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NZ Plants
Sporangium
Ferns are dispersed by air-borne spores which have a thick protective wall that surrounds a living cell. Large numbers of spores are formed inside spore sacs (sporangia) that are usually located on the underside of fronds. As a crest or file of cells (annulus) in the spore sac wall dries out, it contracts, slowly ripping the spore sac in half. A point is reached when the two halves rapidly come together again, throwing the spores out.
Detached sporangia
Pteris tremulus - An unopened sporangium (on the left) has a single file of cells (annulus) seen on its left edge. Contraction of the annulus rips the sporangium into two halves (on the right) .
Several sporangia
Davallia tasmanii - The annulus of each sporangium is brown, the sporangium wall is transparent and the spores inside are blue. (Ultraviolet microscope
In some ferns sporangia are not covered and are randomly arranged on the leaflet
Todea barbara, underside of frond - In some ferns the sporangia are randomly distributed across the leaflet surface. The sporangia of this fern are large and contain a large number of spores, indicating that this is a primitive fern.
In other ferns the sporangia are uncovered but are in groups
Sticherus cunninghamii, underside of frond - In other ferns the sporangia are in groups. Such a group is called a sorus. Each sorus in this more advanced fern consists of 4-5 small sporangia which produce a small number of spores.
In most ferns sporangia are covered by an indusium. The indusium may be circular, covering a cluster of sporangia.
Polystichum vestitum, underside of frond - In many ferns an umbrella-like covering called an indusium conceals each sorus. Here the outermost sporangia of each circular sorus project out from under the edges of each indusium.
The indusium may be elongated, covering a linear group of sporangia.
Deparia petersenii, underside of frond - In some ferns, the indusium is linear and attached to one side of a row of sporangia. In this fern, sori are in two rows at angles to each other.
Sporangia covered by leaf margin
Histiopteris incisa, underside of frond - In other ferns the sorus is in the form of a continuous line of sporangia that is covered by the reflexed leaf margin. (photo, Larry Jensen)
