Strobilus

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

Main article: Conifer cone

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

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.

According to Liddell & Scott, the Greek: strobilos (στρόβιλος) had many meanings, generally of anything twisted up...hence of the hedgehog,... of an egg-shell,... as a name of various twisted or spinning objects. For example:

1. a kind of seasnail...
2. a top...
3. a whirlpool, a whirlwind which spins upwards...
4. the cone of the fir or pine, fir-apple, pine-cone,… also of the tree itself.

References

• Gifford, E. M. & Foster, A. S. (1988). Comparative morphology of vascular plants, 3rd ed. New York: WH Freeman.

References

1. Henry George Liddell, Robert Scott, assisted by Prof. Henry Drisler of Columbia College: Greek-English Lexicon, 8th ed. 1897 p. 1439. Pub:Harper New York 1882.