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A strobilus (plural: strobili) is a structure present on many land
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
species consisting of
sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...
-bearing structures densely aggregated along a stem. Strobili are often called cones, but some botanists restrict the use of the term cone to the woody seed strobili of conifers. Strobili are characterized by a central axis (anatomically a
stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...
) surrounded by spirally arranged or
decussate Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term chiasma is named aft ...
structures that may be modified leaves or modified stems. Leaves that bear sporangia are called ''
sporophylls A sporophyll is a leaf that bears sporangia. Both microphylls and megaphylls can be sporophylls. In heterosporous plants, sporophylls (whether they are microphylls or megaphylls) bear either megasporangia and thus are called megasporophylls, or ...
'', while sporangia-bearing stems are called '' sporangiophores''.


Lycophytes

Some members of both of the two modern classes of
Lycopodiophyta The lycophytes, when broadly circumscribed, are a vascular plant (tracheophyte) subgroup of the kingdom Plantae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest ...
(
Lycopodiopsida Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
and
Isoetopsida Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
) produce strobili. In all cases, the lateral organs of the strobilus are
microphyll In plant anatomy and evolution a microphyll (or lycophyll) is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of ...
s, bearing
sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...
. In other lycophytes, ordinary foliage leaves can act as
sporophyll A sporophyll is a leaf that bears sporangia. Both microphylls and megaphylls can be sporophylls. In heterosporous plants, sporophylls (whether they are microphylls or megaphylls) bear either megasporangia and thus are called megasporophylls, or ...
s, and there are no organized strobili. Diphasiastrum complanatum strobilus (01).jpeg, Strobili of ''
Diphasiastrum ''Diphasiastrum'' is a genus of clubmosses in the plant family Lycopodiaceae. In the Pteridophyte Phylogeny Group classification of 2016 (PPG I), it is placed in the subfamily Lycopodioideae. It is closely related to the genus '' Lycopodium'' ...
'' Lycopodium saururus.JPG, Strobili of ''
Lycopodium ''Lycopodium'' (from Greek ''lykos'', wolf and ''podion'', diminutive of ''pous'', foot) is a genus of clubmosses, also known as ground pines or creeping cedars, in the family Lycopodiaceae. Two very different circumscriptions of the genus are i ...
'' Lycopodium strobilus wm.jpg, Liquid-preserved strobili of ''Lycopodium'', showing reniform sporangia through translucent sporophylls Lycopodium strobilus mosbo6.jpg, Micrograph of the strobilus of ''Lycopodium'' sp., showing spores borne in sporangia Selaginella pilifera0.jpg, Strobili of ''
Selaginella ''Selaginella'' is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses. This family is distinguished from Lycopodiaceae (the clubmosses) by having scale-leaves bearing a ligule and by having ...
'' Selaginella strobilus wm.jpg, Liquid-preserved strobili of ''Selaginella'', showing mega- and microsporangia through translucent sporophylls


Sphenophytes

The single extant genus of
Equisetophyta Equisetidae is one of the four subclasses of Polypodiopsida (ferns), a group of vascular plants with a fossil record going back to the Devonian. They are commonly known as horsetails. They typically grow in wet areas, with whorls of needle-lik ...
, ''
Equisetum ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass Eq ...
'', produces strobili in which the lateral organs are sporangiophores. Developmental evidence and comparison with fossil members of the group show that the sporangiophores are reduced stems, rather than leaves. Sporangia are terminal. Image:Equisetum_hyemale.jpg, Strobilus of ''Equisetum'' Image:Equisetum sylvaticum 240405.jpg, Strobilus of ''Equisetum'' Image:Equisetum strobilus wm.jpg, Liquid-preserved strobilus of ''Equisetum'', showing sporangiophores Image:Equisetum strobilus xs of wm showing sporangiophores.jpg, Cross-section of liquid-preserved strobilus of ''Equisetum'', showing sporangiophores bearing sporangia


Seed plants

With the exception of flowering plants, seed plants produce
ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the fe ...
s and
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
in different structures. Strobili bearing microsporangia are called ''microsporangiate strobili'' or ''pollen cones'', and those bearing ovules are ''megasporangiate strobili'' or ''seed cones'' (or ''ovulate cones'').


Cycads

Cycadophyta Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants o ...
are typically dioecious (seed strobili and pollen strobili are produced on separate plants). The lateral organs of seed strobili are megasporophylls (modified leaves) that bear two to several marginal ovules. Pollen strobili consist of microsporophylls, each of which may have dozens or hundreds of abaxial microsporangia. Image:Dioon edule04.jpg, ''
Dioon edule ''Dioon edule'', the chestnut dioon, is a cycad native to Mexico, also known as ''palma de la virgen''. Cycads are among the oldest seed plants and even pre-date the dinosaurs.The United States Botanical Garden It belongs to the Zamiaceae plant ...
'', seed cone Image:Cycas circinalis - sago palm - desc-top of trunk.jpg, ''
Cycas circinalis ''Cycas circinalis'', also known as the queen sago, is a species of cycad known in the wild only from southern India. ''Cycas circinalis'' is the only gymnosperm species found among native Sri Lankan flora. Taxonomy ''C. circinallis'' is native ...
'', pollen cone Image:Cycas microsporangia.jpg, Abaxial surface of ''
Cycas revoluta ''Cycas revoluta'' (Sotetsu apanese ソテツ sago palm, king sago, sago cycad, Japanese sago palm) is a species of gymnosperm in the family Cycadaceae, native to southern Japan including the Ryukyu Islands. It is one of several species used f ...
'' microsporophyll, showing microsporangia


Ginkgos

The single living member of the
Ginkgophyta Ginkgoales are a gymnosperm order containing only one extant species: ''Ginkgo biloba'', the ginkgo tree. It is monotypic, (the only taxon) within the class Ginkgoopsida, which itself is monotypic within the division Ginkgophyta . The order inc ...
, ''
Ginkgo biloba ''Ginkgo biloba'', commonly known as ginkgo or gingko ( ), also known as the maidenhair tree, is a species of tree native to China. It is the last living species in the order Ginkgoales, which first appeared over 290 million years ago. Fossils ...
'' produces pollen strobili, but the ovules are typically borne in pairs at the end of a stem, not in a strobilus. When there are more than a pair of ovules in ''G. biloba'', however, or when fossil taxa bearing large numbers of ovules are examined, it is clear that the paired ovules in the extant species are a highly reduced strobilus. Image:Ginkgo biloba Inflorescences.jpg, Pollen cones of ''Ginkgo'' Image:Ginkgo pollen cone wm.jpg, Pollen cone of ''Ginkgo'', showing microsporophylls each with two microsporangia


Conifers

Pollen strobili of
Pinophyta Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extan ...
are similar to those of cycads (although much smaller) and Ginkgoes in that they are composed of microsporophylls with microsporangia on the abaxial surface. Seed cones of many conifers are ''compound strobili''. The central stem produces bracts and in the axil of each bract is a ''cone scale''. Morphologically the cone scale is a reduced stem. Ovules are produced on the adaxial surface of the cone scales.


Gnetophytes

Gnetophyta 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 rel ...
consists of three genera, ''
Ephedra Ephedra may refer to: * Ephedra (medicine), a medicinal preparation from the plant ''Ephedra sinica'' * ''Ephedra'' (plant), genus of gymnosperm shrubs See also * Ephedrine Ephedrine is a central nervous system (CNS) stimulant that is of ...
'', ''
Gnetum ''Gnetum'' is a genus of gymnosperms, the sole genus in the family Gnetaceae within the Gnetophyta. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms, they possess vessel elements in the xylem. Some species have been ...
'' and ''
Welwitschia ''Welwitschia'' is a monotypic gymnosperm genus, comprising solely the distinctive ''Welwitschia mirabilis'', endemic to the Namib desert within Namibia and Angola. ''Welwitschia'' is the only living genus of the family Welwitschiaceae and orde ...
''. All three are typically dioecious, although some ''Ephedra'' species exhibit monoecy. In contrast to the conifers, which have simple pollen strobili and compound seed strobili, gnetophytes have both compound pollen and seed strobili. The seed strobili of ''Ephedra'' and ''Gnetum'' are reduced, with ''Ephedra'' producing only two ovules per strobilus and ''Gnetum'' a single ovule. Image:Welwitschia-bug Probergrothius-Angolensus.jpg, Seed cones of ''Welwitschia'' Image:XN Welwitschia mirabilis 02.jpg, Pollen cones of ''Welwitschia'' Image:Ephedra pollen cones.jpg, Pollen cones of ''Ephedra'' Image:Ephedra pollen cone wm.jpg, Pollen cone of ''Ephedra'' showing microsporangia Image:Female cone of Ephedra intermedia - journal.pone.0053652.g002-C.png, ''
Ephedra intermedia ''Ephedra intermedia'', with the Chinese common name of Zhong Ma Huang, is a species of '' Ephedra'' that is native to Siberia, Central Asia, Iran, Afghanistan, Pakistan, the western Himalayas, Tibet, Mongolia, and China. Description ''Ephedra i ...
'' seed cone. Image:Gnetum gnemon BotGardBln1105C.JPG, Seed cones of ''Gnetum''


Flowering plants

The
flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...
of
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s is sometimes referred to as a
bisexual Bisexuality is a romantic or sexual attraction or behavior toward both males and females, or to more than one gender. It may also be defined to include romantic or sexual attraction to people regardless of their sex or gender identity, whi ...
strobilus.
Stamen The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filame ...
s include
microsporangia Microsporangia are sporangia that produce microspores that give rise to male gametophytes when they germinate. Microsporangia occur in all vascular plants that have heterosporic life cycles, such as seed plants, spike mosses and the aquatic fer ...
within the
anther The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filam ...
, and
ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the fe ...
s (contained in
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils'' ...
s) contain
megasporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life c ...
.
Magnolia ''Magnolia'' is a large genus of about 210 to 340The number of species in the genus ''Magnolia'' depends on the taxonomic view that one takes up. Recent molecular and morphological research shows that former genera ''Talauma'', ''Dugandiodendro ...
has a particularly strobiloid flower with all parts arranged in a spiral, rather than as clear whorls. A number of flowering plants have
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 o ...
s that resemble strobili, such as
catkin A catkin or ament is a slim, cylindrical flower cluster (a spike), with inconspicuous or no petals, usually wind-pollinated (anemophilous) but sometimes insect-pollinated (as in ''Salix''). They contain many, usually unisexual flowers, arranged cl ...
s, but are actually more complex in structure than strobili. Image:Alnus rubra 9819.JPG, Staminate catkins of
alder Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...
Image:Alnus rubra 0020.JPG, Pistillate catkins of alder Image:Casuarina cunninghamiana fruit and leaves01.jpg, Pistillate catkins of ''
Casuarina ''Casuarina'' is a genus of 17 tree species in the family Casuarinaceae, native to Australia, the Indian subcontinent, southeast Asia, islands of the western Pacific Ocean, and eastern Africa. It was once treated as the sole genus in the fami ...
''


Evolution of strobili

It is likely that strobili evolved independently in most if not all these groups. This evolutionary
convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen *Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that ...
is not unusual, since the form of a strobilus is one of the most compact that can be achieved in arranging lateral organs around a cylindric axis, and the consolidation of reproductive parts in a strobilus may optimize spore dispersal and nutrient partitioning.


Etymology

The word ''strobilus'' is related to the ancient Greek ''strobilos'' = whirlwind. The
Hebrew Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...
word for conifer cone, ''itstrubal'', is an ancient borrowing from the Greek.


References

* Gifford, E. M. & Foster, A. S. (1988). ''Comparative morphology of vascular plants'', 3rd ed. New York: WH Freeman. {{Authority control Plant morphology