What are pits?
Pits are the characteristic depressions on the cell walls of plant cells. They act as the channels for the transport of water and minerals between adjacent cells. Pits of two neighboring cells are usually located opposite to each other and these opposite pits together are called pit pair. Each pit has a cavity called pit cavity. Pit cavity opens internally to the lumen of the cells. The pit cavities of a pit pair are separated by a thin membrane called pit membrane. Pit membrane composed of the middle lamella and the primary cell wall of corresponding cells.
What are primary pit fields?
Pits of primary and secondary cell wall show considerable differences. Well organized pits can be seen on the secondary cell wall. Pits of primary cell wall look like depressions on the cell wall. These depression like pits on the primary cell wall are called primary pit fields or primordial pits or primary pits. The cells with only primary cell wall, such as the parenchymatous cells of meristem, have only primary pit fields. The sectional view of a primary cell wall with primary pit fields looks like ‘beads on string’ like appearance. This is because; the primary pit fields are formed as depression on the primary cell wall, moreover, the primary cell wall of primary pit fields is relatively thin than other portions. The wall of the primary pit field is traversed by many plasmodesmatal canals. Plasmodesmata provide cytoplasmic connectivity between adjacent cells. Through plasmodesmata, the protoplasm of living cells are interconnected.
Simple pits Vs Bordered pits
During the development of pits, the secondary cell wall may over arch the pit cavity forming a border, leaving an inner opening called pit-aperture. Such pits with borders are called bordered pits. Two opposite bordered pit are called bordered pit pair. Pits which lack the borders are called simple pits. Two opposite simple pits are called simple pit pair. Simple pits are formed on the secondary cell wall of extra xylary fibres (fibers present outside xylem).
Some other terms related to pits
@. Half bordered pits: When a bordered pit is opposed by a simple pit
@. Blind pit: if a pit has not opposing pit, occurs if pit opens into intercellular space
@. Ramified pits: branched pits, formed by the fusion of may pits due to the increase in thickness of cell wall
@. Unilateral compound pitting: a large pit is opposed by two or more small pits
@. Vestured pits: pits with minute outgrowth from wall surface of pit chamber
Ultra-structure of bordered pit:
Bordered pits have a complex structure. The pit cavity which is enclosed by the overarching borders is called pit chamber. The pit aperture in bordered pits may be circular, linear, oval or irregular in shape. When the border of the pit it is much thick a pit canal appears which forms a passage between the pit chamber and the cell lumen. Such a canal has an outer aperture facing the pit chamber and an inner aperture facing cell lumen. Most commonly the inner aperture is large and lenticular and the outer aperture is small and circular. Such pits give the appearance of compressed funnels.
Ultra-structure of Bordered pits in Gymnosperms and Angiosperms: A comparison:
Both gymnosperms and angiosperms possess bordered pits, however they shows considerable variation in their ultra-structures. The variations are mainly shown in the ultra-structure of pit membrane. In Angiosperms, the pit membrane is homogenous where as in Gymnosperms, the pit membrane is heterogenous. Structure of bordered pits in the tracheids of gymnosperms is more elaborate. The middle of the pit membrane forms a circular thickening structure called torus. Torus is larger in diameter than the pit aperture and is composed of primary cell wall materials. The remaining part of the pit membrane surrounding the torus is called margo. Margo is flexible and thin. Under certain circumstances, the margo moves towards one or the other pit aperture closing the same with the torus (as in the figure). In this condition water does not move through the pit and is called aspirated. Thus the margo and torus together act as valves. Water flows thought the margo, but in the presence of an embolism, the pit membrane is displaced or aspirated such that the torus blocks the aperture. The pith membrane of Angiosperms is not differentiated into torus and margo, however, they can also regulate water flow by rather a simple mechanism by the movement of pith membrane in and out (as in the picture).
Distribution of pits
Pits are distributed differently in different types of cells. Pattern of distribution may vary in the same cell. In tracheary cells, the bordered pits are arranged in four characteristic patterns.
(1). Scalariform pitting: If pits are elongate and are arranged in a ladder like series
(2). Opposite pitting: Pits are arranged in horizontal rows or pairs. In opposite pitting, the pits are so closely placed and hence the outline of the pits become rectangular in surface view
(3). Alternate pitting: Pits are arranged in diagonal rows, in alternate pitting, the pits are so closely placed and hence the outline of the pits become hexagonal
(4). Sieve pitting: small pits seen in clusters, looks like sieves
Difference between Simple Pits and Bordered Pits
|Simple pits occurs mainly in parenchymatous cells and rarely in sclerenchymatous cells
|Occurs in sclerenchymatous cells only. Absent in parenchymatous cells
|Found in medullary rays, extra-xylary fibres, companion cells and in tracheids of some angiosperms
|Abundantly found in vessels of angiosperms and tracheids of gymnosperms and ferns
|Pit cavity remains in the same diameter throughout
|Size and shape of pit cavity varies
|Closing membrane of pit remain in same diameter throughout
|Closing membrane of pit varies in their diameter during development
|Simple organization of pit membrane
|Pit membrane with complex organization
|Pit border absent
|Pit border present, formed by the overarching of secondary cell wall
|Pit aperture absent
|Pit aperture present, formed due to presence of pit borders
|Pits may be circular, oval, polygonal, elongated or irregular in facial view
|Pit aperture may be circular, linear or oval in facial view
|Simple pits occurring in the thin walls are shallow where as in thick wall the pit cavity may have the form of a canal passing from the lumen of the cell towards the closing or common pit membrane
|In the case of thick secondary wall, the border divides the cavity into two parts- the space between the closing membrane and the pit aperture is called pit chamber, and the canal leading form pit chamber to the lumen of the cell is called pit canal
|Pit membrane homogenous
|Pit membrane homogenous or heterogenous. Homogenous in angiosperms and heterogenous in gymnosperms
|Torus is absent
|Torus is present ,torus is the thickening of pit membrane which act as valves
|Valve like opening and closing mechanism absent
|Torus and margo together with pith aperture acts like valves and which regulate the opening and closing of pits
|Plasmodesmatal connections are seen in pit membrane
|Plasmodesmatal connections are absent in the pit membrane
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