biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditar ...

, a rachis (from the grc, ῥάχις [], "backbone, spine")Henry George Liddell, Robert Scott, A Greek–English Lexicon, ράχις
/ref> is a main axis or "shaft".

In zoology and microbiology

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

s, ''rachis'' can refer to the series of articulated

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

, which encase the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

. In this case the ''rachis'' usually forms the supporting axis of the body and is then called the spine or

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

. ''Rachis'' can also mean the central shaft of pennaceous feathers. In the gonad of the invertebrate nematode ''

Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...

'', a rachis is the central cell-free core or axis of the gonadal arm of both adult males and

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...

s where the germ cells have achieved pachytene and are attached to the walls of the gonadal tube. The rachis is filled with

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

In botany

plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...

s, a rachis is the main axis of a compound structure. It can be the main stem of a compound

leaf A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

, such as in ''

Acacia ''Acacia'', commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australasia. The genus n ...

'' or

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except ...

s, or the main, flower-bearing portion of an

inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed ...

above a supporting

peduncle Peduncle may refer to: *Peduncle (botany), a stalk supporting an inflorescence, which is the part of the shoot of seed plants where flowers are formed *Peduncle (anatomy), a stem, through which a mass of tissue is attached to a body **Peduncle (art ...

. Where it subdivides into further branches, these are known as rachillae (singular rachilla). A ripe head of wild-type wheat is easily shattered into dispersal units when touched or blown by the

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...

. A series of abscission layers forms that divides the rachis into dispersal units consisting of a small group of flowers (a single spikelet) attached to a short segment of the rachis. This is significant in the

history of agriculture Agriculture began independently in different parts of the globe, and included a diverse range of Taxon, taxa. At least eleven separate regions of the Old World, Old and New World were involved as independent centers of origin. The development ...

, and referred to by archaeologists as a "brittle rachis", one type of shattering in crop plants.

References

{{Reflist Vertebrate anatomy Plant morphology

In zoology and microbiology

In botany

See also

References