Strobilus

A strobilus (plural: strobili) is a reproductive structure found in many land plants, formed when sporangia-bearing parts are tightly clustered along a stem. While strobili are commonly called cones, some botanists reserve the word "cone" specifically for the woody seed strobili of conifers. The structure has a central axis (which is anatomically a stem) around which modified leaves or stems are arranged in spirals or opposite pairs. Leaves that carry sporangia are known as sporophylls, while stems that bear sporangia are called sporangiophores.

Lycophytes

Among lycophytes, both modern classes—Lycopodiopsida and Isoetopsida—include species that produce strobili. In these cases, the side organs are microphylls (small leaves) that hold sporangia. In other lycophytes, ordinary leaves can act as sporophylls without forming organized strobili.

Sphenophytes

In the single living genus of sphenophytes, Equisetum, strobili have sporangiophores as their lateral organs. Evidence from development and from fossil relatives indicates that these sporangiophores are reduced stems, not leaves, and the sporangia sit at their tips.

Seed plants

For seed plants other than flowering plants, ovules and pollen develop on separate structures. Strobili that bear microsporangia are called microsporangiate strobili or pollen cones, while those bearing ovules are megasporangiate strobili or seed cones.

Cycads

Cycads are typically dioecious, meaning seed strobili and pollen strobili grow on different plants. The side organs of seed strobili are megasporophylls (modified leaves) that carry two to several ovules along their edges. Pollen strobili consist of microsporophylls, each of which may have dozens or hundreds of microsporangia on their lower surfaces.

The only living ginkgophyte, Ginkgo biloba, produces pollen strobili, but its ovules usually appear in pairs at the tip of a stem, not inside a strobilus. When more than two ovules occur in G. biloba, or when fossil species with many ovules are studied, it becomes clear that the paired ovules in the living species represent a highly reduced strobilus.

In conifers, pollen strobili resemble those of cycads and ginkgoes—they are made of microsporophylls with microsporangia on the underside—but are much smaller. Many conifer seed cones are compound strobili: the central stem produces bracts, and in the axil of each bract sits a cone scale. Morphologically, the cone scale is a reduced stem, and ovules develop on the upper surface of these scales.

The three genera of gnetophytes—Ephedra, Gnetum, and Welwitschia—are usually dioecious, though some Ephedra species are monoecious. Unlike conifers, which have simple pollen strobili and compound seed strobili, gnetophytes have compound strobili for both pollen and seeds. The seed strobili of Ephedra and Gnetum are reduced, with Ephedra producing just two ovules per strobilus and Gnetum only one.

In flowering plants, the flower is sometimes called a bisexual strobilus. Stamens contain microsporangia inside the anther, and ovules (enclosed in carpels) contain megasporangia. Magnolia has a particularly strobiloid flower, with all parts arranged in a spiral rather than in clear whorls. Some flowering plants have inflorescences that look like strobili—for example, catkins—but these are structurally more complex.

It is likely that strobili evolved independently in most, if not all, of these groups. This convergence is not surprising, since the strobilus form is one of the most compact ways to arrange lateral organs around a cylindrical axis, and concentrating reproductive parts in a strobilus may improve spore dispersal and nutrient distribution.

The word "strobilus" comes from the Ancient Greek στρόβιλος (strobilos), meaning "something that turns, twists, spins, or rolls into a rounded form." The Hebrew word for conifer cone, איצטרובל (itstrubal), is an ancient borrowing from Greek. Liddell & Scott list several related meanings—including hedgehogs, eggs, and the cone of fir or pine—as well as the tree itself.