Equisetaceae L.C.Richard ex DeCandolle, Fl. Fr. 2: 580.

Rhizomatous perennials with stems characterized by a jointed appearance, with leaves small, whorled, and fused into a nodal sheath. A series of ridges and valleys traverse the internodes, and continue up into the nodal sheaths, alternating in successive internodes. Stems in cross-section usually with a prominent central canal and small vallecular (under the valleys) and carinal (under the ridges) canals. Reproduction by spores borne in elongate, longitudinally dehiscing sporangia on sporophylls in cones terminal on the vegetative stem, or occasionally on branches, or in some species on a specialized cone-bearing stem. Spores spherical, green, of one kind, with elaters (hapters); gametophytes green, terrestrial. Chromosome number n = 108. Monogeneric.

Anatomy and Morphology. Recent accounts by Bierhorst (1971) and Foster and Giflford (1989) summarize the morphology and anatomy of Equisetum. The rhizome system is extensive, and of great importance in the persistence and spread of this genus. It bears roots, tubers (in some species), and aerial branches. These initiate as nodal buds, at the base of the internode above and enclosed by the nodal sheath. Each bud has a root apex and a shoot apex, but often only one will develop further. All are determinate structures, the tuber developing only one or two internodes which become gorged with starch.

The aerial shoots have sharply defined nodes and internodes because the leaves are whorled and fused into a nodal sheath. The internode has ridges and grooves running its length, and the ridges become the midribs of the leaves (sheath segments) and continue up into their unfused tip portions (sheath teeth). Leaves, and hence the internode ridges, alternate at adjacent nodes. Stomata are found along the grooves, either scattered or in bands on either side, or in single rows on each side.

A transverse section of the internode shows a pattern of collenchyma (often mistakenly called scleren-chyma) and chlorenchyma under the epidermis, vallecular canals in the cortex under the grooves, and carinal canals under the ridges. These are protoxylem lacunae. The vascular system is a ring of bundles, each with a small amount of phloem flanked by xylem, and with the protoxylem lacuna internal to the phloem. Associated with the vascular system is an endodermis, which appears as an individual endodermis around each bundle, or a single outer endodermis enclosing the ring of bundles, or a double common endodermis, one outside and the other inside the ring of bundles. The pith region is occupied by a large central canal. The collenchyma may be massed under the grooves, under the ridges, or both. The epidermis is covered with a silica layer which develops characteristic mi-cromorphology (Hauke 1979).

Branches develop regularly in some species, irregularly in others, and only as an injury response in others. These branches repeat the morphology of the main stem. At the base of the internode is an intercalary meristem which develops from the lower part of the leaf primordium. Buds initiate exogenously from the uppermost cell of the sheath primordium, between leaf segments. These are enclosed by the nodal sheath and erupt through it, alternating with the leaves. With its often dormant nodal buds and intercalary meristems, this genus can be very plastic in gross morphology, and as a result a large number of meaningless tax-onomic names have been published (Hauke 1963, 1966,1979).

Cones terminate the stems. In most species these are the vegetative stems, but in a few, the cone-bearing stems lack chlorophyll, are unbranched and succulent, with enlarged sheaths. These may be ephemeral, or they may persist and become chlorophyllous and branched after the spores are shed. The cone consists of whorls of sporophylls, each a peltate structure with about six elongate sporangia pendant from its adaxial surface. Sporangia dehisce longitudinally to release


f ig. 15 A-G. Equisetaceae. A-D Equisetum ramosissimum. A Habit (x 0.6). B Node with branch (x5.25). C Strobilus (x4). D Schematic transection through stem (x8.25). K-G Equisetum arvense. E Habit of sterile plant (x0.5). f' Schematic transection through stem (x 12). G Apex of fertile stem with strobilus ( x 1) (A-F from Madalski 1954)

f ig. 15 A-G. Equisetaceae. A-D Equisetum ramosissimum. A Habit (x 0.6). B Node with branch (x5.25). C Strobilus (x4). D Schematic transection through stem (x8.25). K-G Equisetum arvense. E Habit of sterile plant (x0.5). f' Schematic transection through stem (x 12). G Apex of fertile stem with strobilus ( x 1) (A-F from Madalski 1954)

the spherical, alete, chlorophyllous spores. These have n perispore which becomes four strap-like hygroscopic olaters (hapters) and an unmarked spore wall; SEM pictures reveal a granulate surface with a scattered spherical deposit. The spores have a limited viability (about 1 week) and require light for germination.

Gametophytes are epigeal, may grow to about 1 cm in diameter, and have a parenchymatous basal portion with unicellular rhizoids ventrally and plates of chlorophyllous tissue dorsally. Although the spores are liomosporous, the gametophytes are unisexual in most cases (Duckett 1970, 1972, 1979; Hauke 1969, 1977,

1980). The archegonial gametophytes, if unfertilized, commonly change over to antheridial, and during a short period may be functionally bisexual and self-fertile.

Ecology and Distribution. Equisetum is primarily a plant of open, sunny sand banks along river and lake margins, in marshes, and in other wet places. One grows in open, standing water and several inhabit shaded, wet woods. Because of the widespread filling of low, wet places to make roads and railroad beds, some species have become common along roadsides and railroad embankments. This genus is found around the world from the southern portions of South America and Africa to above the Arctic Circle. The greatest concentration of species are found between 40° and 60° north latitude. Of the major land masses, only Australia, New Zealand, and Antarctica have no representatives.

Affinity. This family with its one genus stands alone among extant vascular plants. The many unique features of its anatomy and morphology justify its being segregated as a distinct class, subdivision, or even, by some authors, a division of the plant kingdom.

Karyology and Hybridization. The chromosome number of all species that have been counted (12 of 15) is « = 108. There is some indication of slight differences in the form of the chromosomes of the two subgenera. Numerous hybrids have been described between species within each subgenus, but none are known between the two subgenera. Evidence for dip-loidy, notwithstanding the high chromosome number, was presented by Soltis (1986).

Equisetum L. Fig. 15

Hippochaete Milde (1865).

Presla Dulac (1867).

Characters of the family.

Key to Subgenera.

1. Stomata superficial, usually scattered or in bands 2 or more stomata wide; cones non-apiculate; aerial stems mostly annual, regularly branched, with an outer common endoder-mis; antheridia with more than two cover cells

Subgenus Equisetum

- Stomata sunken, usually in single lines; cones mostly apic-ulate; aerial stems usually perennial, often unbranched, with a double common endodermis; antheridia with only 2 cover cells Subgenus Hippochaete

Although a number of workers have elevated these to genera, the only morphological character which sepa rates all the species of one from all the species of the other is the superficial vs. sunken stomatal apparatus.

Subgenus Equisetum: Eight species, predominately northern in distribution, with only one species south of the 20° north latitude.

Subgenus Hippochaete (Milde) Baker: Seven species, widespread in both southern and northern hemispheres.

Selected Bibliography

Duckett, J.G. 1970. Sexual behaviour of the genus Equisetum, subgenus Equisetum. Bot. J. Linn. Soc. 63:327-352.

Duckett, J.G. 1972. Sexual behaviour of the genus Equisetum, subgenus Hippochaete. Bot. J. Linn. Soc.65: 87-108.

Duckett, J. G. 1979. Comparative morphology of the gameto phytes of Equisetum subgenus Hippochaete, etc. Bot. J. Linn. Soc. 79:179-203.

Foster, A S., Gifford, E. M. 1989. Comparative morphology of vascular plants, 3rd edn., Chap. 10: The Sphenophyta. San Francisco: W. H. Freeman & Co.

Hauke, R. L. 1963. A taxonomic monograph of Equisetum subgenus Hippochaete. Beih. Nova Hedw. 8:1-123.

Hauke, R. L. 1966. A systematic study of Equisetum arvense. Nova Hedw. 13: 81-109.

Hauke, R. L 1969. Gametophyte development in Latin American horsetails. Bull. Torrey Bot. Club 96: 568-577.

Hauke, R. L. 1977. Experimental studies on growth and sexual determination in Equisetum gametophytes. Amer. Fern J. 67: 18-31.

Hauke, R. L 1979. A taxonomic monograph of Equisetum subgenus Equisetum. Nova Hedw. 30 (1978): 385-455.

Hauke, RL 1980. Gametophytes of Equisetum diffusum. Amer. Fem J. 70: 39-44.

Soltis, D. E. 1986. Genetic evidence for diploidy in Equisetum. Amer. J. Bot. 73: 908-913.


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