FERN SPORANGIA
The sporangia are on the lower surface of the leaf. Sporangia may be scattered over the entire surface of the leaf (diffuse arrangement), or localized in groups called sori. The sori may be naked (without a covering of tissue called an indusium), or covered with an indusium (refer to Raven 7th, p. 394, Fig. 17-28; 8th, p. 421, Fig. 17-33).
Below is an example of indusiate sori. The indusium is a thin membrane which covers the sorus, after a time it withers away.
Here is a close up of a sorus showing you the sporangia. You can see the chain of cells which run in a line over the sporangium. This is the annulus which you have heard so much about.
Here is a picture of sori with indusia which are umbrella shaped (peltate).
Some indusia are pocket shaped.
This picture illustrates more “pocket-type” indusia.
In this fern (Adiantum) the protective covering over the sori is called a false indusium because it is actually the margin of the leaf that overarches the sorus.
Not all sori have indusia. This is a picture of a sorus of Polypodium. It has a “naked sorus” because it is without an indusium.
Here you can see sporangia a little closer.
An even closer look. Note the row of thick walled cells (annulus). These cells function to disperse the spores by opening and rapidly closing the sporangium. You can probably observe this by placing a piece of fertile leaf on a slide and letting it dry out.
SPOROPHYTE INTRO
FRONDS
RHIZOME
SPORANGIA