Monday, 13 April 2015

A Closer Look at a Fern

.....specifically, the Sori and Sporangia.

I spent some time photographing a fern today for the purpose of learning more about these pteridophytic plants. I particularly wanted to have a closer look at the little spore containing receptacles that are found on on the underside of fertile fronds.

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. These capsules open when mature. The microscopic spores are then released to be spread by the wind.

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