Sheaths urceolate, 4-7 mm long (m 5), 2.0-4.5 mm wide (m 3.4), with teeth 2-4 mm long (m 3.1), narrow, and dark, the stem ridge grooves continuing up the center of each sheath segment and to the end of the teeth.
Branches ascending, in regular whorls, or irregular, or sometimes absent on coniferous stems, with the first internode about equaling the subtending stem sheath, 3-7 mm long (m 4.8). Ridges 4, these prominently grooved and bearing a silica profile of 2 rows of closely set, blocky swateeth. Sheath segments grooved, the grooves continuing into the triangular, narrowly pointed teeth, commissure to 0.6 mm long, with prominent furrow of anchor cells. Valleys rounded, with stomata scattered throughout. Silica pilules covering stomata and distinctly outlining them. Stoma lined with a single row of somewhat rod-like pilules. Mamillae transversely aligned, usually distinct. Branches solid.
Cones 25-60 mm long (m 39.5), narrow, on peduncles 20-30 mm long (m 25.3).
Rhizome shiny, brown, covered with hairs, occasionally bearing tubers.
Spores 30-41 µm in diameter (m 34).
Page (1974) described a new species of horsetail, E. mekongense, with vegetative stems like E. diffusum but heterophyadic like E. arvense. Micromorphologically it is indistinguishable from E. diffusum. On p. 41-42, after describing its micromorphology, which resembles that of E. diffusum, he says "The combination of these micromorphological features with the presence of two-keeled sheath teeth allow E. mekongense to be clearly distinguished from both E. arvense and E. diffusum even in the absence of information on the type of fertile shoot." Since E. diffusum has two-keeled sheath teeth (and a commis sure with anchorcells just like that in Page's Fig. 9e) Hauke is unaware of any character other than the fertile shoot by which E. mekongense differs from E. diffusum. The description of E. mekongense on p. 36 primarily emphasizes the heterophyadic character, and Page says (p. 37), "I have seen only one collection of this unusual species, which superficially gives the appearance of young vegetative shoots of E. diffusum connected to ephemeral spring shoots of the E. arvense type." Page earlier (1972a) had downgraded the importance of heterophyadic vs. homophyadic shoots as a taxonomic character at the sectional level in Equisetum. Hauke thinks it loses its significance here as well in light of several observations. 1) there was recently reported by Van Hoek (1976) the occurrence of heterophyadic shoots in E. fluviatile, apparently resulting from disturbance related to transplantation. 2) E. pratense normally shows considerable variation in the degree of dimorphism of its coniferous stems - at times they are little different from the vegetative other than being unbranched, at other times they are quite distinctive. 3) The sheaths immediately below the cone of E. diffusum are commonly enlarged, a character associated with coning stems in heterophyadic species. 4) The coniferous stems of E. mekongense are described as "ephemeral", but since this species is known only from the type specimen, the persistence of the coniferous stems cannot be determined. Coniferous stems of E. sylvaticum collected when young also appear "ephemeral", but after the spores are shed they become green, branched, and persistent.
Equisetum mekongense Page appears to me to be an example of not only one character taxonomy, but also one specimen taxonomy. Hauke believes that the type specimen of E. mekongense (on the basis of Page's description, photograph, and a general knowledge of the genus Equisetum) is merely an aberrant specimen of E. diffusum and so place it in synonymy. (The only specimen of E. diffusum Hauke saw coning in April was also from Yunnan, the month and region of collection of E. mekongense).
Hauke observed that specimens of E. diffusum from Kweichow, China, bear some resemblance to E. palustre. Page pointed out that a specimen of E. diffusum from Yunnan had micromorphology tending toward that of E. palustre.
HAUKE, R.L. (1978)
A taxonomic monograph of the genus Equisetum subgenus Equisetum.
Nova Hedwigia 30, p385.