Living sphenophytes have stems containing a large central pith region which in mature plants is hollow. Surrounding the pith cavity are discrete bundles of vascular tissue; this arrangement of conducting tissue is known as a eustele. The bundles contain both xylem and phloem, and are marked by the presence of large canals known as carinal canals, which also function in water conduction. External to the vascular bundles is another set of canals, the vallecular canals or cortical canals. These canals line up with the depressions between the ribs on the surface of the stem. Most fossil sphenophytes had a very similar stem morphology. One Pennsylvanian genus, Sphenophyllum, had a solid stem with abundant secondary xylem -- i.e. wood.
The most obvious external characteristic of sphenophytes is their "jointed" branching pattern: leaves, branches, and roots branch from the stem at evenly spaced points called nodes, separated by lengths of bare stem called internodes. Leaves or roots form circular whorls around each node. Some species frequently branch at the nodes as well, while in others branching is rare or absent. When branching does take place, the branch is not formed in the axil of a leaf (as is the case in most other vascular plants), but instead takes the place of a leaf. While the alternation of nodes and internodes is very characteristic of sphenophytes, it is not limited to sphenophytes: a number of flowering plants, notably grasses, such as bamboo, and their relatives have a similar arrangement of nodes and internodes. Unique to sphenophytes, however (although not found in all fossil forms) is their pattern of vascular anastomosis: at each node, each separate bundle of conducting tissue branches into three. The middle branch leaves the stem and forms the midrib of a leaf, while the other two branches join with their neighbors to reform a full-sized vascular bundle. This creates a "zig-zag" pattern at the nodes of a sphenophyte stem; it can be seen easily in this internal mold of a fossil sphenophyte stem.
Equisetum leaves are either long and slender, as in the tropical Mexican species pictured at the left, or scalelike. Their contribution to photosynthesis is minimal; most photosynthesis takes place in the outer tissues of the stem. Most fossil sphenophytes had similar small or slender leaves, but the Pennsylvanian genus Sphenophyllum had broad, wedge-shaped leaves.
Both living and fossil sphenophytes form underground stems known as rhizomes. Aerial stems periodically branch from the rhizomes, which in some living species grow up to six feet (two meters) below the surface (this explains why it's nearly impossible to get rid of Equisetum once it invades your lawn). Isolated fragments of a rhizome can give rise to new individuals. In those living species of Equisetum that are placed in the subgenus Hippochaete, the aerial branches are perennial, and any aerial branch can form reproductive organs. In the other species, placed in the subgenus Equisetum, vegetative and reproductive branches are separate.
At the tips of reproductive branches are the "cones," or strobili, which consist of tightly packed appendages called sporangiophores. A number of fossil sphenophyte strobili show subtending bracts, leaflike structures in rows that alternate with rows of sporangiophores; Equisetum strobili lack bracts. Each sporangiophore bears several sporangia, in which spores develop. Peltate sporangiophores -- umbrella-shaped structures with sporangia hanging from their edges -- are very typical of the Sphenophyta.
Mature spores bear four long, spoon-shaped appendages known as elaters; these are originally coiled around the spore itself but uncoil when the spores are released, and probably allow the spore to be carried by wind. Most sphenophytes are homosporous; they produce only one type of spore, which germinates into a small gametophyte that bears either male or female sex organs. Some of the great calamite trees of the Pennsylvanian, however, were heterosporous, producing two distinct spore types: small spores that develop into male microgametophytes, and large spores that develop into female megagametophytes.