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Phylum monilophyta: class psilotopsida (whisk ferns)

While most ferns form large leaves and branching roots, the whisk ferns , Class Psilotopsida, lack both roots and leaves, probably lost by reduction. Photosynthesis takes place in their green stems, and small yellow knobs form at the tip of the branch stem and contain the sporangia. Whisk ferns were considered an early pterophytes. However, recent comparative DNA analysis suggests that this group may have lost both vascular tissue and roots through evolution, and is more closely related to ferns.

 Photo shows a whisk fern with many green stems that have small knobs along their length.
The whisk fern Psilotum nudum has conspicuous green stems with knob-shaped sporangia. (credit: Forest&Kim Starr)

Phylum monilophyta: class psilotopsida (ferns)

With their large fronds, ferns are the most readily recognizable seedless vascular plants. They are considered the most advanced seedless vascular plants and display characteristics commonly observed in seed plants. More than 20,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist, shaded places. Ferns made their appearance in the fossil record during the Devonian period and expanded during the Carboniferous.

The dominant stage of the lifecycle of a fern is the sporophyte, which consists of large compound leaves called fronds. Fronds fulfill a double role; they are photosynthetic organs that also carry reproductive organs. The stem may be buried underground as a rhizome, from which adventitious roots grow to absorb water and nutrients from the soil; or, they may grow above ground as a trunk in tree ferns ( [link] ). Adventitious organs are those that grow in unusual places, such as roots growing from the side of a stem.

 Photo shows a potted tree fern.
Some specimens of this short tree-fern species can grow very tall. (credit: Adrian Pingstone)

The tip of a developing fern frond is rolled into a crozier, or fiddlehead ( [link] a and [link] b ). Fiddleheads unroll as the frond develops.

Fiddleheads at the top of a maturing fern curl into a structure that resembles their namesake.
Croziers, or fiddleheads, are the tips of fern fronds. (credit a: modification of work by Cory Zanker; credit b: modification of work by Myriam Feldman)

The lifecycle of a fern is depicted in [link] .

Art connection

 The fern life cycle begins with a diploid (2n) sporophyte, which is the fern plant. Sporangia are round bumps that occur on the bottom of the leaves. Sporangia undergo mitosis to form haploid (1n) spores. The spores germinate and grow into a green gametophyte 1n that resembles lettuce. The gametophyte contains antheridia that produce, sperm and archegonia that produce eggs. Inside the archegonium the sperm fertilizes the egg, forming a diploid (2n) zygote. The zygote undergoes mitosis to form a 2n sporophyte, ending the cycle.
This life cycle of a fern shows alternation of generations with a dominant sporophyte stage. (credit "fern": modification of work by Cory Zanker; credit "gametophyte": modification of work by "Vlmastra"/Wikimedia Commons)

Which of the following statements about the fern life cycle is false?

  1. Sporangia produce haploid spores.
  2. The sporophyte grows from a gametophyte.
  3. The sporophyte is diploid and the gametophyte is haploid.
  4. Sporangia form on the underside of the gametophyte.

To see an animation of the lifecycle of a fern and to test your knowledge, go to the website .

Most ferns produce the same type of spores and are therefore homosporous. The diploid sporophyte is the most conspicuous stage of the lifecycle. On the underside of its mature fronds, sori (singular, sorus) form as small clusters where sporangia develop ( [link] ).

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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