strobilus

A strobilus (plural: strobili) is a structure present on many land plant species consisting of sporangia-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) surrounded by spirally arranged or decussate structures that may be modified leaves or modified stems.

Leaves that bear sporangia are called ''sporophylls'', while sporangia-bearing stems are called '' sporangiophores''.

Lycophytes

Some members of both of the two modern classes of Lycopodiophyta (Lycopodiopsida and Isoetopsida) produce strobili. In all cases, the lateral organs of the strobilus are microphylls, bearing sporangia. In other lycophytes, ordinary foliage leaves can act as sporophylls, and there are no organized strobili.

Diphasiastrum complanatum strobilus (01).jpeg, Strobili of ''Diphasiastrum''
Lycopodium saururus.JPG, Strobili of ''Lycopodium''
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 strobilus wm.jpg, Liquid-preserved strobili of ''Selaginella'', showing mega- and microsporangia through translucent sporophylls

Sphenophytes

The single extant genus of Equisetophyta, ''Equisetum'', 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 ovules and pollen 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 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'', seed cone
Image:Cycas circinalis - sago palm - desc-top of trunk.jpg, ''Cycas circinalis'', pollen cone
Image:Cycas microsporangia.jpg, Abaxial surface of ''Cycas revoluta'' microsporophyll, showing microsporangia

Ginkgos

The single living member of the Ginkgophyta, ''Ginkgo biloba'' 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 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 consists of three genera, ''Ephedra'', ''Gnetum'' and ''Welwitschia''. 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'' seed cone.
Image:Gnetum gnemon BotGardBln1105C.JPG, Seed cones of ''Gnetum''

Flowering plants

The flower of flowering plants is sometimes referred to as a bisexual strobilus. Stamens include microsporangia within the anther, and ovules (contained in carpels) contain megasporangia. Magnolia has a particularly strobiloid flower with all parts arranged in a spiral, rather than as clear whorls.

A number of flowering plants have inflorescences that resemble strobili, such as catkins, but are actually more complex in structure than strobili.

Image:Alnus rubra 9819.JPG, Staminate catkins of alder
Image:Alnus rubra 0020.JPG, Pistillate catkins of alder
Image:Casuarina cunninghamiana fruit and leaves01.jpg, Pistillate catkins of ''Casuarina''

Evolution of strobili

It is likely that strobili evolved independently in most if not all these groups. This evolutionary convergence 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 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.

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Plant morphology