Station 7: Clade Tracheophyta - Class Polypodiopsida (Whisk Ferns, Ferns and Horsetails)

The Class Polypodiopsida is composed of the horsetails, whisk ferns, and ferns, with most of the roughly 12,000 Polypodiopsid species being ferns.

The Polypodiopsids are believed to be the sister group to the seed plants. Ferns are different from mosses in that they have true leaves, roots and vascular tissue.  They still retain some of their ancestral characteristics, however, like flagellated sperm.

For the ferns we are using a variety of ferns as well as the specific genus, Polypodium, to represent the anatomical features of this group.  For horsetails you will study the genus Equisetum and for Whisk Ferns you will examine Psilotum.

A fern is a member of a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having certain tissue that conducts water and nutrients, and having branched stems. Like other vascular plants, ferns have leaves. Most ferns are termed "true ferns"; they produce what are called "fiddleheads" that uncoil and expand into fronds. The group includes about 10,560 known extant species.

Fern fronds may develop spores, in structures called sori, on their underside and these fronds are called a sporophylls.  A sorus (pl. sori) is a cluster of sporangia (structures producing and containing spores). These form a yellowish or brownish mass on the edge or underside of the sporophyll. In some species, they are protected during development by a scale or film of tissue called the indusium, which forms an umbrella-like cover.

Sori occur on the sporophyte generation, the sporangia within producing haploid meiospores. As the sporongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores.  The shape, arrangement, and location of the sori are often valuable clues in the identification of fern taxa. The presence or absence of indusium is also used to identify fern taxa.

Polypodium Life Cycle

Now that you have examined the gametophyte and sporophyte of a fern it is time to look at it as part of a whole in the life cycle.  Like the mosses, club mosses and all of the other plants, ferns go through an alternation of generations life cycle.  This means you will find the same stages but they are now modified from what the mosses have but are similar to the Lycopodiopsids in many respects.  The sporophyte is the most conspicuous part of the life cycle and is considered the dominant stage.   

Horsetail, (genus Equisetum), also called scouring rush, is the only living genus of plants in the class Equisetopsida. Horsetails grow in moist, rich soils in all parts of the world except Australasia. Some species produce two kinds of shoots: those with conelike clusters (strobili) of spore capsules and those lacking such structures. Some are evergreen; others send up new shoots annually from underground root stalks. Their hollow, jointed, ridged stems contain silicate and other minerals. The leaves are reduced to sheaths that clasp and encircle the shoots.

Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests. Some equisetids were large trees reaching to 30 m (98 ft) tall. 

Clade Tracheophyta: Phylum Tracheophyta: Class Polypodiopsida: Genus Equisetum (Horsetail)

You can find Equisetum plants usually in large numbers in ditches or along the edge of collected water (ponds, rivers, etc.). 

Equisetum Strobilis

You have already examined strobili in the Lycopods and know that they are an important part of the story of plant evolution.  However, the Equisetum strobilus poses an interesting question. Is this strobilus just a continuation of the evolution of the strobilus from the lycopods or is it convergent evolution?  The question has to be asked because if lycopods are ancestral to the polypods (members of the polypodiopsids) then why do Psilotum and Polypodium not have a strobilis?  

Evidence that this is convergent evolution is that the sporangia in the Equisetum strobilus form at the base of stems while those in lycopods develop at the base of leaves (microphylls).

Current research suggests that the strobilus first developed in the lycopods but may have independently arose in several groups as an environmental response for efficient spore dispersal.