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Spore-bearing plants with woody growth
Spore-bearing plants with woody growth
The progymnosperms are the basal-most members of the lignophyte clade. This means that they were some of the first plants on Earth to produce robust wood from a cambium, similar to modern day trees. This group was spore-bearing, which differs from modern woody plants (modern woody plants are seed-bearing). This group is thought to be a paraphyletic grade of plants. All members of this group were extinct by the end of the Permian. The progymnosperms were the first true trees, and probably the first woody vines on the Earth. Ancestral members of this group were leafless, but derived members exhibited laminate leaves, which were probably homologous to the leaves on seed plants. The increased size of these plants, and the significant working of soils by their root systems, increased weathering during the Late Devonian. This increased weathering, caused runoff, and the marine burial of massive quantities of organic carbon and inorganic carbonates. This substantially reduced atmospheric CO2 levels, causing severe global cooling, contributing to a large extinction event at the end of the Devonian.
Definition of progymnospermopsida
Definition of progymnospermopsida
Bonamo (1975) created the strictest definition of progymnosperms: woody habit, pycnoxylic wood with narrow rays, complex branching systems, little differentiation among successive orders of branches, terminal sporangia, a free-sporing habit, and fertile leaves that are dichotomous and pinnate.
Beck (1976) used a broader definition in which progymnosperms were plants with free sporing reproduction and gymnosperm-like wood.
Ecology & Habit
Ecology & Habit
Range of forms from woody vines to arborescent forms
First true trees on Earth (Archaeopteris)
True trees are lignophytes, producing wood and girth from a vascular cambium
Stems
Stems
Robust secondary xylem (wood) produced from a bifacial (2-faced) cambium
Secondary xylem (or wood) produced toward the center [centripetally]; secondary phloem produced toward the periphery [centrifugally]
Leaves
Leaves
Ancestral members lack leaves (e.g. Aneuophytales), possessing photosynthetic stems
All other members have laminate leaves, also called megaphylls
Reproduction
Reproduction
Spore-bearing plants; homosporous and heterosporous forms exist
Aneurophytales are homosporous; all others are heterosporous
Sporangia are terminal on axes born on fertile leaves
Fertile leaves are dichotomous or pinnate
Geologic Age
Geologic Age
Classification
Classification
└Progymnosperms †
Diversity
Diversity
Incertae sedis taxa
Incertae sedis taxa
Cecropsis luculentum †
Cecropsis luculentum †
Late Pennsylvanian of eastern Ohio
Protostelic shoot tip with mesarch maturation and 5 cauline protoxylem strands
Dense wood surrounds protostele
Helically arranged vegetative leaves are pinnately-dichotomously branched into terete lobes, and form a 2/5 phyllotaxis
Helically arranged sporophylls are more webbed and bear adaxial globose heterosporous eusporangia with terminal dehiscence
In megasporangia, there is only one radial, trilete megaspore. This is an example of extreme heterospory, which is mostly found only in seed plants
Above: Reconstrution of Cecropsis
Reimannia aldenense †
Reimannia aldenense †
Arnold 1935; Matten 1973; Stein 1982
Middle Devonian (Givetian) of New York
Perminerlaized axes with 3 orders of branching containing only primary tissues: 1st & 2nd-order steles are 3-ribbed; Circular in 3rd-order steles
Xylem maturation is mesarch
Outermost cortex contains hypodermal sclerenchyma
Above: Cross-sections of Reimannia aldenense (From Figs 1-5, Matten 1973)
Yiduxylon trilobum †
Yiduxylon trilobum †
Upper Devonian (Famennian) Tizikou Formation of Hubei Province, China
Helically arranged and bifurcate fronds with two orders of pinnae and planate pinnules
Both secondary pinnae and pinnules are borne alternately
Stems contain a small protostele with three primary xylem ribs possessing a single peripheral protoxylem strand
Thick secondary xylem displays multiseriate bordered pitting on the tangential and radial walls of the tracheids, and has biseriate to multiseriate and high rays
A narrow cortex consists of inner cortex without sclerotic nests and sparganum-type outer cortex with peripheral bands of vertically aligned sclerenchyma cells
Two leaf traces successively arise tangentially from each primary xylem rib and they divide once to produce four circular-oval traces in the stem cortex
Four vascular bundles occur in two C-shaped groups at each petiole base with ground tissue and peripheral bands of sclerenchyma cells
Yiduxylon shares many definitive characters with Calamopityales and Lyginopteridales, further underscoring the anatomical similarities among early seed plants
The primary vascular system, pycnoxylic-manoxylic secondary xylem with bordered pits on both tangential and radial walls of a tracheid and leaf trace divergence of Yiduxylon suggest transitional features between the early spermatophytes and ancestral aneurophyte progymnosperms
Above: Reconstruction of a forest of Archaeopteris trees
Above: Reconstruction of Archaeopteris branches attached to Callixylon wood
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