-shaped cap of living parenchyma cells that protects the RAM as the root pushes through the soil.
Functions:
Produces mucilage for lubrication.
Contains statocytes with statoliths for gravity perception.
Replenishes worn-out cap cells through distal meristem activity.

Axillary Meristem

Located at the axils of leaves (between the leaf and stem).
Function: Produces lateral branches and buds; arises from the shoot apical meristem.

Intercalary Meristem

Found between mature tissues, typically at the bases of leaves or internodes.
Function: Enables continued growth after damage (e.g., grazing, mowing).
Common In: Grasses and other monocots.

Lateral Meristem

Meristem located along the sides of stems and roots.
Function: Responsible for secondary growth—increasing girth (thickness) of stems and roots.
Includes: Vascular Cambium and Cork Cambium (Phellogen)

Vascular Cambium

A lateral meristem specifically responsible for secondary growth of plants.
Function: Produces woody tissues (secondary xylem and phloem) that contribute to the thickness of stems and roots.
Cell types:Fusiform Stem Cells: Elongated cells that divide longitudinally. Produce conducting elements.
Ray Stem Cells: Radially oriented. Produce parenchyma rays for storage and lateral transport.

Cork Cambium (Phellogen)

A lateral meristem that forms the periderm (protective outer layer).
Produces:Phellem (cork): Dead protective cells to the outside.
Phelloderm: Living parenchyma-like cells to the inside.
Location: Below the epidermis in mature stems.

Permanent Tissue

Tissues that have completed differentiation and lost the ability to divide.
Function: Specialized for storage, support, conduction, or secretion, depending on the cell type and location.

Types of Permanent Tissue

1. Simple Permanent Tissue – Made of one type of cell:Parenchyma, Collenchyma, Sclerenchyma
2. Complex Permanent Tissue – Made of multiple cell types:Xylem, Phloem

Phloem

A complex permanent tissue responsible for the transport of sugars (mainly sucrose) and other organic compounds from leaves to other parts of the plant.
Also transports: Amino acids, hormones, and some inorganic ions (e.g., K⁺, Mg²⁺).

Phloem Functions

Translocation of Photosynthates (sugars from leaves to roots and growing tissues).
Nutrient Transport (amino acids, hormones).
Support for Growth (supplying nutrients to developing tissues).
Regulation of Metabolism by directing the flow of energy sources.

Sieve Tube Elements (Angiosperms)

Main conducting cells in phloem of flowering plants.
Aligned end-to-end to form long tubes.
Lack nucleus and some organelles to maximize transport space.
Connected by sieve plates, allowing sap to flow efficiently.

Companion Cells

Nucleated cells found adjacent to sieve tube elements.
Function:Regulate metabolism and loading/unloading of substances into sieve tubes.
Maintain the sieve tube elements since they lack nuclei.

Phloem Fibers (Bast Fibers)

Sclerenchyma cells found in phloem that provide mechanical support.
Characteristics: Thick-walled, elongated, and dead at maturity.
Example: Prominent in flax and hemp.

Phloem Parenchyma

Living parenchyma cells in the phloem tissue.
Functions:Storage of nutrients.
Lateral transport of organic materials.
Support the conducting function of phloem.

Xylem

A complex permanent tissue responsible for the transport of water and dissolved minerals from roots to the rest of the plant. Also provides mechanical support.

Tracheary Elements

The water-conducting cells of xylem, which include:

1. Tracheids
2. Vessel elements

Tracheids

Elongated xylem cells with tapered ends and thick secondary walls perforated with small pits.
Function: Conduct water and provide support; less efficient than vessels.
Found in: All vascular plants, especially gymnosperms.

Vessel Elements

Shorter, wider xylem cells with perforated end walls, stacked end-to-end to form vessels.
Function: Provide a low-resistance pathway for water flow.
More efficient than tracheids.
Characteristic of: Angiosperms.

Other Xylem Components

Xylem Parenchyma: Living cells for storage and lateral transport.
Xylem Fibers: Dead sclerenchyma cells that provide mechanical support.