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PBIO 3080/5080 – STRUCTURAL BOTANY
LABORATORY 7: EARLY LIGNOPHYTES
THE LIGNOPHYTES: PROGYMNOSPERMS & SEED PLANTS
Lignophytes are a clade of vascular plants that consists of seed plants and
progymnosperms, a paraphyletic assemblage from which the seed plants arose. Based on the
transition from free-sporing heterospory to gymnospermous reproduction, it is thought that the
gymnosperms were derived from the progymnosperms. In this lab we will characterize the
progymnosperms by the features of Archaeopteris (Order Archaeopteridales) and then look at the
characters that changed in the evolution of the earliest seed plants, the seed ferns. The latter are
represented in this lab by the most ancient seed fern that has been reconstructed to date, Elkinsia
polymorpha, from the Upper Devonian. The general relationships among progymnosperms and
seed plants are illustrated in the following cladogram from a cladistic analysis of lignophytes
(Fig. 4 of Hilton & Bateman, 2006).
CLASS PROGYMNOSPERMOPSIDA
The Class Progymnospermopsida was proposed by Charles B. Beck (1960) to include
“plants having gymnospermic secondary wood and pteridophytic reproduction” (Beck 1962).
There are three orders of progymnosperms: Archaeopteridales, Aneurophytales and
Protopityales.
Archaeopteris
As illustrated in Fig. 21.5 (p. 266) of Stewart & Rothwell (1993), Archaeopteris was a
large, multi-branched tree with small fan-shaped leaves (also illustrated in Figs. 18.5 and 18.6,
pg. 433 of Raven, 8th ed.). The leaves of some species were highly dissected (see Fig. 21.12 in
Stewart and Rothwell). Based on plant habit, Archaeopteris looks a lot like a conifer, but much
of the resemblance is superficial. While Archaeopteris had a eustele and lateral growth from a
bifacial vascular cambium and cork cambium, it did not have axillary branching. Instead,
branching was apical, typical for pteridophytes, and the branches were borne in the same helix
with the leaves. We do not know if Archaeopteris had bipolar growth, or whether it had a
globular embryo or a cotyledonary embryo. How do you think these features would affect the
way an Archaeopteris tree grew?
Examine a cross section of an Archaeopteris branch. Although the stem is flattened, you
can make out the pith region and the zone of wood. Look at the inner margin of the wood and
identify the mesarch bundles of the eustele that characterize Archaeopteris. Diagram the stem in
cross section.
Examine a leafy branch of Archaeopteris (see Fig. 21.1 in Stewart & Rothwell, 1993).
Locate the leaves and try to interpret how they were arranged. Draw the leafy branch system.
Now examine a fertile branch system of Archaeopteris (Figs. 21.2 and 21.13 in Stewart
and Rothwell). Note that the elongated sporangia stand upright from the adaxial surface of the
sporophylls. Megasporangia and microsporangia apparently are mixed on the leaves (Fig. 21.12
of Stewart and Rothwell), but it is difficult to interpret which is which until you macerate the
spores from the sporangia. Draw a segment of the fertile branch system. Now draw a close up of
one fertile leaf that shows several sporangia.
Callixylon
The wood of Archaeopteris is preserved in large logs that are given the name Callixylon.
It was Beck (1960) who made the connection that these two morphogenera actually represented
the same plant! The wood looks like conifer wood, but it has bordered pits arranged in distinctive
groups on the radial walls of the tracheids (see Fig. 21.4 in Stewart & Rothwell; Fig. 18-4, pg.
432 in Raven, 8th ed.). This grouped pitting is a characteristic feature of Callixylon wood, and is
not found in the wood of gymnosperms.
Examine a block of Callixylon wood. Note how much like conifer wood it appears.
Now examine a radial section of the wood, and find the grouped pitting. Draw a segment
of the wood that shows both the grouped pitting of the tracheids and also some ray cells.
THE MOST ANCIENT SEED PLANTS:
ORDER PTERIDOSPERMALES
Members of the extinct Order Pteridospermales are commonly referred to as “seed ferns.”
They lived from the Upper Devonian to the Cretaceous. Seed ferns formed the ancestral group
from which the Cycadales and all other seed plants arose. In cladistic terminology, seed ferns are
a polyphyletic assemblage and most occur at several nodes near the base of the stem on the
lignophyte tree (see the lignophyte cladogram from Hilton & Bateman, 2006 above). In this
laboratory we will examine material from Elkinsia polymorpha, the most ancient seed fern plant
from the Upper Devonian of West Virginia. We will also examine specimens that show various
degrees of megasporangial enclosure by integumentary lobes (i.e., a transformational series
illustrating the origin of the integument). This brief treatment of an extremely large and diverse
group will serve to give you an idea of the earliest seed plants, and some feeling for the evolution
of the integument.
Elkinsia polymorpha
Examine the reconstruction of the Elkinsia plant as well as the other papers in which
various parts of the plant are detailed. As the plant has been reconstructed from both compressed
and anatomically preserved specimens (Serbet & Rothwell, 1992), it consists of an unbranched
stem from which fern-like fronds diverge in a helical arrangement.
Examine specimens of Elkinsia stem and frond material that are compressed on the rock
surface, and those that have been removed from the rock surface for further preparation. Note the
slender, unbranched nature of the stem and the helical arrangement of the fronds. Also note that
the fronds are dimorphic—that is to say that some are vegetative, terminating in pinnately
arranged pinnules, while others are forked throughout and terminate either in cupulate preovules
or in clusters of pollen sacs.
**Be extremely careful with the Elkinsia specimens! It is the only material of the genus,
and the material upon which the concept of the most primitive seed ferns is based!**
Examine the demonstration specimens and slides of Elkinsia vegetative and fertile
remains. Note the anatomy of the stem. Is there secondary xylem (wood)?
Now examine preparations and reconstructions of the cupulate preovules. Be sure that
you understand what the various parts are, and how they work. Make drawings of those organs
that you consider necessary to help you remember the important features of Elkinsia. Particularly
important are the feature of the preovules. These will be used as an early stage in the
transformational series that leads to fully integumented, indehiscent megasporangia (seeds).
What was the propagule and how was it sealed after pollination in Elkinsia?
EVOLUTION OF THE INTEGUMENT
Seeds or ovules of “hydrasperman seed ferns” (Rothwell, 1986) illustrate stages in the
evolution of the integument. The whorl of integumentary lobes that surround the nucellus of
Elkinsia polymorpha (that you have already examined) reflects the morphology of the earliest
stage in the series. More derived morphologies are represented by the morphogenera for ovules,
Tyliosperma and Conostoma. In the genus Tyliosperma, the lobes of the integument are fused at
the base of the preovule, and free at the distal end. Is there a micropyle in this type of preovule?
Examine a transverse section of Tyliosperma near the base of the preovule.
Now examine an oblique section from closer to the apex. Diagram the ovule as seen in
each section, and then draw the ovule from the outside as you have reconstructed it in your mind.
Ovules with completely fused integumentary lobes are more like the seeds of extant plants.
Examine a demonstration specimen of the genus Conostoma in mid-longitudinal section.
Sketch the ovule. Identify the integument, micropyle and nucellus. Note the complicated
structure of the hydrasperman pollen chamber.
Review the structure, growth, and life cycles of a free-sporing pteridophyte versus a seed
plant to be sure you understand the similarities and differences. Also, review the structure of an
ovule or seed to be sure you understand the homologies of the parts. Ask if you don't understand!
View the animation of pollination biology for hydrasperman seed ferns that is found on
the course web site. How does this type of reproduction compare to that of “seed-free”
heterosporous plants? How does it compare to reproduction in living seed plants? Think in terms
of similarities and differences in each case, and of the relative advantages/disadvantages to the
plants that display each type of reproduction. (Wouldn’t this make a good exam question?)
REFERENCES
Beck, C.B. 1960. The identity of Archaeopteris and Callixylon. Brittonia 12: 351-368.
Beck, C.B. 1962. Reconstruction of Archaeopteris and further consideration of its phylogenetic
position. American Journal of Botany 49: 373-382.
Hilton, J. & R.M. Bateman. 2006. Pteridosperms are the backbone of seed-plant phylogeny.
Journal of the Torrey Botanical Society 133: 119-168.
Rothwell, G.W. 1986. Classifying the Earliest Gymnosperms. In: Spicer, R. A., and B. A.
Thomas eds., Systematic and Taxonomic Approaches in Paleobotany. The Systematics
Association Special Volume No. 31, Linnean Society of London, Academic Press,
London, pp. 137-161.
Serbet, R. & G.W. Rothwell. 1992. Characterizing the most primitive seed ferns. I. A
reconstruction of Elkinsia polymorpha. International Journal of Plant Science 153: 602621.