Strobilus

A strobilus (plural strobili) is an organ of many plants that contains the reproductive structures. Strobili are ordinarily called cones in many of these groups. Strobili are characterized by a central axis (anatomically a stem) surrounded by spirally arranged or decussate structures that bear sporangia or other reproductive parts.

The nature of the lateral organs varies among different plant lineages, being derived from either leaf or stem. Leaves that bear sporangia are often called "sporophylls", whereas stems are called "sporangiophores".

Lycophytes

Some members of two of the three modern classes of Lycopodiophyta (the Lycopodiopsida and the Selaginellopsida 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.

phenophytes

The single extant genus of the 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.

eed 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".

Cycads

Plants of the Cycadophyta are ordinarily dioecious; seed cones and pollen cones are produced on separate plants. The lateral organs of seed cones are megasporophylls, modified leaves that bear two to several marginal ovules. Pollen cones possess microsporophylls, each of which may have dozens or hundreds of abaxial microsporangia.

Ginkgos

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

Conifers

Pollen cones of the 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 espedially notable 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 itself is a reduced strobilus. Ovules are produced on the adaxial surfaces of the cone scales. There are no megasporophylls.

Gnetophytes

The phylum Gnetophyta consists of three genera, "Ephedra", "Gnetum", and "Welwitschia". All three are ordinarily dioecious, although some "Ephedra" species exhibit monoecy. In contrast to the conifers, which often have compound seed cones, gnetophytes have compound pollen cones. The seed cones of "Ephedra" and "Gnetum" are very reduced, with two and one ovules, respectively, per cone.

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.

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.

References

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