A Closer Look at a Fern

…..specifically, the Sori and Sporangia.

Have you ever noticed those little spots on the underside of a fern frond. Most of you will be aware that they carry the spores that go on to make new ferns. A while back I spent some time photographing ferns for the purpose of learning more about these pteridophytic plants. I particularly wanted to have a closer look at these little spore containing receptacles.

_{Fronds of the Kangaroo Fern – Microsorum pustulatum}

The Kangaroo Fern, Microsorum pustulatum, in the above photo is growing on the trunk of a Tree Fern, Dicksonia antarctica. If you look closely at the enlarged photo you can see the rhizomes working their way through the fibrous trunk of the Tree Fern. On the upper surface of the fronds you can see little dimples marking the locations of the reproductive structures which are on the underside. These are the spore containing structures that I wanted to have a closer look at.

Turning the frond over we can see these raised spots arranged along the underside. These are known as sori, or in the singular, a sorus. On this specimen the sori are about 4mm in diameter. . So where are the actual spores? Let’s look closer!

_{Each sorus is tightly packed with sporangia}

This photo shows that the sorus is actually tightly packed with little capsules known as sporangia (singular, sporangium). Notice that the sporangia are open to the air. In some ferns there is a little cap that covers them until they are mature. The cap is known as an indusium. The presence or absence of an idusium can be a useful feature in identifying ferns. Another useful feature for identifying ferns is the shape and position of the sori.

_{Closer view of the sporangia}

Getting up close you can see that each sporangium is a little capsule. The capsules in the above photo are slightly transparent and if you look closely you can see some of the ovoid spores within. These capsules open when mature. The microscopic spores are then released to be spread by the wind.

So, next time you see some ferns, have a closer look at those spots (sori) that are found on the fertile fronds.

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