Questions? AskAuckland

NZ Plants


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

(photo, John Braggins)

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) .

More on this group of ferns 

Several sporangia

(photo, Bill and Nancy Malcolm)

Davallia tasmanii - The annulus of each sporangium is brown, the sporangium wall is transparent and the spores inside are blue. (Ultraviolet microscope

More on Davallia

In some ferns sporangia are not covered and are randomly arranged on the leaflet 

Fertile leaflet, lower surface with scattered sporangia
(photo, L Jensen)

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.

More on Todea

In other ferns the sporangia are uncovered but are in groups

Leaflet lower surface, unopened sporangia
photo, I MacDonald

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.

More on this group of ferns.

In most ferns sporangia are covered by an indusium. The  indusium may be circular, covering a cluster of sporangia. 

Young sori with indusium covering unopened sporangia
(photo, L Jensen)

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.

More on this group of ferns

The indusium may be elongated, covering a linear group of sporangia.  

Sori with indusium covering and mature sporangia
(photo, L Jensen)

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.

More on this group of ferns

Sporangia covered by leaf margin

Fertile leaflet, lower surface with marginal sori
photo, I MacDonald

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)

More on this group of ferns