Astrobilus (pl.:strobili) is a structure present on many landplant species consisting ofsporangia-bearing structures densely aggregated along a stem. Strobili are often calledcones, 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 astem) surrounded by spirally arranged ordecussate structures that may be modified leaves ormodified stems.
Leaves that bear sporangia are calledsporophylls, while sporangia-bearing stems are calledsporangiophores.
Some members of both of the two modern classes ofLycopodiophyta (Lycopodiopsida andIsoetopsida) produce strobili. In all cases, the lateral organs of the strobilus aremicrophylls, bearingsporangia. In other lycophytes, ordinary foliage leaves can act assporophylls, and there are no organized strobili.
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The single extant genus ofEquisetophyta,Equisetum, produces strobili in which the lateral organs aresporangiophores. Developmental evidence and comparison with fossil members of the group show that the sporangiophores are reduced stems, rather than leaves. Sporangia are terminal.
With the exception of flowering plants, seed plants produceovules andpollen in different structures. Strobili bearing microsporangia are calledmicrosporangiate strobili orpollen cones, and those bearing ovules aremegasporangiate strobili orseed cones (orovulate cones).
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.
The single living member of theGinkgophyta,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 inG. 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.
Pollen strobili ofPinophyta 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 arecompound strobili. The central stem produces bracts and in the axil of each bract is acone scale. Morphologically the cone scale is a reduced stem. Ovules are produced on the adaxial surface of the cone scales.
Gnetophyta consists of three genera,Ephedra,Gnetum andWelwitschia. All three are typically dioecious, although someEphedra 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 ofEphedra andGnetum are reduced, withEphedra producing only two ovules per strobilus andGnetum a single ovule.
Theflower offlowering plants is sometimes referred to as abisexual strobilus.Stamens includemicrosporangia within theanther, andovules (contained incarpels) containmegasporangia.Magnolia has a particularly strobiloid flower with all parts arranged in a spiral, rather than as clear whorls.
A number of flowering plants haveinflorescences that resemble strobili, such ascatkins, but are actually more complex in structure than strobili.
It is likely that strobili evolved independently in most if not all these groups. This evolutionaryconvergence 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 sporedispersal and nutrient partitioning.
The wordstrobilus is related to the ancient Greekstrobilos = whirlwind. TheHebrew 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... 6. the cone of the fir or pine, fir-apple, pine-cone,… also of the tree itself.[1]
