Plant Tissue Systems: Structure and Functions

The Epidermal Tissue System: Protective Layers

The epidermal tissue system consists of tegumentary tissues that provide protection. These include the epidermis of young plants and the periderm, found only in plants with secondary growth. This layer eventually forms the suber or cork.

1. Epidermis

In herbaceous plants, the protective tissue is reduced to the epidermis, which includes a single layer of cells. In stems and leaves, these cells secrete a protective cover called the cuticle. This waterproof layer protects these organs and defends against external agents. Some epidermal cells differentiate into trichomes or hairs.

2. Periderm

The periderm is a tissue composed of many layers of cells formed under the epidermis, which also serves a protective function. It replaces the epidermis in the stems and roots of woody plants, forming the outer crust or bark.

The Ground Tissue System: Functions and Support

Often referred to as the key system, this includes the most fundamental tissues of the plant.

1. Parenchyma

This is the most abundant plant tissue and performs many important functions such as photosynthesis, storage, and secretion. According to the type of cells it contains, parenchyma is classified as:

• Chlorophyllous (Chlorenchyma): Formed by cells with numerous chloroplasts, located under the epidermis in leaves and stems, responsible for performing photosynthetic functions.
• Storage Parenchyma: Located in roots and stems. These cells serve as reservoirs for nutrients such as starch, salts, oils, and water.
• Glandular Parenchyma: Responsible for secreting various substances.

2. Support Tissues: Collenchyma and Sclerenchyma

These tissues constitute the skeleton that provides consistency to plants. There are two main types:

• Collenchyma: A tissue of immature support cells that are poorly differentiated with walls reinforced with cellulose. It is the primary supporting tissue in herbaceous plants.
• Sclerenchyma: A tissue composed of dead cells with two cell walls: one composed of cellulose fibers and one reinforced with lignin, which provides great rigidity.

The Vascular System: Conductive Tissues

The vascular system is responsible for the transport of substances and collaborates in the structural support of the plant. There are two types of vessels:

1. Xylem or Woody Vessels

Xylem is responsible for transporting crude sap. The system consists of dead cells without separation walls between them. These cells develop a secondary cell wall, attached inside the primary wall, which is permeated by substances like lignin to provide significant hardness.

2. Phloem or Liberian Vessels

Phloem transports the produced sap (nutrients). It consists of living cells with vessel walls separating them. These cells lose the majority of their cellular organelles, including the nucleus; therefore, they cannot perform all vital functions independently. The Liberian vessel areas contain perforated sections called sieve plates (plates criboses) that allow the passage of nutrients between cells.