Summary: Plant Tissue

Table of Contents
1. Introduction
2. Plant Tissue
3. Meristematic Tissues
4. Permanent Tissues
5. Simple Permanent Tissues
6. Protective Tissues
7. Complex Permanent Tissues
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Introduction

• All living organisms are made of cells. Multicellular organisms show division of labour because cells specialise by Tissue formation via differentiation.
• The branch of biology that studies tissues is called Histology.

Plant Tissue

• Plants and animals have similar life processes but different tissues due to differences in organisation and growth.
• Plant tissues are of two types: Meristematic tissues and Permanent tissues.

Meristematic Tissues

• Groups of thin-walled, compact cells that are dividing or can divide. Cells are small, with dense cytoplasm, large nucleus and few or no vacuoles.
• Classified by position:
  • Apical Meristem - at root and shoot tips; increases length of stem and root.
  • Lateral Meristem - on sides of stem and root (cambium, cork cambium); increases girth; present in dicots; includes vascular and cork cambium.
  • Intercalary Meristem - at bases of leaves and internodes; increases internode length; common in monocots.

Permanent Tissues

• Derived from meristematic cells that have lost the ability to divide and undergone differentiation. Cells may be living or dead and walls may be thin or thick.
• Two main groups: Simple Permanent Tissue and Complex Permanent Tissue.

1. Simple Permanent Tissues

• Parenchyma - living, thin-walled, isodiametric cells that form basic packing tissue. Loosely packed with intercellular spaces. Main function: storage of food.
• Chlorenchyma - parenchyma with chlorophyll; site of photosynthesis.
• Aerenchyma - specialised parenchyma in aquatic plants with large air spaces for buoyancy and gas storage.
• Collenchyma - living cells with uneven thickening at corners (pectocellulose); provides mechanical support and flexibility so plants can bend without breaking.
• Sclerenchyma - usually dead, thick, lignified cells with narrow lumen that give strong mechanical support. Two types:
  • Fibres - elongated, spindle-shaped.
  • Sclereids - short, broad cells occurring singly or in groups (also called stone cells).

Protective Tissues

• Cork - outer protective tissue of older stems and roots formed by cork cambium; produces phelloderm (inside) and phellem or cork (outside). Cork cells are multilayered, impregnated with suberin (impermeable). Lenticels allow gas exchange. Older cork cells die and fill with tannins, resins and air. Cork is light, impervious and used commercially.
• Epidermis - outer protective single layer (usually) of stems, leaves and roots that prevents pathogen entry. Cells are elongated, closely packed; outer walls have cutin. Interrupted by stomata. In xerophytes epidermis may be multi-layered. Root epidermis forms root hairs to increase absorption. Aerial epidermis may have cutinised hairs called trichomes that form an insulating air layer.

2. Complex Permanent Tissues

• Xylem - conducts water and minerals from roots to plant parts. Composed of four elements: tracheids, vessels, xylem fibres and xylem parenchyma.
• Tracheids, vessels and xylem fibres are dead; xylem parenchyma is living.
• Tracheids - long, tubular dead cells with lignified walls and pits that allow water movement; abundant in pteridophytes and gymnosperms.
• Vessels - long tubes formed by end-to-end union of dead cells with lignified walls and pits; characteristic of angiosperms.
• Xylem parenchyma stores food; xylem fibres provide mechanical support.
• Phloem - conducts and translocates food made in leaves to other parts. Phloem cells are living and include sieve tubes/sieve cells, companion cells (for short-distance transport), parenchyma (storage) and phloem fibres (support).