BOT 201 Sphenophya: Equisetum Lab

General Features

We will study specimens of Equisetum. It is the only extant genus in this Division. Related plants once formed substantial populations, and some were tree-like. However, during the course of evolution these plants disappeared from the landscape but have been preserved as fossils.

Equisetum grows in moist environments. However, it has many adaptations for survival in dry habitats. The typical plant has a subterranean stem called a Rhizome. The rhizome produces adventitious roots at each node. Aerial stems have preformed root apices at each node, as well. These are activated when part of the stem falls down or when part of the stem becomes detached. The stems are jointed. This is like having one straw on top of another & another. Branch primordia are also present at each node. In some species these produce branches as part of normal growth. However, there are species which will not branch unless part of the stem is dislodged from the main stem. There is a weak point at the base of each internode because Equisetum has an Intercalary Meristem. The dividing cells are at the node. These have thin walls and will pop apart if the stem is stretched. In addition, the stems are hollow, except at the nodes where there is a solid septum. Thus, each internode-node complex can float because it is light and hollow, and can form roots and shoots. This is a good strategy for plants that live near water wherein asexual propagules can float away to unoccupied habitats where they can put down roots and puns. Why might Intercalary Growth be a hindrance to growing vertically and Standing Tall?

The Roots have a simple anatomy. An Apical Cell produces the Root Cap and Root Body. The stele is centrally located. An endodermis is present as are a cortex and epidermis.

An Apical Cell at the opposite end of the plant produces the leaves and shoot. The Leaves are thin and minute with one Leaf Trace per Microphyll. The microphylls are fused at each node and form a sheath which is tightly pressed to the epidermis of the stem. They are not important for photosynthesis.

The stems are grooved like Psilotum, however, there are more groves present with Equisetum. Stomata are located in the low points of each groove. This is a xeromorphic trait (a trait which has adaptive value for dry habitats). Why is this important?

The Epidermis has a thick, rough cuticle. The cell walls are impregnated with Silicon which contributes to its rough texture and brittleness. Thick walled Sclerenchyma Fibers are present in the subepidermal region opposite each ridge. These have lots of Silicon in their walls but are NOT Lignified. Photosynthetic Chlorenchyma occupies the subepidermal regions in the valleys between the ridges. Could this location have some significance? There is also a canal in this area. It is called the Vallecular Canal. However, we can call it the Cortical Canal since it is in the Cortex. The Vascular Bundles are directly opposite the surface ridges. These are collateral. This means that they have xylem & phloem which remain more or less opposite one another. The xylem in Equisetum surrounds the Phloem on three sides, however. The arrangement of vascular bundles in one circle is called a Eustele, and is similar to Angiosperm Eustele. However an Endodermis is present in the stem. It may surround each bundle or may skirt each side of the bundle. Angiosperms do not have anything like this except with a few species. There is a canal in each vascular bundle. This is caused by the rupture of primary xylem cells during elongation. This is the carinal or vascular or xylem canal. The large hollow space in the center of the stem is called the Central Canal.

The Sporangia are attached to Sporangiophores. The Sporangiophores are assembled into Cones or Strobili. Equisetum is Homosporous and the spores are green. They germinate to produce a protonema which eventually forms a three dimensional, photosynthetic gametophyte. Antheridia develop first on slightly elevated lobes of the gametophyte. The archegonia are produced later in the vicinity of the basal Rhizoids. Fertilization occurs when flagellated sperm swim to the egg. Embryo development is immediate and Exoscopic. The embryo contains Root and Shoot Apical Meristems and a Foot.