Lycopods (Lycophyta)

The lycopods include a small group of modern plants (between 950 and 1,200 species) that have a long fossil history.  Within modern lycopods, there are three major clades (often recognized at the rank of order) that bear the common names clubmosses, spike mosses, and quillworts.

Unlike the mosses, liverworts, and hornworts the dominant generation of a lycopod is the sporophyte.  Lycopod sporophytes have true tissues (including the vascular tissues xylem and phloem), and are thus described as vascular plants.  They have true stems, leaves and roots.  Lycopod leaves have a different evolutionary history than all other leaves.  They evolved from extensions of the stems (enations) that became vascularized.  Such leaves are called microphylls.

Lycopod sporophytes reproduce by forming spores in sporangia that are situated on the upper (adaxial) surface of special leaves (sporophylls), near the leaf base.  Sporangia are borne one sporangium per sporophyll.  Sporophylls may be morphologically identical to vegetative microphylls, or alternatively may be very small (reduced) and clustered at the tips of stems to form a “cone” or strobilus.  Plants may be homosporous (forming just one type of spore) or heterosporous (forming two types of spores: smaller microspores and larger megaspores).  Spores are released to the environment (lycopods are free-sporing), and gametophytes live independently from the sporophytes.  Homosporous lycopods form bisexual gametophytes while heterosporous lycopods form male gametophytes (from microspores) and female gametophytes (from megaspores).

Gametophytes are morphologically variable, but are usually small and short-lived.

Monilophytes (Monilophyta)

The monilophyte clade is large (with about 12,000 species) and diverse.  It includes the familiar “filicalian” ferns, as well as tree ferns and heterosporous water ferns.  This group also includes the whisk ferns (12 species) and horsetails (15 species).  Monilophytes have a long fossil record, second only to the lycopods. Like the lycopods, these are free-sporing vascular plants with a dominant sporophyte generation and a free-living but reduced gametophyte division.  Most monilophytes have stems, leaves, and roots.

Monilophyte leaves are more complex than the simple lycopod microphylls, and are considered to have had a different evolutionary origin.  Monilophyte leaves (and the leaves of all vascular plants other than lycopods) evolved from lateral branching systems that became overtopped, planated (i.e. flattened), and webbed.  Such leaves are called megaphylls.

Ferns

Most familiar fern sporophytes have stems (usually called rhizomes) that bear roots and megaphyll-type leaves (called fronds).  The rhizomes may be subterranean or above ground.  The fronds may be small and simple, or large and complex.  Fern fronds often “unroll” as they mature (developing leaves are often referred to as fiddleheads).  Many fern fronds are subdivided into smaller units (leaflets).  Ferns are often grown ornamentally for their attractive foliage.

Sporangia are often formed on the lower (abaxial) surface of sporophylls.  Sporophylls may be morphology similar to vegetative leaves, or strikingly different.  Numerous sporangia are often clustered into a zone called a sorus (plural sori), and the sori are often protected by a flap of leaf tissue called the indusium (plural indusia).  In some ferns, the margin of leaflets is rolled or folded over the sporangia to make a “false indusium.”  Most ferns are homosporous.  Spores are released from the sporophyte and develop into small, free-living gametophytes.

Gametophytes are variable in their features, but are commonly small, flattened, photosynthetic and roughly heart-shaped.  Fern gametophytes attach to the substrate by rhizoids, and form the antheridia and archegonia on their lower surfaces (typically the archegonia close to the “notch” of the heart, and antheridia closed to the base).  Syngamy results in the formation of new sporophytes within the archegonia.  Initially the sporophytes are nourished by gametophyte, but soon put out their own leaves and roots, causing the destruction of the tiny gametophyte.

Whisk Ferns

Whisk ferns are a small group of simple monilophytes consisting of 12 species in two genera.  The only common species is Psilotum nudum.   The sporophyte of these plants lacks leaves and roots, consisting entirely of stems that bifurcate at regular intervals.  Stems bear small scale-like appendages (often called prophylls), but because they lack vascular tissue they are not considered to be true leaves.

Sporangia are borne along the aerial stems in groups of three that are fused into a single synangium.  Whisk ferns are homosporous.  Whisk fern gametophytes are subterranean and cylindrical and form mutualistic relationships with mycorrhizal fungi

Horsetails

There are 15 species of extant horsetails, all belonging to the genus Equisetum.  Although a small clade, they have a long fossil history extending back to Devonian time. Horsetail sporophytes have roots, stems, and leaves.  The stems are hollow and roughened with crystals of silica oxide.  Like ferns and whisk ferns, these plants spread by subterranean stems (rhizomes) and often form dense stands of individuals.  Leaves are tiny, often not photosynthetic, and fused together to form a grayish band around each node.  The stems are the photosynthetic organs of these plants.  All parts of horsetails are whorled, meaning that numerous leaves (fused into a band, as mentioned above) or branches are formed at each node. Sporangia are borne on structures called sporangiophores that form a strobilus at the tip of some branches.  All modern horsetails are homosporous.  Horsetail gametophytes are very similar to those of ferns.

Equisetum sporophytes fall into two morphological categories. One group of species forms unbranched, photosynthetic, aerial stems.  These stems are perennial, and stay green through the winter.  Pioneers referred to these as “scouring rushes.”

A second group of species is dimorphic, forming separate reproductive and vegetative stems that appear quite different.  The reproductive stems are formed early in the spring, and lack photosynthetic pigments.  They are short, unbranched, and each bears a strobilus.  They die shortly after releasing their spores.  The vegetative stems are formed a little later and bear numerous smaller branches at each node.  This results in a “plume-like” growth form that gives rise to the common name horsetails.  Plants with this mode of growth die back to the ground during the winter.