Chapter 21: Vascular Plants Without Seeds (The Megaphyll Line of Evolution…
Chapter 21: Vascular Plants Without Seeds
: having only one multicellular generation
Ex: some algae, especially
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.
: 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
: 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:
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"
: have vascular tissues, megaphyllous leaves (euphylls, evolved from branches) but do not produce seeds
Living ferns never produce wood.
A member of the monilophyte clade.
: the vascular plants that do not produce seeds, such as lycopods, horsetails, and ferns
: pteridophytes of other classes than Filicopsida
Early Vascular Plants
: group of early vascular plants named after the principal genus
: a small, projecting flap of tissue, thought to have been the ancestor of leaves in the lycophytes
: the common name for
and its close relatives, the earliest vascular plants
Xylem Structure of Early Vascular Plants
: protoxylem is located in the center and metaxylem differentiates on the outer edge of the xylem mass
: with metaxylem located in the center of the xylem mass and protoxylem on the edges as several groups next to the phloem
: a vascular cylinder that has no pith; common in roots and early vascular plants
: a vascular cylinder that contains pith; common in stems but absent in early vascular 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
The Microphyll Line of Evolution: Lycophytes
: a compact collection of reproductive structures on a short axis
: synonym for cone
: in selaginellas, a small flap of tissue on the upper surface of a leaf
: the type of leaf that evolved from an enation; present in lycophytes
The Megaphyll Line of Evolution: Euphyllophytes
: plants that have megaphylls (synonym:
), the horsetails, ferns, and seed plants
: synonym for a plant that develops wood. This is used as an informal name for the clade that contains the woody plants
: the clade containing ferns in a broad sense
, Leptosporangiatae, Marattiales, and Ophioglossales)
: a main trunk, lateral branches, true leaves, and true roots
Classified as division Arthrophyta (also called Sphenophyta)
They consist of several genera of extinct plants and one genus,
, with 15 extant species known as:
: a horsetail with a very rough ridged stem, formerly used for scouring and polishing
: a nonflowering plant with a hollow jointed stem that bears whorls of narrow leaves, producing spores in cones at the tips of the shoots
: in the arthrophytes (sphenophytes), a stalked, umbrella-like structure that bears sporangia
Origin of Megaphylls (Euphylls)
: plants that have megaphylls (synonym: euphyll), the horsetails, ferns, and seed plants
: a leaf that bears sporangia
: in the telome theory of the origin of megaphylls, the concept that the lamina originated by the production of parenchyma cells between the telomes
: in the telome theory of the origin of megaphylls, the concept that all branching occured in one plane, resulting in a flat system
: a leaf that has evolved from a branch system
Present in ferns and all seed plants.
: in a plant with dichotomous branching, the last two twigs produced by the last bifurcation
: the theory that leaves (megaphylls) of arthrophytes, ferns, and seed plants evolved from branch systems (telomes) by overtopping, planation, and webbing
: a vascular bundle that extends from the stem vascular bundles through the cortex and enters a leaf
Two small genera contain the simplest of all living vascular plants.
: known to be highly derived, highly simplified ferns
They have an epidermis, cortex, and a simple vascular cylinder with no pith-a protostele.
They are small plants with prostrate rhizomes and upright stems that branch dichotomously.
Constructed very much like
, and until the mid 1970s, they were often placed together, even though
: limited to Australasia, primarily Australia and other South Pacific islands
: in fern vascular tissue, an area above a leaf trace where there is no conducting tissue
Contain two types of sporangia that differ in fundamental aspects of their development:
: initiated when a single surface cell divides periclinally forms a small outward protrusion
: initiated whens several surface cells undergo periclinal divisions, resulting in a small multilayered plate of cells
: created for types of isolated organs, tissues, spores, or pollen
: in ferns, a cluster of sporangia on the underside of leaves
: 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
: a type of sympodial branching that strongly resembles monopodial branching, having what appears to be one main shoot