Several other small plant groups that share many characteristics in common with the true ferns are often collectively referred to as the fern allies. Members of the divisions Psilotophyta (whisk ferns), Sphenophyta (horsetails), and Lycophyta (clubmosses and spikemosses) are all spore-producing vascular land plants with a dominant sporophyte generation. In this week's lab, you will be introduced to members of the Psilotophyta and Sphenophyta; next week, we will cover the Lycophyta along with the gymnosperms, or cone-bearing plants.

Division Psilotophyta

Members of the Division Psilotophyta (also sometimes spelled Psilophyta) are uncommon, unusual-looking plants. The whisk fern (genus Psilotum)occurs in the southeastern United States and may occasionally be found in Texas. The plant body of Psilotumconsists of a dichotomously-branched aerial stem and a subterranean rhizome that produces rhizoids. True leaves are lacking, although the stem bears small, scale-like appendages termed enations. The sporangia are borne near the tips of the branches and are compound in structure, each consisting of three fused sporangia. These compound sporangia are called synangia. An illustration of the stems and synangia of Psilotumis provided below:
 

Dichotomously branching stems of Psilotum nudum, showing the three-lobed synangia and enations.

Division Sphenophyta

Members of the Division Sphenophyta are also unusual-looking plants, although they are not as uncommon as are the whisk ferns. The division comprises a single living genus (Equisetum)with about 25 species that are commonly known as horsetails or scouring rushes. The plants can frequently be found growing along stream and river margins in Texas.

The stems of horsetails are ribbed and have a segmented appearance. The leaves are scale-like, non-photosynthetic, and produced in whorls at the nodes of the stem. As in Psilotum,the stems are the principal photosynthetic organ of the plant body. Some species of Equisetumproduce two different kinds of aerial stems. The sterile shoots are green, photosynthetic, and produce whorls of branches at the nodes, but do not produce any reproductive structures. The fertile shoots are brown and non-photosynthetic, and usually produce a single terminal strobilus or cone-like structure.
 

Photosynthetic stems of Equisetumshowing the whorled branches produced at each node and non-photosynthetic stems bearing terminal strolbili.	Close-up of the terminal strobilus of Equisetum.

The gametophytes of both Psilotumand Equisetumare bisexual, like those of the true ferns. Each gametophyte produces both antheridia and archegonia. Flagellated sperm cells fertilize the eggs, and the diploid zygote formed as a result of fertilization develops into the next sporophyte generation.
 

This page last updated: 09 Nov 1999