Plant Tissues Overview

Chapter 4: Plant Tissues

Introduction to Tissues

• Plants have three or four major groups of organs:

  • Roots

  • Stems

  • Leaves

  • Flowers

• Each organ is composed of tissues.

• Definition of Tissue: A tissue is a group of cells performing a similar function.

• There may be more than one tissue per organ.

Roots and Stems

• Three basic tissue patterns in roots and stems:

  • Woody dicots

  • Herbaceous dicots

  • Monocots

Meristematic Tissues

• Definition of Meristems: Permanent regions of growth and active cell division.

• Characteristics of Meristematic Cells:

  • Typically small, 6-sided, box-like shape.

  • Large nuclei and small to absent vacuoles.

  • As cells mature, they assume different shapes;

  • Vacuoles increase in size and may occupy more than 90% of the volume of the cell.

Types of Meristems

Apical Meristems

• Function: Roots and shoots increase in length as the apical meristems produce new cells, known as primary growth.

• Location: Found at the tips of roots and shoots.

• Outcomes of Apical Meristems:

  • Produce three primary meristems that give rise to:

  • Embryo

  • Leaves

  • Buds

Primary Meristems

• Definition: Primary meristems are called meristems that produce primary tissues.

• Types of Primary Meristems:

  • Protoderm

  • Ground Meristem

  • Procambium

Lateral Meristems

• Function: Produce secondary tissues that increase the girth of roots and stems, known as secondary growth.

• Includes:

  • Vascular cambium

  • Cork cambium

Cambium Details

• Individual cells of the cambium:

  • Called initials (the remaining cells).

  • Their sister cells are called derivatives.

• Vascular Cambium: Sometimes simply referred to as cambium.

  • Produces secondary tissues that function primarily in support and conduction.

  • Structure: Composed of a thin cylinder of brick-shaped cells extending the length of stems and roots.

  • Cambial cylinder often branches, except at the tips.

  • Tissues produced are responsible for most of the increase in girth as plants grow.

Cork Cambium

• Structure: A thin cylinder running the length of roots and stems of woody plants.

• Position: Lies outside the vascular cambium and just inside the outer bark.

Intercalary Meristems

• Location: Present in the vicinity of nodes (leaf attachment areas) along stems.

• Function: Add to stem length.

• Note: Grasses and related plants do not have vascular cambium nor cork cambium; they have only apical meristems.

Tissues Produced by Meristems

• Types:

  • Simple Tissues: Composed of one type of cell.

  • Complex Tissues: Composed of two or more cell types.

• After arising from meristems, maturing cells assume various shapes and sizes, with their shape relating to function.

Simple Tissues

Parenchyma

• Definition: Composed of parenchyma cells.

• Characteristics of Parenchyma Cells:

  • Thin, pliable walls.

  • Typically 14-sided at maturity.

  • Living cytoplasm, often containing large vacuoles and various secretions.

  • May remain alive for a long time.

Types of Parenchyma Tissue and Cells

• Aerenchyma: Parenchyma tissue with extensive connected air spaces, usually in aquatic plants.

• Chlorenchyma: Parenchyma cells containing chloroplasts that function in photosynthesis.

• Transfer Cells: Develop irregular extensions of the inner wall that greatly increase the surface area of the plasma membrane (e.g., nectaries of flowers).

Collenchyma Tissue

• Characteristics of Collenchyma Cells:

  • May remain alive for a long time.

  • Cell walls thick, with uneven thickness.

  • Provide flexible support due to pliability.

  • Living cytoplasm.

Sclerenchyma Tissue

• Characteristics of Sclerenchyma Cells:

  • Thick, tough secondary walls, normally impregnated with lignin.

  • Dead at maturity.

  • Function: Support.

• Two Types:

  • Sclereids: Scattered in tissue; include stone cells; size - cells as long as wide.

  • Fibers: Found in roots, stems, leaves, and fruits; much longer than wide; contain lumen (tiny cavity); have been used for thousands of years as rope, string, canvas, textiles, etc.

Complex Tissues

• Definition: Composed of two or more kinds of cells.

• Some complex tissues are produced by apical meristems.

• Include:

  • Xylem

  • Phloem

  • Epidermis

  • Periderm

Xylem and Phloem

• Function: Transport water, ions, and soluble food throughout plants.

• In woody plants, most complex tissues are produced by the vascular cambium and are called vascular tissues.

Epidermis

• Definition: A protective layer covering all plant organs.

• Composition: Consists of parenchyma or parenchyma-like cells.

• Specialized cells within the epidermis regulate the movement of gases in and out of the plant, and may include secretory glands, hairs, crystalline isolating cells, and cells that increase the surface area of roots.

Periderm

• Definition: Comprises the outer bark of woody plants.

• Composition: Mainly composed of cork cells and has pockets of parenchyma-like cells.

Xylem Details

• Chief Conducting Tissue: For water and minerals absorbed by the roots.

• Composition includes parenchyma cells, fibers, vessels, tracheids, and ray cells.

Vessels

• Definition: Long tubes made of vessel elements.

• Characteristics of Vessel Elements:

  • Thick secondary cell walls.

  • Open at both ends.

  • Secondary walls develop irregularly.

  • Perforation plates between end walls.

Tracheids

• Characteristics:

  • Tapered at the ends with pairs of pits that allow water to pass from cell to cell.

  • Pits: Areas without secondary cell walls.

  • Dead at maturity; have thick secondary cell walls.

  • May have spiral thickening on cell walls.

Rays

• Function: Lateral conduction and food storage.

• Composition: Composed of long-lived parenchyma cells that radiate out from the center like spokes of a wheel.

• Produced in horizontal rays by ray initials of the vascular cambium.

Phloem

• Function: Conducts dissolved food materials produced by photosynthesis throughout the plant.

• Composition includes fibers, parenchyma cells, and ray cells.

• Comprised of sieve tube members and companion cells.

Sieve Tube Members

• Characteristics of Companion Cells:

  • Lack secondary cell walls and nuclei.

  • Lay end to end to form sieve tubes.

  • Walls have sieve plates with small pores.

  • Callose forms a callus plug to prevent leaking of sieve tube contents when a cell is injured.

  • They are narrow, tapered cells associated with sieve tube members.

Epidermis Details

• Protective Layer: That is one cell-layer thick, covering all plant organs.

• Components of the Epidermis:

  • Usually one cell thick.

  • Cutin: Fatty substance on the surface of outer walls of epidermis forming the cuticle.

  • Wax secreted on cuticle: Prevents water loss by evaporation.

  • Some plants produce velamen roots, which are several cells thick.

  • Others produce leaves with a multiple-layered epidermis that are resistant to bacteria and other disease organisms.

Other Epidermal Components

• Root epidermal cells produce root hairs to increase the absorptive area of the root surface.

• Above-ground parts of plants may have trichomes, which are outgrowths consisting of one to several cells.

Stomata and Other Modifications

• Leaves have stomata bordered by pairs of guard cells.

Glands or Modified Hairs

• Some epidermal cells are modified as glands or modified hairs.

Periderm Details

• Definition: Replaces the epidermis when the cork cambium begins producing new tissue.

• Constitutes outer bark and is primarily composed of cork cells which are dead at maturity.

  • The walls make cork cells waterproof and help protect phloem.

  • While still alive, cytoplasm secretes suberin (a fatty substance) into the walls.

Lenticels

• Definition: Loosely arranged pockets of parenchyma cells formed by cork cambium that protrude through the surface of periderm.

• Function: Allows for gaseous exchange.

Secretory Cells and Tissue

• Function: Secretory cells may function individually or as part of a secretory tissue (e.g., flower nectar, resins, citrus oils).