The horsetails comprise photosynthesising, "segmented", hollow stems, sometimes filled with pith. At the junction ("node", see diagram) between each segment is a whorl of
leaves. In the only extant genus
Equisetum, these are small leaves (
microphylls) with a singular vascular trace, fused into a sheath at each stem node. However, the leaves of
Equisetum probably arose by the reduction of
megaphylls, as evidenced by early fossil forms such as
Sphenophyllum, in which the leaves are broad with branching veins. The
vascular bundles trifurcate at the nodes, with the central branch becoming the vein of a microphyll, and the other two moving left and right to merge with the new branches of their neighbours. The vascular system itself resembles that of the vascular plants'
eustele, which evolved independently and
convergently. Very rapid internode elongation results in the formation of a pith cavity and a ring of
carinal canals formed by disruption of the primary
xylem. Similar spaces, the
vallecular canals are formed in the cortex. Due to the softer nature of the phloem, these are very rarely seen in fossil instances. In the
Calamitaceae, secondary xylem (but not secondary
phloem) was secreted as the
cambium grew outwards, producing a woody stem, and allowing the plants to grow as high as 10m. All extant species of
Equisetum are herbaceous, and have lost the ability to produce secondary growth. The underground parts of the plants consist of jointed
rhizomes, from which roots and aerial axes emerge. The plants have
intercalary meristems in each segment of the stem and rhizome that grow as the plant gets taller. This contrasts with most seed plants, which grow from an apical meristem - i.e. new growth comes only from growing tips (and widening of stems). Horsetails bear cones (technically
strobili, sing.
strobilus) at the tips of some stems. These cones comprise spirally arranged
sporangiophores, which bear
sporangia at their edges, and in extant horsetails cover the spores externally - like sacs hanging from an umbrella, with its handle embedded in the axis of the cone. In extinct groups, further protection was afforded to the spores by the presence of whorls of
bracts - big pointed microphylls protruding from the cone. The extant horsetails are
homosporous, but extinct
heterosporous species such as
Calamostachys casheana appear in the fossil record. The sporangia open by lateral
dehiscence to release the spores. The spores bear characteristic
elaters, distinctive spring-like attachments which are
hygroscopic: i.e. they change their configuration in the presence of water, helping the spores move and aiding their dispersal. ;
sporangiophores, with attached
sporangia (spore capsules) full of spores, can be discerned. of
E. braunii, terminal on an unbranched stem == Taxonomy ==