Theca

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In biology, a theca (pl.: thecae) is a sheath or a covering.

Botany

In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common area of dehiscence called the stomium.^[1] Any part of a microsporophyll that bears microsporangia is called an anther. Most anthers are formed on the apex of a filament. An anther and its filament together form a typical (or filantherous) stamen, part of the male floral organ.

The typical anther is bilocular, i.e. it consists of two thecae. Each theca contains two microsporangia, also known as pollen sacs. The microsporangia produce the microspores, which for seed plants are known as pollen grains.

If the pollen sacs are not adjacent, or if they open separately, then no thecae are formed. In Lauraceae, for example, the pollen sacs are spaced apart and open independently.

The tissue between the locules and the cells is called the connective and the parenchyma. Both pollen sacs are separated by the stomium. When the anther is dehiscing, it opens at the stomium.

The outer cells of the theca form the epidermis. Below the epidermis, the somatic cells form the tapetum. These support the development of microspores into mature pollen grains. However, little is known about the underlying genetic mechanisms, which play a role in male sporo- and gametogenesis.

The thecal arrangement of a typical stamen can be as follows:

Divergent: both thecae in line, and forming an acute angle with the filament
Transverse (or explanate): both thecae exactly in line, at right angles with the filament
Oblique: the thecae fixed to each other in an oblique way
Parallel: the thecae fixed to each other in a parallel way

Zoology

In biology, the theca of follicle can also refer to the site of androgen production in females. The theca of the spinal cord is called the thecal sac, and intrathecal injections are made there or in the subarachnoid space of the skull.

In human embryogenesis, the theca cells form a corpus luteum after a Graafian follicle has expelled its secondary oocyte arrested in second meiosis.

Thecal shape is also important in graptolite and pterobranch taxonomy.

In dinoflagellates that are armoured their covering is made up of thecal plates.

In other taxonomic groups

In microbiology and planktology, a theca is a subcellular structural component out of which the frustules of diatoms and dinoflagellates are constructed.

References

^ Larry Hufford, "The origin and early evolution of angiosperm stamens" in The Anther: form, function, and phylogeny, William G. D'Arcy and Richard C. Keating (editors), Cambridge University Press, 1996, 351pp, p.60, ISBN 978-0-521-48063-5 (from Google Books)

Microbiology: Protistology: Protists

Former
classifications

Morphology

Archaeplastida

Chloroplastida	"green algae": Phycoplast Phragmoplast Flagellar apparatus
Glaucophytes:	Cyanelles Phycobilisomes
Red algae:	Pit connection Phycobilisomes

Hacrobia

Cryptophyta:	Mastigoneme Periplast
Haptophyte:	Coccolith Haptonema

Stramenopiles

General:	Mastigoneme
Diatoms:	Frustule
Brown algae:	Lamina Pneumatocyst

Alveolata

General:	Alveoli Trichocyst
Dinoflagellates:	Dinokaryon Dinocyst Theca
Ciliates:	Cilium Cirrus Macronucleus Micronucleus
Apicomplexans:	Rhoptry Apicoplast Microneme

Rhizaria

Phytomyxea	Cruciform division

Excavate

Kinetoplastids:	Kinetoplast Glycosome
Euglenoidea:	Periplast/pellicle

Amoebozoa

Dictyostelids:	Macrocyst Sorocarp

Opisthokonta

Choanoflagellates:	Collar of microvilli

General

Levels of organization	Unicellular Monadoid Amoeboid Coccoid Amoeboflagellate Colonial s.s. Colonial flagellated Tetrasporal/capsal/palmelloid Sarcinoid Coenobial Filamentous/trichal/hyphal Parenchymatous Pseudoparenchymatous/plektenchymatic Membranous/thalloid/foliaceous Multinucleated Syncytial Coenocytic Siphonous Siphonocladous Multicellular s.s./tissular/histonal
Cell surface structures	Simple cell membrane Mucilage Scale Frustule Cell wall Lorica Skeleton Test Theca Periplast/pellicle
Locomotion	Flagellum Cilium Pseudopodia Gliding motility
Mitochondria	Hydrogenosome Mitosome
Nucleus	Nucleomorph Multinucleate cells Plasmodium Dikaryon Heterokaryon Mitosis in protists Open Closed Orthomitosis Pleuromitosis Meiosis in protists Gametic Zygotic Sporic
Other	Cyst Cytostome Fimbriae Extrusome Contractile vacuole Eyespot apparatus Pyrenoid Axostyle Mastigont system

Ecology and
physiology

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