Psilotaceae Kanitz, Noveny. Att: 43 (1887); White (1977). Tmesipteridaceae Bertrand, Koidzumi (1939).

Small to medium-sized, often epiphytic plants with subterranean, dichotomously to laterally branched stems with endophytic mycorrhiza, without roots, and aerial, simple or dichotomously branched stems; stele protostelic, or solenostelic at the transition of subterranean and aerial stem. Aerial stems green, erect or pendulous, glabrous, slender, angular to sulcate or flattened, the basal part often with rudimentary leaves, the upper part leafy; leaves spirally arranged or distichous, flaccid or firm, narrow, simple or once forked, one-veined or veinless. Sporangia sessile on or above the base of forked sporophylls, thick-walled (euspor-angiate), 2- or 3-celled and -lobed, each cell dehiscing longitudinally with a slit. Spores numerous, small, achlorophyllous, pale, bean-shaped, monolete. Gamet-ophyte subterranean, heterotrophic.

Morphology and Anatomy. The external and internal morphology and anatomy have been extensivly studied (though mostly in Ps. nudum only) because of the enigmatic and still controversial systematic position of the family. The poorly defined leaves have been interpreted as reduced lateral axes of limited growth; or the entire aerial stems have been regarded as homologous to certain kinds of so-called non-ap-

1 In isolated taxa, which occupy a high taxonomic rank, we do not attempt to give diagnoses for all subordinate categories, thus avoiding the farce, as Burtt has said, of trying to decide which characters of a single genus should diagnose a family, order, class, etc.

pendicular fern leaves as found in, e. g., Stromatopteris (Bierhorst 1968 and later). This was much contested, especially by Kaplan (in White 1977) and is not generally accepted at present. The sporangia are generally called "synangia", which seems plausible; but, as pointed out by Eames (1936), the evidence for secondary fusion is slight, and the term "synangium", with its morphophyletic implications, is avoided here.

Much of the interpretation of the morphology also depends on whether the Psilotaceae are regarded as primitive or as derived and simplified (or as a combination of both; e.g. Jonker 1973). The complete lack of fossil remains makes the decision highly uncertain. The strongly developed mycotrophy is very suggestive of reduction, as found in many mycotrophic angio-sperms. For a full account of the morphology and anatomy, see especially Bierhorst (1954 b, 1956, 1971) and various authors in White (1977). For the spores see Lugardon (1973,1979).

Gametophyte. See again Bierhorst (1954 a, 1971). The prothallium in both genera is subterranean, non-green, mycotrophic, ± cylindric and often branched, radially organized, and consists only of parenchymatic, mycor-rhizal parenchyma or has a very weakly developed, often discontinuous, central stele; it sometimes bears gemmae. Antheridia and archegonia are both borne diffusely and in large number all over the surface. The antheridia are rather like those of primitive ferns, with a one-layered wall of many cells and an opercular cell; the inner mass may be up to 512-celled; the numerous spermatozoids produced are multiciliate. The archegonia are sunken, with a short, thick neck most of which is shed towards maturity (Bierhorst 1954 a).

Karyology and Hybridization. The chromosome number is known for both genera, the diploid number being based on n = 52, polyploids (up to octoploid level) being more frequent. No hybrids have so far been reported.

Ecology and Distribution. Both genera may grow terrestrially but are more often epilithic or epiphytic. Tmesipteris often occurs on tree-fern trunks. Both genera mostly grow at lower elevations. See further under the genera.

Affinity. Ideas about this have changed much over the years. Pritzel (in Engler and Prantl 1902) upheld

Sterile Appendages Tmesipteris

M Fig. 2 A-G. Isoetaceae, Lycopodiaceae, spores. A Isoetes cubana, megaspore (xl50). B Isoetes macrospora, megaspore (x 100). C Isoetes andícola, microspore (x 1000). D Lycopodium deuterodensum (xlOOO). E Lycopodium clavatum ( X1000). F Lycopodium obscurum (x 1000). G Huperzia lini-folia (x 1000). Phot. A. F. Tryon

Fig. 3 A-C. Selaginellaceae, spores. A Selaginella apoda, microspore (xlOOO). B Selaginella galeottii, megaspore with microspore attached (xlOO). C Selaginella selaginoides, microspore (c. x 100). Phot. A. F. Tryon

Selaginella Selaginoides Megaspores

the idea of affinity to Lycopodiales, taken up again by, e.g. Jonker (1973). Following a suggestion by Lam (1948), Ps. were compared with Psilophyta (perhaps prompted to some extent by the similarity in name) which is now generally rejected. Bierhorst (1968,1973, 1977) emphasized similarity to ferns, especially Stro-matopteris, which had not been previously considered.

Fig.4A-F. Psilotaceae. A, B Psilotum complanatum. A Part of branch system (x0.8); B Dehisced sporangium (xl2). C, D Tmesipteris truncata. C Part of branch system (xl.l). D Dehisced sporangium ( x 12). E, F Tmesipteris sp. E Ga-metophyte (g) with young sporophyte (sp) (enlarged). F Apex of gametophyte with archegonia (or) and antheridia (an) (enlarged) (A-D from Brownlie 1977; E, F from Eames 1936)

This idea was much opposed (see above) also on phyto-chemical grounds (Cooper-Driver 1977; Wallace et al. 1983). But the idea of a certain degree of affinity to ferns gained some ground (e.g. Tryon and Tryon 1982), also because of similarity in spore structure (Lu-gardon 1973, 1979). The Ps. diverge, however, too far from ferns to be included in this class and merit recognition as a class in its own right (Wagner in White 1977).

Key to the Genera.

1. Leaves scale- or awl-like, without vein, or with one at the extreme base; sporangium 3-celled, on the base of the bifid sporophyll 1. Psilotum

- Leaves tongue-shaped to lanceolate, one-veined; sporangium 2-ceIled, inserted on the sporophyll above its base, at the bifurcation 2. Tmesipteris

Psilotum Sw., Schrad. J. Bot. 18002: 8, 109 (1802); David

Subterranean stems much branched, with gemmae. Aerial stems slender, well-developed, to c. 1 m long but mostly shorter, dichotomously branched, with awl-shaped to squamiform leaves, these without veins or with a vein in the extreme basal part. Sporangia (sub)sessile, globose, later 3-lobed, 3-celled. Spores ellipsoidal, coarsely rugose.

Several species described, but probably only two really distinct. Pantropic to warm-temperate except in dry areas; extending to the SE United States, extreme SW Europe, central Japan, and South Korea; occurring on many remote, especially Pacific, islands. The most widespread is PS. nudum (L.) Pal. Beauv.

2. Tmesipteris Swurtz Figs.4C-F, 5D

Tmesipteris Swartz, Schrad. J. Bot. 18002: 131 (1802); Reed

(1966); Chinnock (1975,1976); Braithwaite (1986).

Terrestrial or more often epiphytic (often on tree-fern trunks) plants with little branched underground stems without gemmae. Aerial stems often short but may reach c. 'A m, often simple, erect to drooping or pendulous, very leafy except at base; in some species indefinite, in others terminated by a leaf; leaves subsessile, narrowly ovate to lanceolate, or sigmoid, obtuse to acute, often mucronate. Sporophylls with deeply bifid lamina, aggregated either in the apical or in the basal part of the stem, the lobes similar in shape to sterile leaves. Sporangia sessile at the bifurcation, 2-celled, the cells rounded or apiculate, equal or unequal. Spores ellipsoidal, shallowly rugose.

C. 10 species, some as yet imperfectly known, in SE Asia-Australasia, concentrated in Australia and New Zealand, extending to New Caledonia, New Guinea, the Philippines, east to the Marquesas and Tahiti.

Selected Bibliography liierhorst, D. W. 1954 a. The gametangia and embryo of Psilotum nudum. Amer. J. Bot. 41: 274-281. liierhorst, D.W. 1954b. The subterranean sporophytic axes of Psilotum nudum. Amer. J. Bot. 41: 732-739. liierhorst, D. W. 1956. Observations on the axial appendages in the Psilotaceae. Phytomorphology 6:176-184. liierhorst, D.W. 1968. On the Stromatopteridaceae (fam.nov.) and on the Psilotaceae. Phytomorphology 18; 232-268.

liierhorst, D. W. 1969. On Stromatopteris and its ill-defined organs. Amer. J. Bot. 56: 160-174. liierhorst, D. W. 1973. Non-appendicular fronds in the Fili-

cales. Bot J. Linn. Soc. 67 Suppl. 1: 45-57. liraithwaite, A.F. 1986. Tmesipteris in Vanuatu (New Hebrides). Fern Gaz.13: 87-96.

Fig. 5. Psilotaceae. Tmesipteris billardieri epiphytic on stem of Dicksonia; Victoria, Australia. Phot. K. U. Kramer

Chinnock, R. J. 1975. The New Zealand species of Tmesipteris (Psilotaceae). New Zeal. J. Bot. 13: 743-768.

Chinnock, R J. 1976. The identification, typification and origin of Tmesipteris tannensis (Psilotaceae). Taxon 25: 115-121.

Cooper-Driver, G. 1977. Chemical evidence for separating the Psilotaceae from the Filicales. Science 198:1260-1261.

David, A1965. Contribution à l'étude des Psilotales. Lyon.

Eames, A J. 1936. Morphology of vascular plants. Lower groups. New York, London: McGraw-Hill.

Jonker, F. P. 1973. The taxonomic position of the Psilotales in the light of our knowledge of Devonian plant life. Palaeobo-tanist 20: 33-38.

Lam, H.J. 1948. Classification and the new morphology. Acta Biotheor. 8:107-154.

Lugardon, B. 1973. Sur les parois sporales de Psilotum trique-trum Sw. et leur structure fine. C. R. Acad. Sei. Paris Ser. D 276: 1277-1280.

Lugardon, B. 1979. Sur la formation du sporoderme chez Psilotum triquetrum Sw. (Psilotaceae). Grana 8:145-165.

Reed, C.F. 1966. Index Psilotales. Bol. Soc. Brot 11.40: 71-96.

Wallace, J.W., Pozner, R.S., Gomez, L. D. 1983. A phyto-chemical approach to the Gleicheniaceae. Amer. J. Bot. 70: 207-211.

White, R. A (Ed.). 1977. Taxonomic and morphological relationships of the Psilotaceae: introduction to the symposium. Brittonia 29:1-68.

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