THE TAXONOMIC POSITION OF THE PSILOTALES IN THE LIGHT OF OUR KNOWLEDGE OF DEVONIAN PLANT LIFE*
F. P. JONKER Laboratory of Palaeobotany and Palynology, Utrecht, The Netherlands
ABSTRACT exclusively Devonian Psilophytales and of the latter the Rhyniaceae are regarded The order Psilotales, containing the two recent genera Psilatum and Tnzesiptel'is only, is usually usually as the closest relatives. Among classified by taxonomists and palaeo botanists those who take this st?nd I mention G. M. in the phylum Psiiophyta. This concept is, Smith (1955), Magdefra.u in Strasburger in spite of the synangia, based on the primitive (1971) and Lcmoigne (1968c). The concept general appearance reminding one of that of the of the last mentioned author will be dis• Devonian Rhyniaceae. Numerous attempts have been made to derive both genera, including their cussed further on. Darrah (1960) states synangia, from either the Rhvniaceae or from other that the status of the putative primitive Psilophyta. These attempts' sometimes led to the nature of the plant body in the Psilotaceae acceptance of a series of missing Jinks but this concept is, however, not supported bv palaeo· is controversial although there is a strong botanical data. . tendency to accept the family as a pel'sis• In the opinion of the present reporter the order tent remnant of the Psilopsida. Andrews has kept a primitive general appearance of its (1961) states that Psilatum has been reaarded Devonian ancestors but in its synangia and in its monolete spores it is more advanced. by some botanists as a very simpl~ land In its anatomy, its microphyllous leaves, and in its plant, possibly a very ancient type that gametophyte perhaps, it shows more affiinity to the has managed to survive for several hundreds phylum Lycopodiophyta. The present reporter of millions of years. Chadefaud (1950) clssiflCs the two genera as a separate order in the isosporous Lycopodiophyta. In his opinion the even regarded them as ancestral to the order is to be derived from the Protolepido• Bryophytes. Others view its simple organi• dendraceae. zation as a result of degeneration from a Available palaeobotanical data in the Devonian more complex ancestor. Thus the former are in support of this concept. The present author has to admit, however, that between the Devonian viewpoint regards it as primitive and the and recent times no palaeobotanical or palyno• latter as reduced. Nothing is known, how• logical data are known to support his concept. ever, of fossil records and there are no related modern plants which offer much aid in INTRODUCTION settling the problem. There are, however, quite a number of objections, even scruples, to an attribution without more to tne Psilophyta to which and twoTmesiptcrisrecent gennaBernh. PsilatumhaVE duringSw. THEa long time already, ?,ttracted the phylum further only Devonian fossils would interests of plant taxonomists and phylo• belong. Of these objections I now mention genists. Eames ('1936) consideled them the already one, namely thE' IE'aves, Psilatum most primitive vascular plants. Tne pri• has very small, spinelike, microphyllous mitive geneml appearance of, especially, leaves without a midvein. The leaves of Psilatum reminds of the Middle-Devonian Tmesipteris are considerably larger and Rhyniaceae: tIle genera Rhynia Kidst. et provided with a mid vein. For this reason Lang and H arneaphytan Bargh. et Darrah, Lotsy (1909) regarded the Psilatum leaves by the green stiff, at first sight almost as reduced, connected with its xerophytic leafless, strictly dichotonous aeri,,>.!shoots and semisaprophytic life. arising from a rootless creeping rhizome. The upper parts of the shoots of both The two genJra, together forming the Psilatum and Tmesipteris are provided, family Psilotaceae "md the order Psilot?.1es, moreover, with bifurcate leaves of the have accordingly been classified by most same size, supporting a very short stalk authors in the Psilophyta, together with the which bea.rs a trilocular synangium in *Contributed to the Paheobotanical Conference, Birbal Sahni Institute of Palaeobotany Silver Jubilee, December 1971. 33 34 THE PALAEOBOTANIST
Psilotum and a bilocular synangium in fundamentally verticillate in organization T11lesipteris. This led, as no fossils are and this might point to a relationship with available, Professor Lam (1948) to his well• Equisetophyta, for instance Sphenophyllales. known ingenious, theoretical-phylogenetic, As neither Psilotum nor Tmesipteris are but highly hypothetical derivation from articulate or show a verticillate organiza• the Rhyniaceae with their terminal (stachyo• tion in their aerial shoots, I reject this sporous) sporangia. concept, the more as other conformities In this way he defended and tried to with Sphenophyllales are scarcely evident. save the attribution to the Psilophyta In later papers Salmi (1923b; 1925) without taking into account other features brought forward anatomical evidence to which contradict such an attribution as bring the Psilotales nearer to the Devonian expounded below. Needless to say that genus Asteroxylon which, in his opinion, Lam's concept lacks every palaeobotanical supports the old view that the Psilotales basis. Lam, consequently, rejects t11eterm are related to the Lycopods. sporophylls for the bifurcate leaf-like organs In 1930 Campbell defended an attributiOn supporting the synangia. Other features to the Lycopodiophyta and the same concept which might be objections to an attribution was held by Zimmermann (1959). TIle to Psilophyta we find, besides in the le2.ves possible relationship to Lycopodiophyta was and the sporangia, in the anatomy and, discussed by a number of authors without perhaps, in the gametophytes. Next to t2.king a definite standpoint with respect the general appear2.nce the 2.bsence of roots to that. In this paper I am defending an is actually the only fe2.ture that is in favour attribution, as a separate order, to the iso• of an attribution to the Psilophytes. sporous Lycopodiophyta and in the following For these reasons a number of authors I will discuss my concept in comparison defend the attribution to a separate phylum with arguments against a relationship to of Pteridophytes or, if they prefer to con• Psilophyta- Rhyniaceae. sider the Pteridophytes a phylum alto• gether, to a separate classis. Such an is.o• ANATOMY lated taxonomic position h
• most primitive Lycopodiophytic anatomy A still more recent view was started by which is characterized by a massive acti• Lemoigne (1968a, b; 19690.) who found nostelic protostcle without pith. There are archegonia of the Pteridophyte type sunken no points of contact to tne llaplostele of in semiglobular tisme bodies on the stems the Rhyniaceae. of Rhynia gwynne-vaughanii and never on any part of Rhynia major. GAMETOPHYTE He also published still vague indications for the presence of antheridia in small The gametophyte of Tmesipteris ha.s been warts on the aerial shoots of R. gwynne• studied extensively by Lawson (1917), by vaughanii (1969b). As he also states that Holloway (1917, 1921), and by Bierhorst the terminal sporangia are only known in (1953, 19540.). The prothallus of the Psilo• Rhynia major and not in R. gwynne-vau• tales is subterraneous, without chlorophyll, ghanii, he arrives at the conclusion that saprophytic, mOT(}or less cylindrica.l in both species are gametophyte and sporo• shape and sometimes dichotomously bran• phyte respectively of the same species. ched. Tile gametophytes bear some resem• This possibility was already suggested six blance to little rootstocks. In gamctophytes years before by Pant (1962). Rhynia con• of Psilotum annular and scalariform tr acncids sequently shows an almost isomorphic alter• have been found, which may be regarded nation of generations, quite unknown up as a relic of a more isomorpllic alternation till nOw in otner Pteridophytes. I am of generations and, consequently, a primitive astonished that this leads Lemoigne (1968c) feature which is very rare among Pterido• to the conclusion that his discoveries have phytes. made it clear that both Psilotum and Tmesip• In 1958 Merker surprised the palaeo• teris belong together with Rhynia in the botanists by his statement tnat he had dis• same order, Psilotales, of the Psilophyta. covered evidence of the occurrence of He 2rgues this by pointing to the vasculi• sunken archegonia in the rootstocks of tne ferous gametophytes. In nis order Psilo• Rhynia's, i.e. he found embryos and only tales he classifies three families, viz. Rhynia• indications of arcIlCgonium remains. In ceae, Psilotaceae, and Tmesipteraceae. In another paper, the following year (1959), my opinion considerable differences exist he again went into the consequences of his in the gametophytE.s of Psilotum and Tmesip• discovery namely that the Rhynia rhizome teris on the one side and Rhynia on the represents the protnallus. The Rhyniaceae other side if we accept that Rhynia gwynne• have, conseql1ently, a leafless sporophyte vaughanii is the gametophyte of R. major. with terminal sporangia growing on the Psilotum and Tmesipteris have sl1bterra• gametophyte in the way as the sporogones neous, though sometimes slightly vascula• of the Bryophytes are attached to the rized prothallia and the gametophyte of gametophytes. When we combine this con• Rhynia is in that case a dichotomously cept, which has not been proved however, organized erect plant, provided with a with the work of Campbell (1925), of haplostele, a cortex, an epidermis with Chadefaud (1936), and especially with the stomata, and a subterraneous rhiZOme. work of Proskauer (1960) regarding corres• Lemoigne also points to the probable early ponding structures in Rhynia and Iiorneo• decay of the archegonium neck in Rhynia, phyton on the one side and in Anthoceros a feature also present in Psilotum and in on the other side, we may conclude to a Anthoceros but generally rare in Pterido• much closer affinity of the Rhyniaceae to phyta though it is found in Lycopodium. the Bryophytes and especially to Antho• As we have up till now only a vague idea cerotales. In that case tnere is no close of the structure of the Rhynia archegonium relationship of the Rhyniaceae to the and tile early decay of its neck cells is Psilotales as in Psilotum and Tmesipteris hardly more than surmise, one might use the I'hizom<:'is part of the sporophyte and the feature as well to suggest a relationship the gametophyte is a separate prothallus between the recent Psilotales and the Lyco• not persistently bearing the sporophyte. podiophytes. In other word s there is no justification any LEAVES longer for an attribution of the Psilotalcs to the Psilophyta based on a similarity Notwithstanding Lam's effort (1948) to with regard to their general appearance. deny the leaf nature of the Psilotum and 36 THE PALAEOBOTANIST
Tmesipteris leaves we may state that the character, viz., Protolepidodendraceae with two genera have microphyJIous leaves where• bifurcate leaves and Drepanophycaceae with as the Rhyniaceae ale leafless. spine-like leaves. Both are primitive Lyco• In the probably monotypicgenus Tmesipteris podiophyta as the sporangia occur on the has T. tannensis Bernh. rather well-deve• adaxial side of the sporophylls which have loped leaves with a midvein which communi• the same shape as the trophophylls. In cates with the stele by a leaf trace in the these cases the sporophylls occur in the cortex. This means that there is no im• apical parts of the shoots but they are not pediment to regard them as real leaves. arranged in distinct strobili. W. Schmidt Psilotum nudum (L.) Griseb., on the other (1955) described Sugambrophyton pilgeri from hand, has spinelike leaves without midvein the West-German Lower-Devonian and this or leaf trace in the cortex and it has been new genus was characterized by spine-like wondered, for instance by Manton (1950) leaves in the lower parts and bifurcate and Tronchet (1954) whether they are to be leaves in the apical parts of the shoots. regarded as le?ves. He evan observed a repeated bifurcation Lotsy (1909) assumed a reduction of the occasionally. This makes it cle?r that leaves of Psilotum in cOmparison to those the simple or bifurcate leaf is not a good of Tmesipteris, connected with the xero• character to distinguish two families on phytic and semisaprophytic life In Psilo• it as both kaf forms may occur in the same tum complanatum Sw. (=P. jlaccidum Wall), plant. But at the sGme time we may however, a leaf trace in the cortex has been observe the same situation in Psilotum observed, terminating just below the leaf and Tmesipteris with the difference that base This character .is known also in the in these two genera the bifurcate leaves Middle-Devonian genus Asteroxylon Kidst. always support a synangium. This is, et Lang which was, on the ba<;is of its in my opinion, the final reason to consider presumed terminal sporangia, comidered the Psilot?.les members of the Lycopodio• a member of the Psilophyt?, notwithstand• phyta ?nd not Psilophyta, In my concept ing its actinostele and its leaves which the isosporous Lycopodiophyta are to be occur, densely crowded, along the aerial divided into two orders, viz. Lycopodiales shoots like in Lycopodium. Lyon (1964), consisting of three familiEs: Protolepido• however, made clear tn.at in all probability d'mdraseae, Asteroxylaceae and Lycopodia• the obferved sporangia-bearing, naked di• ceae, and Psilotales with a single family chotomous branch0S did not belong to Psilotaceae (genera Psilotum and Tmesip• Asteroxylon. He found among masses of teris), Arguments against an attribution Asteroxylon in the Rhynie chert strobilus• of the Psilotales to the Lycopodiophyta like branches beset with both normal leaves arc the absence of roots (which in my opinion and shortly stalked leaves supporting a is a primitive character that does not turn sporangium. When accepting these organs of the scale) and the synangium which does as fertile parts of Asteroxylon there is no not occur in other Lycopodiophytes but hindr?nce any more to classify Asteroxylon which docs not occur in other Psilophyta within the isosporous Lycopodiophyta. either And, at tl1e same time, tllis is a strong support to consider, tn.e Psilotales Lyco• PRIMITIVITY podiophyta as well. The leaves supporting the synangia of The question arises, finally, whether the the Psilotales are bifurcate, however. It Psilotales arc to be considered very primitive. has been observed occasionally that in In my opinion they show a mixture of pri• Psilotum there m-y occur a repe'ated bifur• mitive and more advanced characters. Tn.e cation and in that case every bifurcation general appearance is of course primitive bears a synangium. Bifurcate leaves and and reminds to the primitive appearance sporophylls occur in the early Devoman of the shoots of Psilophyta, Protolepido• genus Protolepidodendron, family Protolepi• dendraceae, and some Protopteridiales. The dodendraceae. To this family also Drepano• absence of roots is primitive and has not phycus, with spine-like leaves and sporophylls been observed, as far as known to me, in has been attributed, but in the opinion of Lycopodiophytes whereas it IS usual in some authors these differences made it Psilophyta though there is no proof that necessary to base two families on this roots are always absent in Psilophyta. JONKER - THE TAXONOMIC POSITION OF THE PSILOTALES 37
The actinostele which is, at the same A combination of a primitive general time, siphonostelic is more advanced than appearance and advanced characters is not the most primitive steles known in Lyco• very rare. Good examples we find in the podiophytes. The prothallus is, undoubted• Devonian genus Moresn(Jtia which in all ly, primitive. probability produced seeds and in a number The synangium has to be considered of aerial shoots of Devonian Protoptfridiale:;, advanced and so are, in my opinion, the e.g. Archaeopteris (Beck, 1960) and Sval• monolete spores. The spores of Psilophyta bardia (Carluccio, Hueber & Banks, 1966) are trilete as a rule and so are mostly which genera form secondary wood of a Lycopodiophyte spores. Tne microspores very advanced structure. of Isoetes are monolete, however, with With regard to the Psilotales - which occasionally some tendencies to a trilete nave quite a nUmber of primitive characters condition. This character is apparently in common with the early land plants, in not in <>.11cases settling the problem as in combination with more advanced charac• related ferns both conditions arc possible. ters - the f?ct is left that no connecting Kramer (1970) mentioned the occurrence fossil records arc known between the OCCUI• of both monolete and trilete spores rence of their far relativ(}s, the Protolepido• in the fern genera Sphenomeris and dendracea'J in the Lower-Devonian, and the Lindsaea. present.
REFERENCES
ANDREWS, H. N. (1961). Studies in Paleobotany. LAWSON, A. A. (1917). The prothallus of Tmesip• New York & London. teris tann'ensis. Trans. R. Soc. Edinb. 51: BECK, C. B. (1960). The identity of Archaeopteris 785-794. and Callixylon. Brittonia. 12(4); 351-368. Idem (1917/21). The gametophyte generation of BIERHORST, D. ,v. (1953). Structure and develop• the Psilotaceae. Ibid. 52: 93. ment of the gametophyte of Psilotum nudum. LEMOIGNE, Y. (1968a). Observation d'archegones Am. j. Bot. 40(9): 649-658. portes par des axes de type Rhynia gwynne• Idem (1954a). The gametangia and embryo of vattghanii Kidston et Lang. Existence de Psilotum nudum. Ibid. 41(3): 274-281. gametophytes vascularises au devonien. C.R. Idem (1954b). The subterranean sporophytic axes Aced. Sc. Paris. 266: 1655-1657. of Psilotum nudum. Ibid. 41(9): 732-739. Idem (1968b). Observations d'archegones portes Idem (1956). Observations on the axial appen• par des axes vascularises du type Rhynia dages in the Psilotaceae. Phytomorphology. 6: gwynne-vaughanii Kidston et Lang. Existence 176-184. de gametophytes vascularises chez des Psilo• CARLUCCIO,L., F. M. HUEBER & H. P. BANKS phytales du devonien. Bull. mens. Soc. Linn. (1966). Archaeopteris macilenta, anatomy and Lyon. 37(4): 148-149. morphology of its frond. Am. j. Bot. 53: Idem (1968c). Les genres Rhynia Kidston et 719-730. Lang au devonien et Psilotum Seward actuel CHADEFAUD,M. (1950). Les Psilotinees et I'evo• appartiennent-ils au nH~me phylum. Bull. Soc. lution des Archegoniates. Bull. Soc. bot. Fr. bot. Fr. 115: 425-440. 97: 99-100. Idem (1969a). Contributions a la connaissance du DARRAH,W. C. (1960). Principles of Paleobotany. gametophyte Rhynia gwynne-vaughanii Kidston 2nd ed. New York et Lang: Probleme des protuberances et pro• EAMES, A. J. (1936). Morphology of Vascular cessus de ramification. Bull. mens. Soc. Linn. Plants. New Yorl/. Lyon. 38(4): 94-102. EMBERGER, L. (1968). Les piantes fossiles dans Idem (1969b). Organe assimilable a une antheridie leur rapport avec les vegetaux vivants. Paris. et stomates epidermiques portes par des axes GOTHAN, W. & H. WEYLAND (1964). Lehrbuch ramp ants du type Rhynia g,wynne-va'ughanii der PaHiobotanik. 2nd. ed. Berlin Kdiston et Lang. G.R. Aced. Sc. Paris. 269: HOLLOWAY, J. E. (1917). The prothallus and 1393-1395. young plant of Tmesipteris. Trans. Proc. N.Z. LOTSY, J. P. (1909). Vortr::ige tiber botanische Inst. 50; 1-44. Stammesgeschichte, 2e Band Cormophyta Zoido• Idem (1921). Further notes on the prothallus gamia. jena. and young sporophyte of Tmesipteris. Ibid. LYON, A. G. (1964). Probable fertile region of 53: 586-422. Asteroxylon mackiei K. & L. Nature. 203, KRAMER, K. U. (1970). The Lindsaeoid ferns (4949): 1082-1083. of the Old World V. The smaller Pacific MAGDEFRAU, K. (1971), in Strasburger, E., Lehr• Islands. Blumea. 18(1): 157-194. buch der Botanik fUr Hochschulen. 30e Aufl., LAM, H. J. (1948). Classification and the New Stuttgart. Morphology. Acta biotheor. 8(4): 107-154. MANTON, 1. (1950). Problems of cytology and LAWRENCE, G. H. M. (1951). Taxonomy of evolution in the Pteridophyta. Cambridge. vascular plants. New York. University Press. 38 THE PALAEOBOTANIST
MERKER, H. (1958). Zum fehlenden Glied der Idem (1923b). Modern Psilotaceae and archaic Rhynienflora. Bot. Notiser. 111(4): 608-618. terrestrial plants. Nature. 111: 84. Idem (1969). Analyse der Rhynien-Basis und Idem (1925). On Tmesipteris vieillardi Dangeard, Nachweis dcr Gametophyten. Ibid. 112(4): an erect terrestrial species from New Caledonia. 441-452. Ph-il. Trans. R. Soc. London, Ser. B. 213: 143-170. PANT, D. D. (1962). The gametophyte of the SMITH, G. M. (1955). Cryptogamic Botany, Vol. II. Psilophytales. Proc. summ. sell. Bot. Darjeeling. Bryophytes and Pteridophytes. New York. 276-301. TRONCHET,A. (1954). Remarques sur la primaute SAHNI, B. (1923a). On the theoretical significance originelle attribuee a. la tige. Annis. scient. of certain so-called •• abnormalities" in the Univ. Besallvon, 2e Ser. Bot. 1: 3-10. sporangiophores of the Psilotaceae. J. Indian ZIMMERMANN, W. (1959). Die Phylogenie der bot. Soc. 3(7): 185-191. Pflanzen. 2e Aufl., Stuttgart.