HOME

TheInfoList



OR:

Pits are relatively thinner portions of the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...
that adjacent cells can
communicate Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inqu ...
or exchange fluid through. Pits are characteristic of cell walls with secondary layers. Generally each pit has a complementary pit opposite of it in the neighboring cell. These complementary pits are called "pit pairs". Pits are composed of three parts: the pit chamber, the pit aperture, and the pit membrane. The pit chamber is the hollow area where the secondary layers of the cell wall are absent. The pit aperture is the opening at either end of the pit chamber. The pit membrane is the primary cell wall and
middle lamella The middle lamella is a layer that cements together the primary cell walls of two adjoining plant cells. It is the first formed layer to be deposited at the time of cytokinesis. The cell plate that is formed during cell division itself develops ...
, or the membrane between adjacent cell walls, at the middle of the pit chamber. The primary cell wall at the pit membrane may also have depressions similar to the pit depressions of the secondary layers. These depressions are primary pit-fields, or primary pits. In the primary pit, the primordial pit provides an interruption in the primary cell wall that the plasmodesmata can cross. The primordial pit is the only aperture in the otherwise continuous primary cell wall. Pit pairs are a characteristic feature of xylem, as sap flows through the pits of xylem cells.


Types of pits

Though pits are usually simple and complementary, a few more pit variations can be formed: *Simple pits: A pit pair in which the diameter of the pit chamber and the diameter of the pit aperture are equal. *Bordered pits: A pit pair in which the pit chamber is over-arched by the cell wall, creating a larger pit chamber and smaller pit aperture. *Half bordered pits: A pit pair in which a bordered pit has a complementary simple pit. Such a pit pair is called half bordered pit pair. *Blind pits: A pit pair in which a simple pit has no complementary pit. *Compound pits: A pit pair in which one cell wall has a large pit and the adjacent cell wall has numerous, small pits.


Plasmodesmata

Plasmodesmata are thin sections of the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ...
that traverse pits and connect adjacent cells. These sections provide an avenue of transport through the pits and facilitate communication. Plasmodesmata are not restricted to pits however, as plasmodesmata often cross a cell wall of constant width and occasionally the cell wall is even wider in areas where plasmodesmata traverse it.


Torus and margo

The torus and margo are characteristic features of bordered pit-pairs in
gymnosperms The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν� ...
, such as Coniferales, '' Ginkgo'', and '' Gnetales''. In other
vascular plants Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
, the torus is rare. The pit membrane is separated into two parts: a thick impermeable torus at the center of the pit membrane, and the permeable margo surrounding it. The torus regulates the functions of the bordered pit, and the margo is a cell wall-derived porous membrane that supports the torus. The margo is composed of bundles of microfibrils that radiate from the torus. The margo is flexible and can move towards either side of the pit while under stress. This allows the thick, impermeable torus to block the pit aperture. When the torus is displaced so that it blocks the pit aperture, the pit is said to be aspirated.


References


Further reading

* Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: ''Strasburger – Lehrbuch der Botanik.'' Begründet von E. Strasburger. Spektrum Akademischer Verlag, Heidelberg 2008 (36. Aufl.) * Dietger Grosser: ''Die Hölzer Mitteleuropas – Ein mikrophotographischer Holzatlas'', Springer Verlag, 1977. * Rudi Wagenführ: ''Holzatlas'', 6. neu bearb. und erw. Aufl., Fachbuchverlag Leipzig im Carl Hanser Verlag, München, 2007. {{ISBN, 978-3-446-40649-0 Plant anatomy Plant physiology Cell biology Wood