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