SCHECKLER, STEPHEN E.1*, BRIGITTE MEYER-BERTHAUD2, and JEAN GALTIER2. 1Department of Biology, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061-0406, USA; 2Laboratoire de Paleobotanique, Universite Montpellier 2, Place Bataillon, 34095 Montpellier cedex 5, France. - Secondary phloem of the Late Devonian progymnosperm tree Archaeopteris.
Well preserved secondary phloem was found in a trunk-sized axis
(Callixylon trifilievii/henkei-type) and a small branch (C.
trifilievii/zalesskyi-type) of Archaeopteris from Late
Devonian beds of Morocco and New York (early Famennian marine beds
near Jebel El Mrakib, Morocco, and basal Frasnian Oneonta Fm. from
Ashland, NY). Previous reports of bark in Callixylon (Arnold
1929, 1930; Lemoigne, Iurina, and Snigirevskaya 1983) did not clearly
identify secondary phloem so that the existence of this tissue in
Archaeopteris was uncertain, but described as simple. Our new
material shows unequivocally that Archaeopteris has complex
secondary phloem and bifacial vascular cambium (VC) similar to those
of aneurophytes where these tissues are better known. The small (1.5
cm diam) branch has intact VC and a thin layer of secondary phloem.
The large (11 cm diam) branch or trunk lacks preserved VC but has a
thick zone (7 mm) of secondary phloem with conspicuous rays and
concentric tangential bands of fibers and parenchymatous cells. Short
radial files of fibers are abundant in the innermost (0.5 mm) youngest
phloem. Both axes show that phloem rays have similar widths and
heights as xylem rays, have similar but larger procumbent cells, and
lack ray tracheids. Clusters of sclereid-like (tanniniferous?) cells
and elongate sieve cell-like elements occur amongst the axial
parenchyma. Expansion adjustment of older tissues is by enlargement of
axial parenchyma cells, preferentially in the radial direction, so
that concentric fiber bands are spaced farther apart. Secondary phloem
of Archaeopteris is thus histologically similar to that of
aneurophytes, differing only by fibers occurring mostly in concentric
tangential bands in older external phloem. Bifacial VC of
Archaeopteris is also organized like that of aneurophytes.
Despite differences of vegetative and reproductive morphology, the
similar secondary tissues and VC of Archaeopteris and
aneurophytes support interpreting them as closely related basal
lignophyte groups.
Key words: Archaeopteris, Devonian, phloem, progymnosperm