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Biol 3400: Vascular Plant Taxonomy: Ferns

Polypodiophyta

Key words: Eusporangia; Leptosporangia; Megaphyllous

Members of the Polypodiophyta are "true ferns". They form a much more diverse group than the Lycopodiophyta or Sphenophyta - even taking into account the extinct members of each group. What distinguishes the Polypodiophyta from other groups?

• The leaves are megaphyllous
• The sporangia are borne on sporangiophores or abaxially on the leaves
• The spores form free-living, photosynthetic gametophytes
• The stem is usually horizontal and subterranean and often highly reduced, but in tree ferns it becomes vertical and columnar
• In all but two genera, all growth is from primary meristems; there is no secondary growth. Oddly, neither of the two exceptional genera is treelike.

Fern leaves are frequently referred to as fronds. The central axis is the rachis, but the portion below the blade is often called a petiole or stipe. In a compound leaf, the subdivisions are called pinnae. The ultimate divisions of a pinna are called pinnules. This use of special terminology for the leaves of ferns is annoying, but old folk like myself coped with it. I am sure you are equally capable of doing so. I could omit it from this course, but you would not be able to use any references other than those designed specifically for this course, a major handicap.

There are several distinctive groups with the true ferns. The biggest division is into the eusporangiate and leptosporangiate ferns. These terms refer to two different patterns of sporangial development.

Eusporangiate ferns

There are two orders of eusporangiate ferns, the Ophioglossales and Marattiales. It is generally thought that they are more like the ancestral ferns (or pre-ferns) than the leptosporangiate fern orders, but there are respectable taxonomists that think the leptosporangiate ferns are more like the ancestral ferns. There is also support for the idea that the two orders may be more closely related to Cycads than to other ferns.

Eusporangiate ferns in Utah

There is one genus of eusporangiate fern in Utah, Botrychium. It is a member of the family Ophioglossaceae in the order Ophioglossales. There are two other genera in the family. All three have erect vernation and bear their sporangia on stalked spikes or sporangiophores that are attached to the petiole just below the blade.

In Botrychium, the blade is pinnately divided and the venation is reticulate. This last means that the veins form a closed circuit. In both the other genera the venation is open, i.e., the veins simply end in the blade chlorenchyma. One of the other genera has ternately divided blades, the other simple blades.

Taxonomic and key characters for the Ophioglossaceae

Which of the characters mentioned above would be good taxonomic characters for the Ophioglossaceae? Which would be good key characters? Be prepared to defend your answer!

Marattiales

The Marattiales are tree ferns. They are eusporangiate, but the sporangia are located abaxially on the fronds, as in the leptosporangiate ferns with which you are probably more familiar.

Leptosporangiate ferns

Leptosporangiate ferns are far more common than eusporangiate ferns nowadays, particularly at latitudes such as ours. In the 'good old days', most leptosporangiate ferns were put into the family Polypodiaceae. Over the last 2-3 decades, pteridologists (people that study ferns) have become convinced that many, many more families should be recognized. The evidence comes from consideration of a large number of characters, many of which are not very useful as key or diagnostic characters in the field despite their value as taxonomic characters. You need to make a practice of keying ferns using either the hand out that I shall give you (or you may download the Acrobat file here) or, if you are outside northern Utah, the second volume of Bryophyte Flora North America. The following pictures illustrate circinate vernation, a false indusium, real indusia, pinnules, and bracts on on true fern. Be sure you know which is what! (Click on the pictures to enlarge them.)