Rhacophytes †
Rhacophytales
The Rhacophytes are spore-bearing, fern-like plants that existed during the Late Devonian. They were probably tall shrubs that lived in monotypic stands in open floodplain environments (in deltaic marshes and peat-accumulating back-swamps). During the Late Devonian they were co-dominant with Archaeopteris in drier portions of floodplains growin gin the understory contributing to fuel low-intensity fires. They lacked laminate leaves, but ultimate appendages displayed flattening (no webbing) for collecting light and conducting photosynthesis. Internally, these lateral structures have fern-like primary anatomy, but they also exhibit wood which is never found in fern leaves.
Stems
Rhizome
There are some indications that Rhacophyton employed clonal propagation to occupy and monopolize preferred habitats.
Dense mats of adventitious roots are commonly found along the main stem and on the laterals.
Instead of a cluster of separate and erect shrubs, Rhacophyton may have adopted a sprawling habit with the main axis and some laterals functioning like rhizomes
Upright shoots
Tall shrub with a central main stem 1-1.5 m in height (Cornet et al. 1976)
Branch system is semi-erect and frond-like
Laterals exhibit a quadriseriate branching pattern, which exhibit two vertical rows of alternating sets of paired branches
Laterals extended as much as 50 cm and exhibited a variety of morphologies.
Vegetative laterals typically exhibited a biseriate arrangement (two lateral rows forming a plane) of side branches (pinnae) reminiscent of fern fronds
Laterals were true branches containing small amounts of secondary xylem (fern fronds are compound leaves)
Secondary growth may have been from unifacial cambium; only wood production and not secondary phloem
Secondary growth only in periclinal divisions; not anticlinal to add more cambial initials as stem increases in girth
They can also exhibit a 3-dimensional quadriseriate side branching, often at the distal ends of otherwise biseriate laterals
No lamina or blade on stems; photosynthetic terminal axes
The pinnae- and pinnule-like ultimate branches vary in their degree of flattening, but they are not webbed (no laminar tissues extending between veins).
Stem consist of a central, clepsydroid-shaped strand of primary xylem
Primary xylem surrounded by radially aligned rows of secondary xylem
Peripheral loops occur at the end of the primary xylem bar
Leaves
No distinct leaves since there is no lamina or blade on stems
Classification
└? Rhacophytales †
The systematic position of Rhacophyton is uncertain.
Andrews and Phillips (1968) connected this group to progymnosperms.
Cornet et al. (1976) concluded it was neither progymnosperm, nor fern.
Non-fern traits: possesses secondary xylem in its laterals branches.
Fern-like traits: distinctive vascular tissue morphology found only in ferns.
Most place Rhacophyton within the monilophytes, but outside the living fern lineages.
Geologic Age
Fossil found in New York, Virginia, West Virginia, Maine, Belgium, Germany, Norway, and western Siberia.
Reproduction
Three-dimensional structure that appears loosely ball-shaped
Main axis forks many times to produce both long sterile axes and highly branched fertile axes
Probably homosporous
Fertile lateral segments have a 3-dimensional quadriseriate arrangement, typically with two elongate sterile pinnae and two highly branched fertile pinnae. Fertile segments have been found on the distal ends of otherwise sterile laterals, but laterals bearing fertile structures have yet to be found attached to the main stem. Rhacophyton is probably homosprous. In other words, it produces only one type of spore.
Above: Eocladoxylon
Left: Rhacophyton
Diversity
Rhacophyton ceratangium
Andrews and Phillips, 1968; Cornet et al., 1976)
Chlidanophyton dublinensis
Gensel, 1973
Late Devonian - Early Tournaisian
Melvillipteris quadriseriata
A plant of Late Devonian (Famennian) age, from Melville Island, Nunavut, Canadian Arctic Archipelago.
Main axes and two orders of lateral branches are recognized.
The main axes are monopodial in habit, with a zigzag and/or upright appearance and dense adventitious roots attached on some internodes.
First-order branches are inserted on the main axes in distichous pairs, showing a quadriseriate pattern (i.e. alternate pairs); they bear one or two sterile ultimate appendages on the proximal portions, and then distally bear alternate second-order branches.
Second-order branches bear alternate ultimate appendages which may be fertile or sterile; the sterile ultimate appendages are composed of one or two successive dichotomies, terminating in oppositely recurved tips, and fertile appendages show an elaborate branching system.
Each fertile appendage has an initial dichotomy forming two sister branches, each of which further divides three or four times to produce ultimate divisions terminated by sporangia.
Two to four small fusiform sporangia are grouped in a truss which is supplied by penultimate division of the fertile appendage.
The sporangia have a pointed tip and a longitudinal dehiscence.
Eocladoxylon
Middle Devonian Xichong Formation, Yunnan province, China,
Demonstrate three orders of branching.
Sterile branching systems have a pinnate two-dimensional branching pattern with alternate insertion of lower orders.
Up to several times deeply dissected, overall wedge-shaped, flattened appendages are inserted alternately on the third order of branching.
Second and third orders of branching have an opposite pair of similar appendages inserted at the base, perpendicular to the plane of the remaining branching system.
Fertile appendages, consisting of a dichotomous axial system and terminal pairs of sporangia attached closely in groups of eight, replace the sterile appendages in fertile branches.
Anatomy of a probable second-order axis demonstrates a bipolar clepsydroid vascular system.
The sterile branching system and anatomy bear some similarity to the Late Devonian fernlike plant Rhacophyton.
The potential for an ancestor/descendant relationship between Iridopteridales and Rhacophyton
Protocephalopteris
Schweitzer, 1968
Protopteridophyton
Li and Hsu 1987
? Urpicalis steemansii
Gerrienne 1992
Lower Devonian (Emsian) plant of Fooz-Wepion (northern margin of Dinant Synclinorium, Belgium)
Herbaceous plant made of smooth sporangiferous axis systems, isotomously branched, probably borne laterally on wider smooth axes. Distance between two successive isotomies, variable.
Sporangia fusiform to oval-elongated, one being often borne at the axil of the isotomy and the others inserted in one row on the adaxial side of the isotomously branched axes.
Sporangium orientation parallel to the isotomy bisector.
Epidermic cells elongated with tapering ends.
Spores and plant anatomy unknown.
This incertae sedis taxon is reminiscent of the fertile axes of Aneurophyton and may represent one of the first steps leading to the Progymnosperms
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