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Chapter 21: Vascular Plants Without Seeds (The Megaphyll Line of Evolution…
Chapter 21: Vascular Plants Without Seeds
Concepts
monobiontic
: having only one multicellular generation
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.
interpolation hypothesis
: a hypothesis; postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically
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.
Would have resembled a liverwort sporophyte.
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.
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
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:
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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.
The Term "Vascular Cryptogams"
fern allies
: pteridophytes of other classes than Filicopsida
vascular cryptograms
: the vascular plants that do not produce seeds, such as lycopods, horsetails, and ferns
ferns
: have vascular tissues, megaphyllous leaves (euphylls, evolved from branches) but do not produce seeds
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A member of the monilophyte clade.
Living ferns never produce wood.
Early Vascular Plants
Zosterophyllophytes
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
Rhyniophytes
rhyniophytes
: the common name for
Rhynia
and its close relatives, the earliest vascular plants
Xylem Structure of Early Vascular Plants
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
Alternative:
siphonostele
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
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).
equal dichotomous branching
: both branches being of equal size and vigor
Cooksonia
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A genus of extinct plants
Earliest vascular land plant fossil
The Microphyll Line of Evolution: Lycophytes
Heterospory
strobili
: synonym for cone
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cones
: a compact collection of reproductive structures on a short axis
Extant Genera
ligule
: in selaginellas, a small flap of tissue on the upper surface of a leaf
Morphology
microphylls
: the type of leaf that evolved from an enation; present in lycophytes
The Megaphyll Line of Evolution: Euphyllophytes
Monilophytes
monilophytes
: the clade containing ferns in a broad sense
(
Psilotum, Equisetum
, Leptosporangiatae, Marattiales, and Ophioglossales)
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
Equisetophytes
sporangiophore
: in the arthrophytes (sphenophytes), a stalked, umbrella-like structure that bears sporangia
Classified as division Arthrophyta (also called Sphenophyta)
They consist of several genera of extinct plants and one genus,
Equisetum
, with 15 extant species known as:
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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
monopodial growth
: a main trunk, lateral branches, true leaves, and true roots
Origin of Megaphylls (Euphylls)
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
Present in ferns and all seed plants.
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
Ferns
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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:
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
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.
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.
leaf trace
: a vascular bundle that extends from the stem vascular bundles through the cortex and enters a leaf
Trimerophytes
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
