Pits are relatively thinner portions of the cell wall that adjacent

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s 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 inquir ...

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 Cytokinesis () is the part of the cell division process during ...

, 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 Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and spec ...

can cross. The primordial pit is the only aperture in the otherwise continuous primary cell wall. Pit pairs are a characteristic feature of

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

, as

sap Sap is a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, or cell sap; it is a separ ...

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 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, γυμνό� ...

, such as

Coniferales The order Pinales in the division Pinophyta, class Pinopsida, comprises all the extant conifers. The distinguishing characteristic is the reproductive structure known as a cone produced by all Pinales. All of the extant conifers, such as cedar ...

, ''

Ginkgo ''Ginkgo'' is a genus of non-flowering seed plants. The scientific name is also used as the English name. The order to which it belongs, Ginkgoales, first appeared in the Permian, 270 million years ago, and is now the only living genus with ...

'', and ''

Gnetales Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three reli ...

''. 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

microfibril A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently ...

s 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.

Types of pits

Plasmodesmata

Torus and margo

References

Further reading