A strobilus (plural strobili) is a structure present on many plant species consisting of leaf-like, sporangia-bearing structures densely aggregated along a stem. Strobili are often called cones, but many 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.

Contents 1 Lycophytes 2 Sphenophytes 3 Seed plants 3.1 Cycads 3.2 Ginkgos 3.3 Conifers 3.4 Gnetophytes 3.5 Flowering plants 4 Evolution of strobili 5 Etymology 6 References

[edit] 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 meiosporangia. In other members of both classes, ordinary foliage leaves (trophophylls) can act as sporophylls, and there are no organized strobili.

Strobili of Lycopodium

Liquid-preserved strobili of Lycopodium, showing reniform sporangia through translucent sporophylls

Strobili of Selaginella

Liquid-preserved strobili of Selaginella, showing mega- and microsporangia through translucent sporophylls.

[edit] Sphenophytes

The single extant genus of Equisetophyta, Equisetum, produces strobili in which the lateral organs are called sporangiophores. Developmental evidence and comparison with fossil members of the phylum show that the sporangiophores are reduced stems, rather than leaves.

Strobilus of Equisetum

Strobilus of Equisetum

Liquid-preserved strobilus of Equisetum, showing sporangiophores

Cross-section of liquid-preserved strobilus of Equisetum, showing sporangiophores bearing sporangia

[edit] 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).

[edit] 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 possess microsporophylls, each of which may have dozens or hundreds of abaxial microsporangia.

Dioon edule, seed cone

Cycas circinalis, pollen cone

Abaxial surface of Cycas revoluta microsporophyll, showing microsporangia

[edit] Ginkgos

The single living member of the Ginkgophyta, Ginkgo biloba produces pollen strobili, but the ovules are borne in pairs at the end of a stem, not in a strobilus.

Pollen cones of Ginkgo

Pollen cone of Ginkgo, showing microsporophylls each with two microsporangia

[edit] Conifers

Pollen strobili of Pinophyta are similar to those of cycads (although much smaller) and Ginkgoes in that they have microsporophylls with microsporangia on the abaxial surface. Seed cones of many conifers are compound strobili; the central stem produces bracts (free in Pinaceae, and especially prominent in Douglas-fir), but fused to the cone scales at maturity in many conifers; in the axil of each bract is a cone scale, which is a reduced stem. Ovules are produced on the adaxial surface of the cone scales.

[edit] 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.

Seed cones of Welwitschia	Pollen cones of Welwitschia	Pollen cones of Ephedra	Pollen cone of Ephedra showing microsporangia
Seed cones of Gnetum

[edit] 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) are 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, usually catkins, that resemble strobili, but are more complex in structure than strobili.

Staminate catkins of alder

Pistillate catkins of alder

Pistillate catkins of Casuarina

[edit] 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.

[edit] Etymology

The word strobilus is related to the ancient Greek strobilos = whirlwind.

[edit] References

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