Plant Tissue Systems: Structure and Function of Xylem, Phloem, and Parenchyma
Fundamental Tissue System
The fundamental tissue system provides structure, storage, and metabolic functions for the plant.
Parenchyma
Parenchyma is composed of living cells with large vacuoles and thin, strong cell walls. These cells typically cannot divide. Parenchyma carries out photosynthesis and stores substances. Several types are distinguished:
- Chlorophyll Parenchyma (Chlorenchyma): Found in leaves and green stems. Its cells contain chloroplasts and perform photosynthesis.
- Palisade Parenchyma: Located on the upper side of leaves, constituted by elongated and tightly packed cells.
- Spongy Parenchyma: Found in the interior and underside of leaves, formed by irregular cells that leave empty spaces (meatus or air spaces).
- Reserve Parenchyma: Found inside storage organs. Consists of rounded cells with large vacuoles containing many reserve substances, leaving meatus between them. It is abundant in seeds, tubers, and fleshy roots.
- Aquiferous Parenchyma: Stores water and is characteristic of succulent plants.
- Aerenchyma (Aeriferous Parenchyma): Found in marshy or aquatic plants. It has numerous meatus for air circulation and storage.
Collenchyma
Collenchyma is composed of prismatic living cells with cell walls thickened at the vertices. It is located beneath the epidermal tissue where it forms cords, providing support to young stems and leaf petioles.
Sclerenchyma Tissue
Sclerenchyma is formed by bundles of dead cells with thick, woody membranes. These cells have either a polyhedral form (sclereids) or a long form (fibers).
- Fibers: Form bundles or cords in roots and leaves, providing support and elasticity. They are associated with conducting vessels in the xylem and phloem.
- Sclereids: Are either dispersed (e.g., in fruit pulp) or associated, forming protective layers (e.g., nutshells).
Vascular Tissue System
The vascular system is formed by conducting tissues: Xylem (woody tissue, transporting crude sap) and Phloem (liberian tissue, transporting processed sap).
Xylem (Woody Tissue)
Xylem transports crude sap (water and minerals) from the root to the leaves. It is composed of tracheids, trachea (vessels), parenchyma, and sclerenchyma.
- Tracheids: Dead, elongated cells provided with slanting transversal walls containing pits (through which crude sap passes) and thickened walls due to lignin deposits.
- Trachea (Vessels): Found primarily in angiosperms. The transversal walls of tracheids are reabsorbed, leading to continuous intracellular tracheas or wood vessels. These are classified based on how the lignin is deposited (e.g., annular vessels, reticulate vessels).
- Parenchymal Cells: Store substances.
- Sclerenchyma: Fibers and sclereids that support and maintain the plant structure.
Phloem (Liberian Tissue)
The phloem carries prepared sap (water and glucose) from the leaves to the rest of the plant. It is formed by sieve tubes (liberian vessels) composed of living cells without nuclei. These elongated cells are superimposed, featuring perforated partitions (sieve plates) that allow the passage of sap.
Each sieve tube cell is linked to a companion cell (a parenchymatic cell) responsible for its survival.
In autumn, callose is deposited on the sieve plates, cutting off sap flow. In spring, this callose is dissolved, or new sieve vessels are formed.