Chapter 21: Vascular Plants Without Seeds
Concepts
The Term "Vascular Cryptogams"
Early Vascular Plants
The Microphyll Line of Evolution: Lycophytes
The Megaphyll Line of Evolution: Euphyllophytes
Zosterophyllophytes
Rhyniophytes
Heterospory
Extant Genera
Morphology
Monilophytes
Equisetophytes
Origin of Megaphylls (Euphylls)
Ferns #
Trimerophytes
monobiontic: having only one multicellular generation
interpolation hypothesis: a hypothesis; postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically
dibiontic life cycle: each species has a multicellular gametophyte and also a multicellular sporophyte
transformation theory: an alternative hypothesis; postulates that after the dibiontic life cycle originated, both gametophyte and sporophyte became larger, more complex, and vascularized, in a life cycle with an alteration of isomorphic generations
Ex: some algae, especially Coleochaete
Conversion of a monobiontic ancestor into dibiontic plants.
A zygote undergoes only meiosis, producing more spores that will grow into new gametophytes.
Thus, an important step in the evolution of embryophytes.
A zygote cannot undergo mitosis and cannot grow into a sporophyte.
No living plants have gametophytes that look like sporophytes, but many algae do, and some fossil plants did:
Some fossils of early vascular plants bore gametangia and grew among other similar plants that bore sporangia.
Postulates that these early ancestors diverged into two clades: #
1.) Nonvascular plants in which sporophytes became much simpler and dependent on the gametophytes.
2.) The rest of the vascular plants, in which sporophytes became increasingly elaborate whereas gametophytes became very reduced.
rhyniophytes: the common name for Rhynia and its close relatives, the earliest vascular plants
Xylem Structure of Early Vascular Plants
equal dichotomous branching: both branches being of equal size and vigor
endarch protostele: protoxylem is located in the center and metaxylem differentiates on the outer edge of the xylem mass
exarch protostele: with metaxylem located in the center of the xylem mass and protoxylem on the edges as several groups next to the phloem
protostele: a vascular cylinder that has no pith; common in roots and early vascular plants
siphonostele: a vascular cylinder that contains pith; common in stems but absent in early vascular plants
seed plants: any plant that bears seeds as part of its reproductive cycle
Alternative: siphonostele
Seeds may have originated twice in evolution.
If so, then the concept of seed plant is a grade (a level of evolution), not a clade (a group of closely related organisms sharing a common ancestor).
enations: a small, projecting flap of tissue, thought to have been the ancestor of leaves in the lycophytes
zosterophyllophytes: group of early vascular plants named after the principal genus Zosterophyllum
microphylls: the type of leaf that evolved from an enation; present in lycophytes
strobili: synonym for cone #
cones: a compact collection of reproductive structures on a short axis
ligule: in selaginellas, a small flap of tissue on the upper surface of a leaf
pseudomonopodial branching: a type of sympodial branching that strongly resembles monopodial branching, having what appears to be one main shoot
overtopping: in the evolution of unequal branching, the ability of one shoot to grow for a longer time than the other shoot that resulted from the branching
telome theory: the theory that leaves (megaphylls) of arthrophytes, ferns, and seed plants evolved from branch systems (telomes) by overtopping, planation, and webbing
telome: in a plant with dichotomous branching, the last two twigs produced by the last bifurcation
megaphylls: a leaf that has evolved from a branch system
planation: in the telome theory of the origin of megaphylls, the concept that all branching occured in one plane, resulting in a flat system
webbing: in the telome theory of the origin of megaphylls, the concept that the lamina originated by the production of parenchyma cells between the telomes
sporophyll: a leaf that bears sporangia
euphyllophytes: plants that have megaphylls (synonym: euphyll), the horsetails, ferns, and seed plants
Present in ferns and all seed plants.
monilophytes: the clade containing ferns in a broad sense
lignophytes: synonym for a plant that develops wood. This is used as an informal name for the clade that contains the woody plants
euphyllophytes: plants that have megaphylls (synonym: euphyll), the horsetails, ferns, and seed plants
(Psilotum, Equisetum, Leptosporangiatae, Marattiales, and Ophioglossales)
sporangiophore: in the arthrophytes (sphenophytes), a stalked, umbrella-like structure that bears sporangia
Classified as division Arthrophyta (also called Sphenophyta)
monopodial growth: a main trunk, lateral branches, true leaves, and true roots
They consist of several genera of extinct plants and one genus, Equisetum, with 15 extant species known as: #
horsetails: a nonflowering plant with a hollow jointed stem that bears whorls of narrow leaves, producing spores in cones at the tips of the shoots
scouring rushes: a horsetail with a very rough ridged stem, formerly used for scouring and polishing
sorus: in ferns, a cluster of sporangia on the underside of leaves
form genera: created for types of isolated organs, tissues, spores, or pollen
Contain two types of sporangia that differ in fundamental aspects of their development:
leaf gap: in fern vascular tissue, an area above a leaf trace where there is no conducting tissue
Two small genera contain the simplest of all living vascular plants.
leaf trace: a vascular bundle that extends from the stem vascular bundles through the cortex and enters a leaf
eusporangium: initiated whens several surface cells undergo periclinal divisions, resulting in a small multilayered plate of cells
leptosporangia: initiated when a single surface cell divides periclinally forms a small outward protrusion
Tmesipteris: limited to Australasia, primarily Australia and other South Pacific islands
Psilotum: known to be highly derived, highly simplified ferns
Constructed very much like Rhynia and Aglaophyton, and until the mid 1970s, they were often placed together, even though Rhynia and Aglaophyton are extinct.
They are small plants with prostrate rhizomes and upright stems that branch dichotomously.
They have an epidermis, cortex, and a simple vascular cylinder with no pith-a protostele.
fern allies: pteridophytes of other classes than Filicopsida
vascular cryptograms: the vascular plants that do not produce seeds, such as lycopods, horsetails, and ferns
A member of the monilophyte clade.
Living ferns never produce wood.
At a later stage in evolution, the zygote would germinate mitotically and produce a simple sporophyte that in early stages would have consisted of a sporangium and perhapsalso a foot.
With continued evolution, sporophyte becomes progressively more elaborate while gametophytes become simpler.
The very earliest land plants were postulated as having no sporophyte; instead, the zygote "germinated' by meiosis, producing haploid spores that grew into new gametophytes.
Would have resembled a liverwort sporophyte.
A genus of extinct plants
Earliest vascular land plant fossil