(c) Give an account of different types of axial parenchyma found in dicotyledonous woody plants stating their phylogenetic significance. Mention two primitive and two advanced features of ray parenchyma.

Types of Axial Parenchyma in Dicotyledonous Woody Plants and their Phylogenetic Significance

Axial parenchyma cells are vital components of the secondary xylem, primarily involved in storage of carbohydrates (like starch), water, and other metabolic products, as well as in short-distance transport. Their classification is primarily based on their association with or independence from vessels.

I. Apotracheal Parenchyma (Independent of Vessels)

This type of axial parenchyma is not directly associated with the vessels.

• Diffuse: Individual parenchyma cells are scattered singly among the fibers throughout the xylem. They are loosely distributed without forming any specific pattern.
• Diffuse-in-aggregates: Parenchyma cells are scattered but tend to form short, irregular tangential lines or aggregates.
• Banded (Metatracheal): Parenchyma cells form continuous or discontinuous tangential bands that are independent of the vessels. These bands can be of varying widths.
  • Terminal Parenchyma: Forms a continuous band at the end of a growth ring.
  • Initial Parenchyma: Forms a continuous band at the beginning of a growth ring.

II. Paratracheal Parenchyma (Associated with Vessels)

This type of axial parenchyma is closely associated with the vessels, forming sheaths or other patterns around them.

• Scanty Paratracheal: A few parenchyma cells are loosely associated with the vessels, often forming an incomplete sheath.
• Vasicentric: Parenchyma cells form a complete sheath around the vessels, often with a uniform thickness.
• Aliform: A vasicentric sheath around the vessel extends laterally in wing-like projections.
• Confluent: Aliform parenchyma with wing-like extensions from adjacent vessels joining to form short to long tangential bands.
• Banded Paratracheal: Parenchyma forms tangential bands that are consistently associated with vessels, often incorporating multiple vessels within the band.

Phylogenetic Significance of Axial Parenchyma

The distribution patterns of axial parenchyma are significant indicators of evolutionary trends in dicotyledonous wood.

• Primitive Features:
  • Diffuse and Diffuse-in-aggregates apotracheal parenchyma are generally considered primitive. This reflects a less specialized arrangement, where storage cells are broadly distributed.
  • Banded apotracheal parenchyma (metatracheal), particularly when wide and extensive, is also considered a relatively primitive feature.
• Advanced Features:
  • Paratracheal parenchyma, especially the more elaborate forms like vasicentric, aliform, and confluent types, are considered advanced. This arrangement allows for more efficient water storage and solute exchange directly with the conductive elements (vessels), indicating a higher degree of functional specialization. This specialization is often correlated with increased efficiency in water conduction and adaptability to diverse environments.
  • The trend moves from scattered, unorganized parenchyma to more organized patterns, particularly those associated with vessels, suggesting an evolutionary advantage in nutrient and water management.

Primitive and Advanced Features of Ray Parenchyma

Ray parenchyma cells are radially oriented and form vascular rays, which are crucial for radial transport of water and nutrients, and for storage. Their anatomical features also show evolutionary trends.

Feature Category	Primitive Features of Ray Parenchyma	Advanced Features of Ray Parenchyma
Cell Shape and Orientation	Heterocellular rays: Composed of different types of cells, typically including upright (erect) cells and procumbent (radially elongated) cells. This mixed composition is considered primitive (e.g., Kribs' Heterogeneous I, IIA, IIB types). Tall rays with many upright cells.	Homocellular rays: Composed predominantly or entirely of one type of cell, usually procumbent cells. This represents a more specialized and efficient storage and transport system (e.g., Kribs' Homogeneous I, II, III types). Short rays with fewer or no upright cells.
Ray Size and Seriation	High uniseriate rays or very broad multiseriate rays: Often tall and composed of numerous cells. Presence of uniseriate wings in multiseriate rays.	Low uniseriate or multiseriate rays: Shorter and less extensive radially, indicating a reduction in overall ray size for increased mechanical strength of the wood or optimized transport. Reduction or absence of uniseriate wings in multiseriate rays.
Cell Wall Pitting	Large and simple pits on radial walls.	Smaller and more numerous pits, sometimes bordered, facilitating more regulated and efficient intercellular communication and transport.