Anatomy of ferns, life-cycle and biodiversity (with photos, some from a microscope)

When I studied biology at the University of Montreal, I had the pleasant opportunity to take classes at the Botanical Garden, including one class on Plants Anatomy and Morphogenesis. One of the subjects which interested me most is the weird and wonderful class of organisms called Polypodiopsida, or ferns.

Polypodiopsida anatomy and biodiversity

Let's begin with anatomy. This is the typical shape of fern leaves. The leaf, or frond, actually includes everything that is connected to the stalk. In this photo, each pinna (or leaflet) holds about 20 sori (the seed-like structure).

Magnification of a sorus (singular of sori), which is a regroupment of many sporocarps, the structure which produces spores.

Magnification of a sporocarp.

Cute little fern spores! Spores are quite different from seeds, even though their function is the same. Seeds are really an independant organism, containing all the nutrients and protection it needs to survive, and the genetic material needed to form an organism similar to its parent. Spores are much simpler and smaller, being mostly a shell containing cytoplasm and haploid genetic material (meaning it has only half the number of chromosomes that its parent had). Being haploids, spores when growing produce an organism (a gametophyte) that's very different from it's parent (the sporophyte). This process is called alternance of generation.

^{CC photo of a gametophyte with a growing sporophyte, from Wikipedia}
The small gametophyte, growing from a single spore, is anchored in the earth through rhizoids, and produces eggs and/or sperms (usually both in the same organism). When contact occurs between the haploid egg and haploid sperm, a diploid sporophyte grows directly on the gametophyte. The latter usually dies soon after, and the fern grows.

Rather than growing in a linear way like most modern plants, ferns grow through a process called circinate vernation. The growing plant is curled in a fiddlehead, thus protecting its most fragile part, and unfolds as it grows. (bonus info: fiddleheads are delicious! Look for them in spring in mosts woods.. or supermarkets)

On this photo you can see the much bigger fiddlehead of a tree fern!

These trees may seem strange and unusual today, but they used to be very abundant. In the Devonian era (about 400 million years ago), ferns and their ancestors were the first type of plants to form trees and had no significant competition, so they were the dominant species of the earliest forests of Earth. Until spermatophytes (plants who produces seed), in the Permian era (275 million years ago) evolved the ability to form trees as we know them today, some of the earliest being of the ginkgo division and gymnosperms (ancestors of pine trees).

As you guessed these grow very differently than regular trees. Rather than add layer of lignin-rich cells in its stalk and thus adding girth from the inside-out like modern trees do, tree ferns get bigger and support themselves by adding successive layers of lignified above-ground "roots", which grows from the top of the tree.

Another survival strategy of many tropical species of ferns is to grow like vines; instead of investing nutrients to make a strong stalk, they anchor themselves on other trees or rocks, allowing them to quickly reach sunny places and dedicate most of their energy to leaf-growing and reproduction. Plants such as these which use another plant to support themselves are called epiphytic.

And finally these are the puzzle-shaped cells of a fern leaf epidermis. The lip-like structures are stomata, the opening through which plants breathe (which can be closed ie. when the plant is in danger of dessication). This aspect of fern is very similar to that of angiosperms (flower-producing plants).

That is all, I hope you learned something of interest, or at least appreciated the photos!

Thanks for reading.

-@a0i

^{(all photos are mine except the one of the gametophyte. Shot at Montreal's Botanical Garden and at a laboratory of the Institut de Recherche en Biologie Végétale)}