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The Present Status of Rhynia Gwynne-Vaughanii (K

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The Present Status of Rhynia Gwynne-Vaughanii (K THE PRESENT STATUS OF RHYNIA GWYNNE-VAUGHANII (K. AND L.) Y.L. YVES LEMOIGNE Department de Biologie Vegetale, Universite de Lyon, (France) cheids, surrounded by a zone of thin-walled singlefamilygenus,RhyniaceaeRhynia, compriseswith onea phloem. Cortex consisting of an inner THEspecies. It is known only from the and outer zone. Epidermis of aerial stems famous Devonian beds at Rhynie in with cuticularized outer wall and stomata. Scotland. Locality - Muir of Rhynie, Aberdeen• shire. HISTORY Horizon - Old Red Sandstone (not younger than the Middle Division of the The Muir Chert at Rhynie, containing Old Red Sandstone of Scotland). remains of Psilophytes, among them the In addition to these diagnoses Kidston Rhyniales, was discovered by Dr W. and Lang mentioned (1917, p. 765): Mackie in 1910. "Rhizome and aerial stems bore small In the first description of the fossil hemispherical projections which were more plants from this chert, Kidston and Lang or less closely placed without apparent regularity. On some of these bulges tufts (1917) recognized only two species, which of rhizoid-like hairs were borne, while in they called Rhynia gwynne-vattghanii and Asteroxylon mackiei. However, they were other cases the projections developed into adventitious branches, usually attached by aware of the presence of more plants which a narrow base. Some of these branches they intended to describe later. In this first description Kidston and Lang gave appear to have been readily detached, and the following diagnoses (p. 780): their occurence free in the feat suggests "Psilophytales: A Class of Pteridophyta that they served to propagate the plant characterized by the sporangia being borne vegetatively" . at the ends of certain branches of the stem Thus, in their first description Kidston without any relation to leaves or leaf-like and Lang recognized only one species of Rhynia. They explained the differences organs." in the structure of the stele as results of Rhynia gwynne-vaughanii, Kidston and Lang. n. g. and n. sp. differences in position of the sections in different parts of the vegetative body. "That the rhizomes, aerial stems, and Psilophyton princeps, Dawson (pars) sporangia were portions of the same plant RHYNIA, Kidston and Lang. n. g. might have been inferred from their asso• ciation in a bed composed of one type of Diagnosis - Plant gregarious, rootless plant only. Their continuity has, however, and leafless, underground rhizomes with been directly traced, and is established rhizoids, generally situated on large, down• by anatomical evidence" (Kidston & Lang, wardly directed protuberances of the cortex. 1917, p. 766, 1. 4-7). Aerial stems cylindrical tapering upwards, Three years later, in 1920, Kidston and about 8 inches in height, bearing small Lang published an emend.:nent of the genus hemispherical protuberances. Stems spar• Rhynia. They wrote: "In part I (1917) ingly dichotomous and also bearing lateral a general account was given of the silicified adventitious branches. Sporangium large, peat bed found at Rhynie, and one vascular cylindrical and terminating an aerial stem. plant was described in detail under the Sporangial wall thick, of mans layers of name of Rhynia gwynne-vaughanii. Further cells. Homosporous, spores developed in study has shown that they are two species tetrads about 65 [1. in diameter. Stele of Rhynia which we now distinguish as throughout the plant small, cylindrical, Rhynia gwynne-vaughanii and Rhynia consisting of a solid strand of annular tra- major" (Introduction). They further added, ;Ill 40 THE PALAEOBOTANIST "As mentioned above, two species of Rhynia 12 mm. and a diameter of 4 mm. Sporangial have to be distinguished. Their similarity wall about 3-4 mm thick. Spores about in organization is so great that they are 65 fl. in diameter. not always readily separated from one Locality - Muir of Rhynie, Aberdeen• another, and in our former paper they were shire. described together u.nder the name Rhynia Horizon - Old Red Sandstone (not gwynne-vaughanii. The second species, younger than the Middle Division of the which we now separate as Rhynia major, Old Red Sandstone of Scotland). is larger in all its parts, and differs from After the publicRtion of the "Studies" Rhynia gwynne-vaughanii in the absence by Kidston and Lang numerous botanists of hemispherical projections and adventi• and paleobotanists have taken a great in• tious branches from the stems, in the greater terest in the Psilophytales, and all of them size of the stele and xylem strand, and in have wished to know something about the the much larger size of the sporangia and gametophytes of these plants, which for spores" (p. 603). In this second account a long time were regarded as the first Kidston and Lang gave new diagnoses (p. vascular plants and interpreted as the an• 610-611). cestors of the living vascular plants. I Rhynia - Plant gregarious, rootless and shall quote only a few authors and only leafless, consisting of subterranean rhizomes in so far as they have contributed to our attached by unicellular rhizoids, and erect, better understanding of these Devonian dichotomously branched, cylindrical aerial plants: stems. Stomata present. Stele consisting Bower (1935) has suggested that the of a zone of phloem surrounding a strand gametophytes of the earliest colonizers of of tracheids. Sporangia cylindrical, with• the land had probably a robust built out columella, terminal on aerial stems. (particularly the gametophytes of Rhynia). Homosporous. Spores with cuticularized Eames (1936) vaguely speculated that wall, developed in tetrads. "if gametophyte and sporophyte were much Rhynia gwynne-vaughanii: aerial stems alike in early land plants, some of the sterile tapering upwards, probably about 20 cm. fossil branching axes may perhaps be game• high, and ranging in thickness from 3 mm. tophytic" . to under 1 mm. Small hemispherical pro• Fritsch (1945) has with good reasons tuberances of superficial tissues of the stem suggested that "if embryos and gameto• occur, and sometimes, in place of them, phytes of Psilophytales are ever discovered adventitious branches, the stele of the it would not be astonishing to find that branch not being continuous with that of the latter were of the same general nature the main stem. Xylem strand of stele as those of Psilotales; the gametophytes slender, only sometimes showing a distinc• may even have been less red uced" . tion of smaller central and larger peripheral Merker (1959) in his paper entitled tracheids. Tracheids with broad annular "Analyse der Rhynien-Basis und Nachweis thickening. Sporangia about 3 mm long des Gametophyten" first criticized the ter• and 1·5 mm in diameter. Sporangial wall minology of Kidston and Lang, particularly about 2 mm. thick Spores about 40 fl. in because they used the term "rhizome" for diameter. the creeping axes. If the creeping axis Locality - Muir of Rhynie, Aberdeen• had simply been the horizontal subterranean shire. parts of the same plant as the erect aerial Horizon - Old Red Sandstone (not stems one should have expected, in Merker's younger than the MiddIe Division of the opinion, that they had been biCurcating Old Red Sandstone of Scotland). with one of the branches curving upwards, Rhynia major - Plant larger in all its in the same way as in Lycopodi~tm clavatum, parts than R. gwynne-vaughanii, aerial stems where aerial shoots curve up from the hori• tapering upwards, and ranging in thickness zontal creeping axes. Among the from 6 mm. to 1·5 mm. or less. No Rhyniales, however, there is a marked hemispherical projections or adventitious difference between the two parts: the aerial lateral branches. Stele large, xylem strand axis usually forms an almost right angle of numerous tracheids differentiated as to the horizontal one. Also, according to smaller central and larger peripheral tra• Merker, the creeping parts (the rhizomes cheids. Sporangia reaching a length of of Kidston and Lang) of Rhynia are often LEMOIGNE-THE PRESENT STATUS OF RHYNIA GWYNNE-VAUGHANII 41 decaying while the erect axis are not. On (f) The occurence of gametophyte-like the basis of observations and his "intuition" non-vascular axes in R. gwynne-vaughanii. (*) Merker said: "Neue Untersuchungen (g) The absence of actual organic connec• haben nun ergeben, dass die horizon tale tion between the indubitable axes of R. Basis der "Urlandpflanzen" kcin Rhizom gwynne-vaughanii and the sporangia as• sein kann, sondern das vermisste Prothal• signed to it. It thus appears that at least lium darstel1t, das dank seiner weitgehen those axes of R. gwynne-vaughanii which den Bestandigkeit dem vi:illig wurzellosen bear the 'hemispherical projections' or Sporophyten als Brutpflegeorgan und als 'adventitious branches' may well represent Halt in Boden diente" (p. 452).* the gametophytes of either of Psilophytales Two years later, in 1961, Merker main• described from Rhyn,ie" (Pant, 1962, p. tains again that the creeping parts 278). ("rhizomes") of Rhynia major and Rhynia "It is clear from the above account that gwynne-vaughanii are gametophytic. there is a close structural similarity between Pant (1962) published an important paper the gametophytes of Psilotales (more espe• on the problem of the gametophyte of the cially the vascular gametophytes of Psilophytales. He based his conclusions Psilotum) and the axes of R. gwynne• partly on the famous work of Hollaway vaughanii, and also between the develop• (1939) on the vascularized gametophytes ment and structure of 'adventitious of Psilotum. Pant m1.de it his working branches' in R. gwynne-vaughanii and the hypothesis that it should be possible to formation, development and structure of discover the gametophytes of the Psilo• the young sporelings, in Psilotales. Indeed phytales in the m1.terial already described. the resemblance between the fossil and the From this starting point and on the basis living structures is so close that it is hard of his own observations, he arrived at the to explain it on any other basis than by following conclusions: regarding even the fossil "parent axes" as "As a result of our studies it now seems gametophytic.
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