vascular plants without seeds
early vascular plants
Rhyniophytes
First all extant plants( all known fossil plants) have a dibiontic lifecycle: each species has a multicellular gametophyte and also a multicellular sporophyte.
Some algae , especially coleochaete, are monobiontic, having only one multicellular generation: A zygote undergoes only meiosis, producing more spores that will grow into new gametophytes.
The interpolation hypothesis, postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically.
The transformation theory, postulates that after the dibiontic life cycle originated both gametophyte and sporophyte became larger, more complex and vascularized in a life cycle with an alternation of isomorphic generations.
xylem structure of early vascular plants
Each vascular plants had two types of xylem organization.
The center is a solid mass of xylem with no pith, this is a protostele.
In an endarch protostele, protoxylem is located in the center and metaxylem differentiates on the outer edge of the xylem mass.
The other type of stele present in an early vascular plants is an exarch protostele.
another type of stele, which did not evolve until later is the siphonostele, one in which pith is present in the center.
The earliest fossils that definitely were vascular land plants.they had equal dichotomous branching, both branches being of equal size and vigor.
rhyniophytes were homosporous: there were no separate microspores and megaspores.
zosterophyllophytes
Small herbs without secondary growth. their sporangia were lateral, not terminal; sporangia opened transversely along the top edge, and their xylem was an exarch protostele.
Grew as small bunches, only approximately 15 cm high.
The microphyll line of evolution: Lycophytes
Lycophytes represent a distinct line of evolution out of the early land plants that resembled zosterophyllophytes.lateral sporangia and exarch protosteles.
Morphology
The earliest lycophytes were members of the genera drepanophycus and baragwanathia.
there were similar to their zosterophyllophytes .
Their enations were large, up to 4 cm long, and they contained a single well-developed trace of vascular tissue.
lycophyta are called microphylls.
Heterospory
In many extinct and extant lycophytes sporangia are clustered together in compact groups called cones or strobili which protect them.
Extant genera
Lycopodium is fairly common in forest from tropical regions to the arctic.
All living species approximately 200, are small herbs with prostrate rhizomes that have true roots and short upright branches. Selaginellas can be distinguished from lycopodium by a small flap of tissue, the ligule, on the upper surface of leaves.
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
most important is the trend of overtoping: trimerophytes had an unequal branching in which one stem was more vigorous.
pertica displays pseudomonopodial branching, that is, a single main trunk rather than a series of dichotomies.
Origin of Megaphylls(Euphylls)
Megaphylls: leaves that evolved from branch system and are present in all seed, plants, ferns, and equisetophytes.
Megaphyll evolution is summarized by the telome theory.
the ultimate twigs, those of the last dichotomy, are known as telomes.
Sporophyll, a sporangium bearing leaf like structure.
Megaphyllous referred to as the euphyllophytes.
Monilophytes
some euphyllophytes contains two sisters clades, the monilophytes and the woody plants lignophytes.
Equisetophytes
Equisetophytes have been classified as division Arthrophyta.
They consist of several genera of extinct plants and one genus, equisetum, with 15 extant species known as horsetails or scouring rushes. reproductive structure is called sporangiophore.they had true monopodial growth, a main trunk, lateral branches, true leaves, and true roots.
Ferns
Early ferns first appeared in the Devonian period and then diversified greatly.
the rest of the monilophytes have more than 12,000 species and almost all of those are what are called leptosporangiante ferns.
At each node, a leaf trace diverges from the siphonostele, leaving a small segment of the vascular cylinder as just parenchyma. this region is a leaf gap.
Eusporangia and Leptosporangia
The term " vascular cryptogams"
Traditionally, the plants of this chapter have been referred to informally as vascular cryptogams or often as ferns and fern allies.
This name indicates that they have vascular tissue and that because they lack seeds their reproduction is hidden.
The eusporangium is initiated when several surface cells undergo periclinal divisions, resulting in a small multilayered plate of cells.
Leptosporangia are initiated when a single surface cell divides periclinally and forms a small outward protrusion.