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Vascular Plants Without Seeds (The Megaphyll Line of Evolution:…
Vascular Plants Without Seeds
Early Vascular Plants
Rhyniophytes: Plants of Cooksonia had an epidermis with a cuticle, a cortex of parenchyma, and a simple bundle of xylem composed tracheids with annular secondary walls. Plants of Cooksonia were homosporous; there were no separate microspores and megaspores
Zosterophyllophytes: They were small herbs without secondary growth. Many features are similar to rhyniophytes but three character differentiate them. Their sporangia were lateral; not terminal; sporangia opened transversely along the top edge, and their xylem was an exarch prostele
The Microphyll Line of Evolution: Lycophytes:- Lycophytes represent a distict line of evolution out of the early land plants that resembled zosterophyllophyte
Morphology: Earliest lycophytes were members of the genera Drepanophycus and Baragwanathia similar to their presumed ancestor, the zosterophullophytes.
Heterospory: It is a necessary precondition for the evolution of seeds
Extant Genera
Lycopodium: Common in forests from tropical regions to the arctic , are small herbs with prostrate rhizomes that have true roots and short upright branches.
Selaginella: Less common in temperate North America, and its plants are smaller and easily overlooked to be mosses. They can be distinguished from lucopodiums by a small flap of tissue, the ligule.
The Term "vascular Cryptogams": They have vascular tissue and that because they lack seeds their reproduction is hidden
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes: Considered a distinct advancement out of rhyniophytes due to several special feature
Overtoping: It had unequal branching in which one stem was more vigorous
Pseudomonopodial Branching: Single main trunk rather than a series of dichotomies. Plants have small lateral branches, some fertile and bearing sporangia and others sterile and acting as leaves.
Origin of Megaphylls (Euphylls): At least three distinct types of homoplasic structure called leaves occur in plants
Leaves on gametophytes of nonvascular plants
Enations/ microphylls of Zosterophyllophytes and lycophytes
Megaphylls: leaves that evolved from branch systems and are present in all seed plants, ferns and equisetophytes
Monilophytes: They are plants we know as ferns
Euphyllophytes are united by three synapomorphies
Their roots have exarch xylem
They have a 30-kilo base inversion in the large single copy region of their plastid DNA
They have megaphylls
Equisetophytes: Classified as division Arthrophyta, consist of several genera of extinct plants and one genus, Equisetum, with 15 extant species called Horsetails
Ferns: Early ferns apperared in the Devonian Period and then diversified greatly
Psilotum and Tmesipteris: These two small genera contain the simplest of all living vascular plants
Eusporangia and Leptosporangia
Eusporangium: Initiated when several surface sells undergo periclinal divisions, resulting in a small multilayered plate of cells
Outer cells develop into the sporangium wall, and the inner cells proliferate into sporogenous tissue
Leptosporangia: Initiated when a single surface cell divides periclinally and forms a small outward protrusion, undergo several more divisions, which result in a small cell of sporogenous cells and thin covering of sterile cells; only few spores are produced
Xylem Structure of Early Vascular Plants
Endarch Protostele: Protoxylem is located in the centre and metaxylem differentiates on the outer edge of the xylem mass
Exarch protostele: Metaxylem located in the centre of the xylem mass and protoxylem on the edges as several groups next to the phloem
Siphonostele: this is not the evolved one which pith is present in the center as occurs in the stems of ferns and seed plants
