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2 Laboratoire de Paléobotanique, Institut des Sciences de l'Evolution (Unité Mixte de Recherche 5554), Université Montpellier II, Place Bataillon, 34095 Montpellier Cedex 5, France; and 3 Department of Biology—0406, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0406 USA
Received for publication April 29, 1999. Accepted for publication July 23, 1999.
ABSTRACT
A 5 m long trunk of a young Archaeopteris/Callixylon erianum tree from the Late Devonian of Morocco shows new branching patterns for early lignophytes. This progymnosperm tree produces a helical pattern of traces that we infer belonged to reduced, short-lived, primary (apical) branches (type A) as well as two types of adventitious traces (types B and H). We infer that type-B traces supplied branches that initiate close to the site of attachment on the trunk of some, but not all type-A branches in an irregular but nonrandom pattern. Unlike ephemeral type-A branches, those of type B persist and become long-lived, potentially permanent units of the architecture of Archaeopteris trees. Type-H adventitious traces are also short-lived and occur singly or in serial groups, but differ from traces of either type A or B branches by lacking differentiation into a readily identifiable organ category. We interpret type-H traces as supplying latent primordia that could develop into either adventitious roots or shoots depending on extrinsic factors. Our new data suggest that Archaeopteris had a wide range of branch primordium amplitude. Type-B branches compare with axillary lateral branch buds of some Early Carboniferous spermatophytes (Calamopitys) and are a major developmental departure from the strictly apical, pseudomonopodial shoot branching of older aneurophyte progymnosperms. Type-H traces suggest that Archaeopteris trees had some potential for formation of adventitious roots or shoots in response to environmental factors, such as partial burial by overbank sedimentation. Collectively, these novel methods of tree branching may partly explain the extraordinary success and worldwide dominance of Archaeopteris forests on fluvially dominated, Late Devonian floodplains.
Key Words: Archaeopteris • Callixylon • development • Devonian • evolution • lignophytes • progymnosperms • trees
The Late Devonian progymnosperm Archaeopteris (Dawson, 1871) played a prominent role in the phylogeny of land plants. It is the major representative, in terms of abundance, geographical distribution and diversity, of the progymnosperms (Nixon et al., 1994
; Rothwell and Serbet, 1994
). Members of Archaeopteris also comprise the earliest trees of the lignophyte clade, and remains are abundant and distributed worldwide. These trees dominated forest ecosystems at the end of the Devonian and may have played a significant role in biotic crises and global climatic changes that affected the biosphere at this time (Algeo et al., 1995
; Algeo and Scheckler, 1998
).
Beck's (1960a, b
) discovery of the connection of Archaeopteris fern-like branch systems with Callixylon conifer-like axes prompted renewed interest for this genus. But the wealth of information that has accumulated since then principally relates to the distalmost parts of the plant, the so-called "lateral branch systems." Interestingly, despite the pivotal place of Archaeopteris in the evolutionary history of early terrestrial plants, few characters of its vegetative body are considered as evolutionarily significant. Indeed, besides heterospory, the only unequivocal character presently recognized as common to Archaeopteris and the seed plants is the possession of laminated ultimate appendages arranged alternately/helically (Rothwell and Serbet, 1994
).
We believe that many phylogenetically informative characters exist for Archaeopteris and other progymnosperms that will help identify evolutionary connections between these basal taxa and their sister lignophytes, the seed plants. Developmental characters, like the trunk branching we report here and in Meyer-Berthaud, Scheckler, and Wendt (1999)
, and others like vascular cambium mechanics (Scheckler and Galtier, 1998
) or gametophyte cellularization (Scheckler, Snigirevskaya, and Hill, 1997
), as well as wood, secondary phloem, and cortical anatomy will provide additional phylogenetic information for resolving basal lignophytes and spermatophytes, as well as their possible connections to other early, non lignophyte woody plants with which they could easily be confused, such as Zygopteridales, Rhacophytales, and Cladoxylales.
Understanding of the whole Archaeopteris plant is still rather poor, and, since their first publication, Beck's (1962, 1964a, 1971, 1981)
speculative reconstructions of Archaeopteris as a conifer-like tree have rarely been challenged (Snigirevskaya, 1984a, b
; Chaloner, 1999). Recently, a significant advance in the reconstruction of the vegetative body of Archaeopteris came from Trivett's (1993)
structural analysis of a piece of trunk measuring 4 m in length, 0.6 m in diameter, and assigned to the species Archaeopteris/Callixylon whiteanum. Trivett (1993)
erected a theoretical model of the tree, consisting of an orthotropic monopodial trunk with a succession of branches of increasing complexity from base to distal part of the stem. All branches arise from the pseudomonopodial division of the trunk apex. Trivett validated the model using her results from vascular trace analysis. She also identified traces of adventitious organs, interpreted as branches, capable of replacing parts of the tree in case of damage and thus ensuring its prolonged survival.
In this paper, we follow the same type of analysis as Trivett's, but on the distal portion of a young trunk of Archaeopteris/Callixylon erianum. This 5-m long specimen, from an early Famennian locality in southeast Morocco, is decorticated. It tapers from 6.3 x 9 cm in diameter at its base to 3.2 x 5.4 cm distally, bears no branches suggestive of a terminal crown, and is straight (Meyer-Berthaud, Wendt, and Galtier, 1997
). We interpret this axis as a young trunk, rather than a long lateral branch, because it lacks any curvature over its length and secondary xylem shows no eccentricity or other anatomical features of reaction wood, although sediment compaction has diagenetically flattened the axis somewhat.
The small diameter of this trunk facilitated its technical preparation. More importantly, its presumed young age made it especially favorable for analysis of the primary construction of this tree where features related to aging should be either absent or few and weakly developed. Based on vascular trace analysis, three types of organs were identified in the distal part of the Moroccan trunk. Two types at least are interpreted as branches that are morphologically similar to branches previously reported in the literature. Our mapping of the vascular traces allows an accurate reconstruction of their sequence of production and suggests that the most evolutionarily significant characters of the vegetative body of Archaeopteris are primarily developmental rather than either structural or morphological.
MATERIALS AND METHODS
The 5-m long trunk, first reported by Wendt and Belka (1991)
, was collected on the north-west slope of Jebel El Mrakib, in the Eastern Anti-Atlas of Morocco. It occurred in a marine horizon of alternating black limestone beds and dark shales dated as early Famennian age by the associated goniatite and conodont fauna. The specimen is preserved as a calcite petrifaction and was identified as a trunk of Archaeopteris/Callixylon erianum primarily by the structural characters of its secondary xylem (Meyer-Berthaud, Wendt, and Galtier, 1997
). This specimen is entirely decorticated and some features of its branching were briefly described by Meyer-Berthaud, Scheckler, and Wendt (1999)
.
The distalmost 40 cm long portion of the trunk, constituting the focus of the present report, was sectioned into 31 transverse and 30 longitudinal thin sections (Fig. 1). Most were prepared from two regions, one proximal (blocks 33B and 33B1) and one distal (blocks 36A–37A1), which were selected because they showed protruding scars on the outer wood surface.
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The level of the trunk analyzed in this paper corresponds to a menetogenetic phase of development (stationary phase when size and complexity of primary body remain unchanged; Eggert, 1961
) where the diameter of the stelar system and the number of primary xylem strands in the eustele do not vary significantly. The stele has been compressed and is ~21 mm in the largest transverse dimension vs. 5–12 mm in the smallest one. The number of primary xylem strands averages 31. Tangential divisions and fusions of the primary vascular bundles are unpredictable and seem to occur more abundantly on one side of the axis (Fig. 3).
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). They are preserved over a length of ~2.5 cm, after which they terminate and are buried by successive wood layers (Figs. 7–8). In transverse section, the level of their course where they are broken corresponds to a horizontal distance from the margin of the stele of 6 mm or less. Internode length ranges from 8.5 to 15 mm. Type-A traces consist of a cylindical core of primary xylem surrounded by a ring of secondary xylem (Fig. 6). In transverse section, the primary xylem is 450–750 µm wide (tangential dimension) and the secondary xylem 300–400 µm thick. Primary tissues in type-A traces do not vary significantly in cross-sectional area through the course of their departure. The primary xylem strand is mesarch with tracheids averaging 35 µm in diameter. There is one protoxylem strand in the proximal part of type-A traces, that divides into two distally (Figs. 5–6).
Nature of type-A organs
The organs corresponding to type-A traces develop from apically produced primordia and are short lived. Type-A traces resemble leaf traces as they do not vary much in diameter and do not give rise to traces of a higher order in their preserved course of departure. But they are larger than those produced by the Callixylon axes reported by Scheckler (1978)
. They differ also from all other leaf traces described elsewhere in that they are enclosed in a layer of secondary xylem, do not get tangentially elongated and do not divide into two strands distally (Scheckler, 1978; Snigirevskaya, 1982; Kenrick and Fairon-Demaret, 1991
). The size of the traces at the level where they connect to the trunk stele is comparable to that of branches described by Beck (1979)
in several species of Callixylon. In the Moroccan specimen, however, traces do not increase in diameter, do not contain a pith, and show only two protoxylem strands in their distal parts. We interpret them as morphologically comparable to Archaeopteris ultimate branch traces, which, according to Scheckler (1978)
, have a proximal diameter of 300–800 µm, remain cylindrical, and do not produce leaf traces in the proximal 1–3 cm of their course. A less likely alternative, however, is that type-A traces supplied large leaves, perhaps like those of Eddya (Beck, 1967
). If our preferred interpretation is correct, then the lack of leaf traces in the basal part of type-A branches suggests that their development was sylleptic or synchronous with that of the trunk (Hallé, Oldeman, and Tomlinson, 1978
).
Type-B traces
In cross section, the proximal end of type-B traces occurs close to the margin of the trunk primary vascular body, at a horizontal distance of 3 mm or less from the latter (Figs. 10–15). We are uncertain, from the present evidence, whether type-B traces connect to the stele but, if so, the connection is inconspicuous and unlike that of type-A traces (Figs. 14–15). Type-B traces diverge at ~30° from horizontal and their course extends through the whole thickness of the wood (Fig. 10). They protrude on the external surface of the specimen as oval structures that are up to 10 mm wide and 20 mm high. The three traces identified in the proximal zone are distributed evenly around the trunk section; although not strictly opposite, the two traces in the distal zone are separated by a divergence angle exceeding 120°.
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Nature of type-B organs
Based on the structure of their vascular traces, which show a conspicuous pith except at the very base, and on the fact that they produce regularly arranged appendages, type-B organs are interpreted as branches. The large size of their traces and shape of their stele, which is not so prominently ridged as that of Archaeopteris penultimate branches (Scheckler, 1978
), suggest that type-B organs may have produced larger and more complex branches than the latter. They possibly corresponded to axes with types IV/V growth according to Trivett's categories (1993)
.
The occurrence of short internodes at the base of type-B organs suggests that their growth was delayed compared to that of the trunk (Hallé, Oldeman, and Tomlinson, 1978
). We must consider the possibilities that type-B branches were either adventitious or derived from exogenous buds and had a proleptic growth, after a short period of dormancy (Fink, 1983, 1984
). Type-B traces contrast markedly with those of the adventitious organs reported for Archaeopteris/Callixylon whiteanum (Trivett, 1993
), which generally occur in groups, are short and run horizontally through the wood, are apparently devoid of a pith, and do not produce laterals. But type-B organs neither correspond to any of the apically produced branch types previously described in Archaeopteris that might have simply had delayed development. Indeed, in all orders of Archaeopteris axes investigated to date, all apically initiated organs, whether similar or not, are generated successively along one single phyllotactic helix (Carluccio, Hueber, and Banks, 1966
; Beck, 1971
; Scheckler, 1978
). Positional relationships between type-A and type-B traces indicate that this is not the case in the Moroccan specimen and that type-B organs correspond, from a developmental point of view, to a new type of structure, previously unknown in progymnosperms.
Type-H traces
Type-H traces are not connected to the primary vascular system of the trunk and are produced at a minimal distance of 8 mm from the margin of the trunk stele (Figs. 9, 18). Their course is typically horizontal and extends over a length of 5–10 mm within the wood (Figs. 9, 20). Type-H traces are produced irregularly, but transverse sections show that, when they occur, their position is linked to that of type-A traces, as they originate on their abaxial side and along the same radius (Figs. 9, 18). There is no connection, by means of horizontally orientated tracheids, between the two types of traces, which are separated by a variable amount of secondary xylem. Moreover, reconstructions of the course of the two types of traces indicate that type-H ones do not occur as horizontal extensions of type-A ones but are generally produced below the distal end of the latter (Fig. 18). Type-H traces occur singly or in serial groups of 2–5, sometimes more, vertically arranged bundles (Figs. 19, 23).
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Nature of type-H organs
Type-H organs correspond to adventitious appendages that arise after the production of a minimum amount of secondary xylem estimated at ~8 mm in the present specimen. Their production is spatially linked to that of type-A organs, but they do not necessarily occur at all nodes. Traces have a limited extension (Figs. 9, 20). They get occluded by secondary xylem distally, indicating that these organs were short lived. Their arrangement, all along the circumference and length of the specimen, suggests that type-H organs were initiated as a normal process during trunk growth and not as a local response to any external stimulus.
Prior to this study, the only documentation of adventitious organs in Archaeopteris was by Trivett (1993)
from a structural analysis of a large trunk of Archaeopteris/Callixylon whiteanum. From the two categories of traces she described, those that are small (<1 x 2 mm in transverse dimensions) and devoid of secondary xylem compare well with the type-H traces here. The only differences lie in the apparently higher proportion of single type-H traces in the Moroccan specimen. Trivett interpreted the adventitious appendages of A./C. whiteanum as branches. But the type-H traces in the Moroccan specimen do not resemble any branch or root traces described to date in Archaeopteris (Beck, 1953, 1967, 1971, 1979, 1981
; Scheckler, 1978
; Beck and Wight, 1988
; Algeo and Scheckler, 1998
; Algeo, Scheckler, and Maynard, in press
). Instead, they compare positively, in their origin, orientation, and cell arrangement, with the radial bands of tissue that differentiate in the secondary xylem of extant trees in relation to the production of latent primordia from cambial cells (Carlson, 1938, 1950
; Bannan, 1942
; Fink, 1982
). These structures are mostly parenchymatous, but tracheid-like elements have been described in some conifers (Fink, 1982
). The fate of the primordia is not determined a priori. From most observations reported to date on extant trees, they either abort or diffentiate into root meristems, contributing to the formation of adventitious roots on layered branches and stem cuttings.
DISCUSSION
Based on the size, structure, and spatial arrangement of the vascular traces embedded in the wood, at the distal part of the specimen, three categories of appendages are identified. A single category, type-A branches, represents laterals that grow from apically initiated primordia, in a predictable phyllotaxis, and with development that is synchronous with that of the trunk. These organs were probably short-lived. From a developmental point of view, this combination of features suggests that type-A laterals are homologous to leaves, even though we suspect they were radially symmetrical branches (Carluccio, Hueber, and Banks, 1966
; Beck, 1971
; Scheckler, 1978
; Kenrick and Fairon-Demaret, 1991
). This statement is independent of our interpretation of the morphology of type-A organs. Whether the latter are actually similar to the distalmost branches of Archaeopteris lateral branch systems, as we argue earlier, or to simple leaves like those of Eddya (Beck, 1967
), they represent structural units that were initiated exogenously, had a determinate development, and grew like spermatophyte leaves. This view is consistent with Beck's (1971, 1979, 1981; Beck and Wight, 1988)
and Scheckler's (1978)
previous interpretations concerning the branches of Archaeopteris lateral branch systems. From an architectural point of view, type-A branches likely did not play a significant role in the construction of the tree. Physiologically, however, type-A branches may well have been a major photosynthetic surface for the plant. From a morphological point of view, we did not identify any traces to simple leaves directly borne on the 40 cm long portion of trunk analyzed. This interpretation agrees with Beck's (1979)
and Trivett's (1993)
previous statements that some portions of Archaeopteris main stems may have lacked such appendages.
Several features of type-B branches deserve special consideration when discussing the evolutionary significance of such organs. (1) They were not part of the normal sequence of organs produced at the stem apex. Rather, they represent a new category of organ for progymnosperms (Scheckler, 1976, 1978
), produced in addition to the latter. Their origin, either endogenous or from exogeneous buds, is uncertain. (2) The place where they were initiated is related to the site of production of a type-A branch. (3) Their development was delayed compared to that of the trunk and type-A branches. (4) They are the largest and most complex organs borne on the trunk length analyzed herein. They had a long life span, and we hypothesize that they played a significant role in the crown architecture of the Archaeopteris/Callixylon erianum tree.
Within the lignophyte clade, the earliest occurrence of axillary branching was reported in a basal family of putative seed plants, the Early Carboniferous Calamopityaceae (Galtier and Holmes, 1982
; Galtier, 1999
). The few cases described in Calamopitys correspond to branches that were indeed inserted in the axil of megaphylls but with vascular bundles that generally connected to the closest vascular traces and not necessarily to the stele of the main stem, suggestive of some period of branch primordium dormancy. Galtier (1999) suggests that the evolution of axillary branching was a gradual process starting within the seed plant group. According to his scenario, seed plants first evolved megaphylls, from planation of lateral branch systems of progymnosperm type. The new physiological properties of such appendages allowed first the rise of a Calamopitys type of occasional delayed axillary branching, and then the derived type shared by most seed plants since the Late Carboniferous.
The evidence presented in this paper suggests that Late Devonian (Famennian) Archaeopteris evolved a new branching syndrome that necessitated only a few adjustments to fit with the axillary branching of seed plants (Fig. 26). This syndrome involved the evolution of an extra type of branch and spatial dependence on a physiologically leaf type of organ, the latter not necessarily fully planated. The main difference with axillary branching of the Calamopitys type lies in the laterally offset (i.e., nonaxillary) position of the branch relative to the "leaf."
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found comparable adventitious branches in the trunk wood of a large old tree trunk. She attributed them to tree aging and suggested that they grew out as small branches that may have supplemented crown photosynthesis, reproductive effort, or perhaps delayed tree death similar to the small adventitious branches that appear on diseased Ulmus americana. We find, instead, that type-H adventitious traces occur commonly in the wood of a young trunk. This suggests that they arose without significant aging or imminent senescence of a tree and are part of normal biology of some Archaeopteris. Evidence based on present-day spermatophyte trees indicates that, when they do not abort, latent meristems often develop into roots, favoring the efficient propagation of species by means of layered axes or cuttings, in both artificial and natural conditions (Carlson, 1938, 1950
; Bannan, 1942
; Fink, 1982, 1983, 1984
)
If Archaeopteris produced adventitious roots and propagated vegetatively, then, combined with its heterosporous condition, these features may have represented major factors for the worldwide success of this tree in the Late Devonian. The common occurrence of extensive gallery forests of Archaeopteris trees adjacent to the riverine systems of Late Devonian floodplains (Beck, 1964b
; Retallack, 1985
; Scheckler, 1986
; Beerbower et al., 1992
; Algeo and Scheckler, 1998
) suggests that this plant may have been well adapted to the disturbance regimes of bank erosion and overbank sedimentation, thus permitting these trees to thrive and expand in fluvially disrupted habitats.
CONCLUSION
In the portion of trunk described in this paper, a single category of organs, the type-A branches, arise from the pseudomonopodial division of the trunk apex. They are ephemeral and interpreted as developmentally homologous to the megaphylls of early seed plants. If we designate the "node" as the level of the trunk where they are inserted, it is significant that the other two categories of organs that arise successively during the development of this trunk length are produced at nodal regions. The morphogenetic potential of nodal areas is a feature commonly observed in extant spermatophyte organisms (Bell, 1991
).
One of the significant advances of the present work is the identification of the lateral origin of the perennial branches, the type-B ones, interpreted here as major architectural units of the tree. This branching pattern is an important breakthrough from the Aneurophytales where the primary body entirely derives from pseudomonopodial branching (Scheckler, 1976
). Based on two features, the fact that these organs are not inserted in the ontogenetic spiral of type-A "megaphylls," and that they are produced at nodal zones, this Archaeopteris branching syndrome compares closely with the axillary branching pattern of the earliest seed plants (Galtier, 1999).
Our interpretation concerning the adventitious type-H structures arising later in the development of this distal part of a trunk differs somewhat from that of Trivett (1993)
. By comparison with extant plants, we interpret them as latent primordia of undetermined fate that generally die after a certain time but which are potentially capable of forming either roots or branches if activated by external factors. We agree with Trivett that such organs greatly increase tree survival. We further speculate that the potential for forming roots on aerial axes is an element of major ecological importance that may have contributed to the worldwide distribution of Archaeopteris in the Late Devonian.
FOOTNOTES
1 The authors thank Jobst Wendt for providing the trunk; Jean Galtier for critical reading of the manuscript; Muriel Fairon-Demaret, William G. Chaloner and Claude Edelin for helpful advice; and Laurence Meslin, Jacques Guiraud, Michel Pons, and Bernard Orth for technical assistance. This research was supported by sabbatical research leave funds from VPI&SU (to SES), and a grant from the National Science Foundation (NSF-IBN 9728719 to SES). Contribution 99-063 of ISEM (UMR 5554). 
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