How do the reduced leaves of horsetails form a sheath around each stem node?

In the plant class Equisetopsida, the leaves of the living genus Equisetum are greatly reduced and nonphotosynthetic. These leaves are arranged in whorls at the nodes of the jointed stems. To form the characteristic sheath, the individual leaves are united laterally at each node, resulting in a toothed structure that encircles the stem.12