Ch.
21: Vascular Plants without Seeds Transformation Theory
Concepts
Life Cycles
Diabontic • Having multicellular sporophyte and gametophyte
• Manifested in all extant and known fossil plants
Monobiontic • Only have one multicellular generation
• In some algae, esp. Coleochaete
• Zygote undergoes meiosis to produce spores,
growing into gametophytes. It does not undergo
mitosis to produce mitosis to produce sporophytes
Hypotheses/Theories
Interpolation • Earliest land plants did not have a
sporophyte generation; the zygote
germinated by meiosis that produced haploid
spores that grew into new haploid Early Vascular Plants
gametophytes
Rhyniophytes • Earliest fossils of land vascular plants
Transformation • After the dibiontic life cycle originated, both – Cooksonia
gametophyte and sporophyte became larger,
more complex, and vascularized, in a life cycle • Equal dichotomous branching –
with an alternation of isomorphic generations. branches of equal sizes and vigor
• Homosporous, Terminal sporangia.
• Plant body was not differentiated into
roots, stems and leaves.
• Epidermis with a cuticle, cortex of
parenchyma cells, and protostele.
• Endarch protoxylem
• Underground rhizome with rhizoids.
• Protostele consisting of a core of
xylem surrounded by one or two
layers of phloem cells.
• Some had conducting cells similar to
hydroids rather tracheids; they are
called protracheophytes.
Interpolation Theory
• There is evidence of isomorphic Microphyll : Lycophytes
alternation of generations. • Earliest lycophytes were members of the general Depranophycus
• Examples: Rhynia, Cooksonia, and Barawagnathia. Had up to 4 cm long enations (outgrowths
Aglaophyton. that range from quite small to long, thin scales) with a single well-
developed trace of vascular tissue, called microphylls.
Zosterophyllophyte • Share similar features with • Many extinct lycophytes such Lepidodendron, Sigillaria and
s Rhyniophytes but: Stigmaria
• Their sporangia were lateral • Heterosporous – produces megaspores and microspores;
• Exarch protostele necessary for evolution of seeds
• Sporangia opened transversely at • Have a special spore producing body called a strobilus,
the top • Presence of true vascular stems, roots and leaves.
• Prominent orders:
Xylem Structure of Early Vascular plants ➢ Lycopodiales, or club mosses;
o One sporangium per sporophyll, near the base and on
the adaxial side.
o Gametophyte bisexual, either green or subterranean,
non-photosynthetic, mycorrhizal structures, depending
on the genus.
o Sporophyte with true leaves, stems and roots
o Sporophylls, modified fertile microphylls, sometimes
grouped into strobili (strobilus, cluster of overlapping
non-photosynthetic sporophylls).
o Biflagellated sperm requires water to reach the
archegonium.
➢ Selaginellales, or spike mosses;
Protostele - a solid, central mass of vascular
tissue with no pith. o Protostele held in place by trabeculae.
o Rhizophores (modified leafless shoots producing
Endarch - the protoxylem is roots) present or absent, geotropic, borne on stems at
located in the center of the branch forks, throughout, or confined to base of stems.
protostele, and the metaxylem o Strobili sometimes ill-defined, terminal, cylindrical,
surrounds it. quadrangular, or flattened.
Exarch - the metaxylem is located o microsoporophylls and megasporophylls.
in the center and the protoxylem o Endosporic development - Gametophytes develop
grouped into several clusters inside the spore
around the metaxylem ➢ Isoetales, or quillworts.
Siphonostele a central pith surrounded by xylem. o has CAM photosynthesis.
o Each leaf is a potential sporophyll
o Megasporophylls and microsporophylls usually borne in The euphyllophytes consists of two sister clades: monilophytes and
alternating cycles; hardened scales and phyllopodia lycophytes.
occasionally surround leaves.
Megaphyll : Euphyllophytes MONILOPHYTES
Trimerophyta • Many monilophytes are known as ferns
(Leptosporangiate, Ophioglossales, Marattiales); others
• Were larger and more complex than Rhyniophytes and are known as fern allies (equisetophytes, Psilotales).
zosterophyllophytes
EQUISETOPHYTES
• They lacked leaves. • Classified as a Division of Arthrophyta (also called as
• branched pseudomonopodially, that means that the branching Sphenophyta)
was unequal, forming a main stem, or axis, with several smaller • Consists of 15 extat plant species and one genus,
lateral branches. Equisetum known by their common name as horsetails or
scouring rushes
• Lateral branches forked dichotomously several times.
• Have sporangiospores as there reproductive
• They were homosporous. structure arranged in compact spirals – strobilus
• Some branches were vegetative while others bore elongated • Have siphonosteles
sporangia.
• Vascular strand was more massive than that of the rhyniophytes.
• The xylem differentiated centrifugally.
FERNS(Pteredophyta)
• Can be found in almost any habitat
• Perennial and herbaceous
Telome Theory – origin of megaphyll • Sporophyte is differentiated into true roots, stem (rhizome) and
The twigs of the last dichotomy are called telomes leaves (megaphylls).
• The stele varies from a protostele to a complex dictyostele.
➢ One branch • The xylem is composed of tracheids.
overtops the other which • Leaves or fronds develop in curled position at the apex of the
remain small and lateral: rhizome (stem) and uncoil as they mature, a condition called
overtopping. circinate vernation.
➢ The lateral branch • Leaves usually differentiated into stipe (petiole) and blade with
system becomes restricted a central rachis or vein.
in their branching to one • Leaf gaps are conspicuous. They are associated with leaf
plane: planation. traces, the vein that goes into the petiole or stipe.
▪ The spaces • Most ferns are homosporous; a few aquatic genera are
heterosporous.
between close branches
developed a thin sheet of
chlorophyll-containing cells: • Sporangia normally develop on the abaxial (dorsal, lower side)
webbing. or on the margin of the leaf.
• Clusters of sporangia occur in most ferns and are called sori
(sing. sorus).
• Gametophytes are inconspicuous and arise directly from Sporangial development
spores. • The eusporangiate sporangium (eusporangia) has the
• Gametophytes are bisexual in homosporous ferns and sporangial wall developing from several superficial cells and
unisexual in heterosporous ferns. the sporogenous tissue develops from internal cells of the
• Sperms are flagellated and coiled. sporophyll.
• Structure • The leptosporangiate sporangium (leptosporangia) develops
entirely from the periclinal division of a single superficial cell.
Psilotum and Tmesipteris
• It includes two living genera, Psilotum and Tmesipteris,
from tropical and subtropical regions of the world.
• These plants are simplest vascular plants.
THE TERM VASCULAR CRYPTOGAMS
Life Cycle
• Traditionally the plants in this chapter have been referred as
Vascular Cryptogams or Ferns and Fern Allies
• These plants share two ancestral features (symplesiomorphies):
• Because they lack seeds, their reproduction is “hidden” (crypto =
hidden).
• They have vascular tissue.
• Vascular cryptogams are NOT grouped together because ancestral
characters are not used to show relationships between plants.
• To group the vascular cryptogams, monilophytes and lycophytes)
only into a clade would leave the lignophytes out.
• The ancestor of the monilophytes is also the ancestor of the
lignophytes.
