Anatomy
In these plants the leaves are greatly reduced and usually non-photosynthetic. They contain a single, non-branching vascular trace, which is the defining feature of microphylls. However, it has recently been recognised that horsetail microphylls are probably not primitive like in Lycopodiophyta (clubmosses and relatives), but rather advanced adaptations, evolved by the reduction of a megaphyll. They are therefore sometimes actually referred to as megaphylls to reflect this homology.
The leaves of horsetails grow in whorls fused into nodal sheaths. The stems are green and photosynthetic, and distinctive in being hollow, jointed and ridged (with sometimes 3 but usually 6-40 ridges) and these are often played with by children who will separate and then seamlessly rejoin the segments. There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except being smaller and more delicate.
Read more about this topic: Equisetum
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