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Vascular Plants With Seeds (The Megaphyll Line of Evolution:…
Vascular Plants With Seeds
Early Vascular Plants
Rhyniophytes - The common name for Rhyniaand its close relatives
The earliest vascular plants
Equal Dichotomous Branching - Both branches being of equal size and vigor
Found in Cooksonia plants
Refers to a genus of extinct plants that are among the earliest vascular plants
Homosporous plants, meaning that the microspores and megaspores were not separate
Xylem Structure of Early Vascular Plants
Xylem - The water and mineral conducting portion of vascular tissue
Contains either tracheids or vessel elements or both, as well as parenchyma, fibers, and sclereids
Parenchyma - Cells with only thin primary walls, with all other features varying from type to type
Tracheids - a long and tapered xylem conducting cell that never have a complete hole in the primary wall
Endarch Protostele - A stele that forms a solid core of xylem encased by phloem
Located in the center and metaxylem differentiates on the outer edge of the xylem mass
Phloem - The portion of vascular tissue involved in conducting sugars, water, minerals, and other organic compounds
Exarch Protostele - involves the development of protoxylem from the outermost edge of procambial cylinder.
Protoxylem is found outside the metaxylem
Protoxylem - The part of the primary xylem that differentiates early, while adjacent cells are still elongating
Development is from outside to inside.
Siphonostele - A vascular cylinder that contains pith
Common in stems but absent in early vascular plants
Pith - refers to a spongy, central cylinder of tissue found inside the stems of most flowering plants
Seed Plants - Any plant that bears seeds as part of its reproductive cycle
May have originated twice in evolution
Concept of seed plant is a grade not a clade
Grade - A level of evolution
Clade - A group of closely related organisms sharing a common ancestor
Protostele - A vascular cylinder that has no pith
Pith - refers to a spongy, central cylinder of tissue found inside the stems of most flowering plants
Common in roots and early vascular plants
Zosterophyllophytes - The common name for the genus Zosterphyllum
Small herbs without secondary growth
Distinctly different from rhyniophytes plants
Their sporangia opened transversely along the top edge
Their xylem was an exarch protostele
Their sporangia were lateral, not terminal
Enations - A small, projecting flap of tissue, that increased the photosynthetic surface area of some plants
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The Microphyll Line of Evolution: Lycophytes
Morphology - The branch of biology that deals with the form of living organisms, and with the relationships between their structures
Heterospory - A condition in which the life cycle of a plant contains two types of spores
Cones - Compact collection of reproductive structures on a short axis
Strobili - Synonym, or same as, for cones
Two Types: Microspores and Megaspores
Lycophytes - plants that have microphyll leaves
Extinct members include the zosterophyllophytes
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Living members include: Lycopodium, Selaginella, and Isolates
Microphylls - The type of leaf that evolved from enation
Present in lycophytes
Extant Genera - Refers to Genuses that are still alive or existing
Selaginella - Small and easily overlooked or mistaken for mosses
Ligule - A small flap of tissue on the upper surface of a leaf
Heterosporus and the megagametophyte develops inside the megaspore wall
Isoetes - Small, unusual, plants called quillworts, that grow in wet, muddy areas that occasionally become dry
Approximately 60 species still living today
Lycopodium - Small herbs with prostrate rhizomes that have true roots and short upright branches
Approximately 200 living species
Stylites - Very similar to isoetes
Suggested that the 2 species of stylites are extreme forms of isoetes
The Megaphyll Line of Evolution: Euphyllophytes
Origin of Megaphylls ( Euphylls)
Types:
Sporophyll - A leaf that bears sporangia
Sporangiophore - stalked, umbrella-like structure that bears sporangia
Ferns - Have vascular tissue, megaphyllous leaves but do not produce seeds
Seed Plants - Any plant that bears seeds as part of its reproductive cycle
Arthrophytes - Refers to any plant belonging to the former class Arthrophyta, called horsetails
Euphyllophytes - Refers to plants that have megaphylls
Lignophtyes - Synonym for a plant that develops wood
Monilophytes - The clade containing ferns in a broad sense
Megaphylls - A leaf that has evolved from a branch system
Telome Theory - Postulates that leaves of arthrophytes, ferns, and seed plants evolved from branch systems by overtopping, plantation, and webbing
Plantation - The concept that all branching occured in one plane, resulting in a flat system
Webbing - The concept that the lamina originated by the production of parenchyma cells between telomes
Lamina - The broad, expanded part of a leaf
Parenchyma - Cells with only thin primary walls, with all other features varying from type to type
Telomes - In a plant with dichotomous branching, the last two twigs produced by the last bifurcation
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
Overtopping, Plantation, and Webbing
Equisetopytes - Classified as a division Arthrophyta, also known as Sphenophyta
Horsetails - are very primitive plants belonging to the genus Equisetum
The plant consists of long, hollow, narrow stem segments with miniscule, non-photosynthetic leaves
vascular plants that reproduce by spores in a similar fashion to ferns
Scouring Rushes - “scouring rushes” because early pioneers used them to scrub pots and pans
Trimerophytes - Strongly resemble rhyniophytes.
Having terminal sporangia that dehisced laterally, homospory, dichotomous branching, and an endarch vascular cylinder of tracheids
Considered a distinct advancement out of rhyniophytes because of several special features
Pseudomomopodial Branching - A single main trunk rather than a series of dichotomies
have small lateral branches, some fertile and bearing sporangia and others sterile acting as leaves
Overtopping - have an unequal branching in which one stem was more vigorous
so pronounced that the main stems and lateral stems can be easily identified
Ferns - Have vascular tissue, megaphyllous leaves but do not produce seeds
Leaf Gap - In fern, vascular tissue and area above a leaf trace where there is no conducting tissue
Leaf Trace - A vascular bundle that extends from the stem vascular bundles through the cortex and enters a leaf
Leptosporangiate Ferns - Having each sporangium formed from a single epidermal cell; Modern ferns
Life Cycle:
Eusporangia and Leptosporangia
Leptosporangia - Initiated when a single surface cell divides periclinally and forms a small outward protrusion
Having leptosporangia is a derived trait occuring only in the clade Leptosporangiate
This undergoes several more divisions, which result in a small set of sporogenous cells and a thin covering of sterile cells, produces only a few spores
Eusporangium - Initiated when several surface cells undergo periclinal divisions, resulting in a small multilayered plate of cells
This is the fundamental type of sporangium, it exists in virtually all plants other than Leptosporangiate
Outer cells develop into the sporangium wall, and the inner cells proliferate into sporogenous tissue, which results in a relatively large sporangium with many spores
Psilotum and Tmesipteris
Psilotum - small plants with prostrate rhizomes and upright and stems that branch dichotomously
They have an epidermis, cortex, and a simple vascular cylinder
Tmesipteris - The "hanging fork fern", is a genus of fern-like vascular plants
Tmesipteris is from the Greek language, meaning a "cut fern,'' referring to the truncated leaf tips
Theories, Concepts, and “Vascular Cryptograms”
“Vascular Cryptograms” - Vascular plants that do not produce seed
Such as lycopods, horsetails, and ferns
Ferns and Fern Allies - Refers to ferns, which do not produce seeds, flowers, fruits, ect., and other plants like ferns
reproduce using haploid, unicellular spores instead of seeds
Dibiontic Life Cycle - Refers to species as having a multi-cellular gametophyte and a multi-cellular sporophyte
Monobiontic Life Cycle - Having only one multi-cellular generation
A zygote undergoes only meiosis, producing more spores that will grow into new gametophytes
Interpolation Hypothesis - Postulation that small sporophytes came into existence when zygote germinated mitotically instead of meiotically
The First Stage - The very earliest land plants were postulated as to having no sporophyte, instead the zygote germinated by meiosis, producing haploid spores that grew into new gametophytes
Sporophyte - A diploid plant that produces spores
Zygote - The diploid cell formed as the result of the fusion of two gametes
Meiosis - Reduction division, a process in which nuclear chromosomes are duplicated once but divided twice, such that the resulting nuclei have only one half as many chromosomes as the mother cell
Haploid - Refers to one full set of chromosomes per nucleus
The Second Stage - At a later stage in evolution, the zygote would germinate mitotically and produce a simple sporophyte that in the early stages would have consisted of a sporangium and perhaps also a foot, which would have resembled a liverwort sporophyte
Evolution - The change of nucleotide sequences in a species’ DNA through natural selection, genetic drift, or accident
Zygote - The diploid cell formed as the result of the fusion of two gametes
Sporophyte - A diploid plant that produces spores
Sporangium - A structure that produces spores
The Third Stage - With continued evolution, the sporophyte would have become progressively more elaborate while the gametophytes became simpler
Evolution - The change of nucleotide sequences in a species’ DNA through natural selection, genetic drift, or accident
Sporophyte - A diploid plant that produces spores
Interpolation Theory:
Transformation Theory - Postulation that after the dibiontic life cycle originated, both gametophyte and sporophyte became larger, more complex, and vascularized
specifically in a life cycle with an alternation of isomorphic generations
The First Stage - Gametophytes were upright and dichotomously branched, with epidermis, cuticle, and vascular tissue, just like sporophytes
Epidermis - The outermost layer of the plants primary body
Covering leaves, flower parts, young stems, and roots
Cuticle - Layer of cutin on epidermal cells
Reduces water loss but also unavoidably restricts the entry of carbon dioxide
The Second Stage - With time, sporophytes became larger and more complex, and gametophytes became simpler
In certain species gametophytes have become so small that the microgametophyte develops within the spore wall
Microgametophyte - A gametophyte that produces microgametes , sperm only
The megagametophyte protrudes from the spore only slightly
Spore - A single cell that is a means of asexual reproduction, it can grow into a new organism but cannot fuse like a gamete
Megagametophyte - A gametophyte that produces gametes, eggs, only
The Third Stage - With continued reduction, it is possible, but not necessary or inevitable, for the megaspore and its megagametophyte to be retained inside the megasporangium
To remain on the parental sporophyte is an important step in the process of seed evolution
Megaspore - A large spore that grows into a megagamtophyte that produces egg cells
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Megagametophyte - A gametophyte that produces gametes, eggs, only
Megasporangium - A structure that produces megaspores
Sporophyte - A diploid plant that produces spores
Evolution - The change of nucleotide sequences in a species’ DNA through natural selection, genetic drift, or accident
