Pits – Simple and Bordered Pits in plants

What are Pits?

• Pits are specialized structures found within the secondary cell walls of plant cells. These features manifest as unthickened regions, resulting in distinct depressions that can be observed when examining cell walls. The primary function of pits is to facilitate intercellular communication and transport, particularly concerning the movement of water and essential minerals.
• Each pit is not an isolated feature; rather, it is part of a complementary system. Specifically, every pit in one cell aligns with a corresponding pit in the adjacent cell wall, forming what is known as a pit pair. This configuration is crucial for the efficient transport of nutrients and water between neighboring cells, thereby playing a vital role in the overall physiological functioning of plants.
• The architecture of pits includes a cavity known as the pit cavity. This cavity serves as a conduit that opens into the cell’s lumen, thus allowing for the exchange of substances. The pit cavities of a pit pair are separated by a thin, selective barrier called the pit membrane. This membrane is composed of the middle lamella and the primary cell wall material of the adjacent cells, effectively regulating the movement of water and solutes.
• Understanding the structural and functional aspects of pits is essential for comprehending how plants maintain their physiological balance and support growth. Therefore, pits not only serve as transport channels but also as crucial elements that contribute to the integrity and functionality of plant tissues. Their presence highlights the intricacies of plant biology and the mechanisms that enable plants to thrive in various environments.

What are primary pit fields?

• Primary pit fields, also known as primordial pits, are distinctive features located within the primary cell wall of plant cells. These structures appear as depressions on the cell wall, facilitating cellular communication and transport. Their formation is crucial for the intercellular connectivity that is vital for plant development and function.
• A defining characteristic of primary pit fields is the presence of plasmodesmata. These microscopic channels traverse the primary pit fields and establish cytoplasmic connections between neighboring cells. Plasmodesmata enable the protoplasm of living cells to communicate and exchange nutrients, thereby playing a significant role in maintaining the physiological equilibrium within plant tissues.
• The structural organization of primary pit fields is markedly different from that of pits found in secondary cell walls. While secondary cell wall pits are well-defined and organized, primary pit fields exhibit a more irregular, depression-like appearance. In fact, when viewed in cross-section, the primary cell wall with primary pit fields resembles a “beads on a string” configuration. This visual representation highlights the relatively thinner regions of the primary cell wall, where the primary pit fields are located.
• Primary pit fields are typically found in cells that possess only primary cell walls, such as parenchyma cells within meristematic tissue. These cells, being actively involved in growth and differentiation, rely on the connectivity provided by plasmodesmata to coordinate their activities. Therefore, primary pit fields not only serve as structural features of the cell wall but also function as essential conduits for communication and resource sharing among plant cells.

Structure of Pits

Pits are vital components of plant cell walls, playing a crucial role in intercellular communication and the transport of water and nutrients. Their structural complexity allows for efficient functioning, contributing to the overall physiological processes of plants. The following points outline the key structural components of pits:

• Pit Cavity: This structure refers to the cavity created by the absence or thinning of material in the secondary wall of plant cells. The pit cavity serves as a channel through which substances can flow, facilitating the transfer of water, minerals, and other essential compounds between adjacent cells.
• Pit Membrane (or Closing Membrane): The pit membrane is an essential barrier that separates the two pit cavities within a pit pair. It is composed of the primary cell walls and the middle lamella from the neighboring cells. This membrane plays a critical role in regulating the movement of materials between cells, as it selectively permits the passage of certain solutes while restricting others.
• Pit Aperture: The pit aperture is the opening of the pit located on the inner side of the cell wall. This structure provides access to the pit cavity, allowing for the entry and exit of substances. The size and shape of the pit aperture can influence the efficiency of transport processes between cells.

Types of Pits

Pits are specialized structures found in the cell walls of plants, playing a crucial role in water and nutrient transport between adjacent cells. They can be categorized into different types based on their structural features and configurations. The two primary types of pits are simple pits and bordered pits.

• Simple Pits: These are characterized by the absence of borders and are formed on the secondary cell walls of extra xylary fibers, which are fibers situated outside the xylem. Simple pits create openings that allow for the movement of substances between cells but do not have the complex architecture found in bordered pits. When two opposite simple pits are aligned, they are referred to as a simple pit pair.
• Bordered Pits: In contrast to simple pits, bordered pits feature a more complex structure. During the development of bordered pits, the secondary cell wall may arch over the pit cavity, forming a border that creates an inner opening known as the pit aperture. This arrangement results in two opposite bordered pits being classified as a bordered pit pair.

The structural components of bordered pits include the following:

• Pit Chamber: This is the section of the pit cavity enclosed by the overarching borders. It plays a crucial role in housing the fluid that flows between adjacent cells.
• Pit Aperture: This opening is located in the secondary wall and faces the cell lumen. The shape of the pit aperture can vary, being circular, linear, oval, or irregular, which affects the flow of substances through the pit.
• Pit Canal: In instances where the secondary wall is particularly thick, a pit canal may form, connecting the cell lumen with the pit chamber. This canal has two openings: an inner aperture that faces the cell lumen and an outer aperture that faces the pit chamber.

Additional types of pits include:

• Half Bordered Pits: These occur when a bordered pit is opposed by a simple pit, combining features of both types.
• Blind Pits: These pits lack an opposing pit, resulting in an opening into an intercellular space.
• Ramified Pits: Characterized by branches, ramified pits form due to the fusion of multiple pits as the thickness of the cell wall increases.
• Unilateral Compound Pitting: This occurs when a large pit is opposed by two or more smaller pits, demonstrating an intricate arrangement.
• Vestured Pits: These pits are distinguished by the presence of minute outgrowths on the wall surface of the pit chamber, which can influence fluid dynamics.

The ultra-structure of bordered pits reveals considerable differences between gymnosperms and angiosperms. Both groups possess bordered pits, yet they exhibit variations primarily in the structure of the pit membrane. In angiosperms, the pit membrane is homogeneous, while in gymnosperms, it is heterogeneous.

In gymnosperms, the structure of bordered pits is more elaborate. The center of the pit membrane forms a circular thickening known as the torus, which is larger in diameter than the pit aperture and composed of primary cell wall materials. Surrounding the torus is the margo, which is flexible and thin. Under specific conditions, the margo can move toward either pit aperture, effectively closing it with the torus, a state known as aspirated, preventing water flow. This configuration allows the margo and torus to function as valves, regulating fluid movement.

In contrast, the pit membrane of angiosperms lacks the distinct differentiation into torus and margo. However, these membranes can still regulate water flow through a simpler mechanism involving the movement of the pit membrane in and out.

Distribution of Pits

The distribution of pits in plant cells exhibits significant variability, influenced by the type of cell and its specific function. Understanding this distribution is essential for comprehending how plants facilitate the movement of water and nutrients through their vascular systems. The following points detail the distribution patterns of pits, particularly in tracheary cells, which play a critical role in water transport.

• Scalariform Pitting: This arrangement features elongate pits that are organized in a ladder-like series. The scalariform pattern allows for an effective transfer of water between adjacent cells, enhancing the efficiency of vascular tissue.
• Opposite Pitting: In this configuration, pits are aligned in horizontal rows or pairs. The close placement of the pits results in a rectangular appearance when viewed from the surface. This arrangement facilitates rapid water movement across adjacent tracheary cells.
• Alternate Pitting: The pits in alternate pitting are arranged in diagonal rows. Similar to opposite pitting, these pits are also closely positioned, leading to a hexagonal outline when observed from the surface. This pattern contributes to effective connectivity between cells, ensuring efficient fluid transport.
• Sieve Pitting: Characterized by small pits grouped in clusters, sieve pitting resembles the appearance of a sieve. This pattern is especially important for cells involved in the transport of nutrients, allowing for the efficient movement of substances through the plant’s vascular system.

Difference Between Simple Pits and Bordered Pits

The distinction between simple pits and bordered pits is fundamental in understanding the structural and functional diversity of plant cells. Each type of pit serves specific roles in facilitating intercellular communication and nutrient transport. The following points elucidate the differences between these two types of pits.

1. Occurrence: Simple pits are primarily found in parenchymatous cells and are rarely present in sclerenchymatous cells. They are located in medullary rays, extra-xylary fibers, companion cells, and some tracheids of angiosperms. In contrast, bordered pits occur exclusively in sclerenchymatous cells, making them absent in parenchymatous cells. Bordered pits are abundantly found in the vessels of angiosperms and the tracheids of gymnosperms and ferns.
2. Pit Cavity Structure: The pit cavity of simple pits maintains a consistent diameter throughout its length. Conversely, the size and shape of the pit cavity in bordered pits can vary significantly, particularly during their development.
3. Pit Membrane Organization: Simple pits possess a straightforward organization of the pit membrane, whereas bordered pits exhibit a complex organization. This complexity is crucial for their functionality in water transport.
4. Presence of Pit Borders: Simple pits lack a defined pit border. In contrast, bordered pits have a pronounced pit border formed by the overarching of the secondary cell wall, which contributes to their unique structural characteristics.
5. Pit Aperture: Simple pits do not have a pit aperture, while bordered pits feature a pit aperture that results from the presence of pit borders.
6. Facial Appearance: In facial view, simple pits can be circular, oval, polygonal, elongated, or irregular. The pit aperture in bordered pits can be circular, linear, or oval.
7. Secondary Wall Thickness: Simple pits typically occur in thin secondary walls, whereas bordered pits can be found in thick secondary walls. In bordered pits, the pit cavity may have a canal that divides it into two parts: the space between the closing membrane and the pit aperture, which leads from the pit chamber to the cell lumen.
8. Pit Membrane Composition: The pit membrane in simple pits is homogeneous, while in bordered pits, it can be either homogeneous or heterogeneous. Angiosperms tend to have a homogeneous pit membrane, while gymnosperms display a heterogeneous structure.
9. Presence of Torus: Torus, which is a thickening of the pit membrane that acts as a valve, is absent in simple pits. In bordered pits, the torus is present, contributing to the regulation of water flow.
10. Valve Mechanism: Simple pits do not have a valve-like opening and closing mechanism. However, in bordered pits, the torus and margo together with the pit aperture function as valves, regulating the opening and closing of the pits.
11. Plasmodesmatal Connections: Simple pits contain plasmodesmatal connections in their pit membranes, facilitating communication between adjacent cells. In contrast, bordered pits lack these connections.