Seedless vascular plants




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Chapter 21 SEEDLESS VASCULAR PLANTS
Air is drier than water and less buoyant. Land plants had to develop adaptations to conserve and transport water with its solutes, absorb water from the environment, support itself facing the direction of sunlight, solve reproductive problems like fertilization and nourishment of embryo, and dispersal of offspring.


  1. Dominant sporophyte and reduced gametophyte.




  • In bryophytes, the gametophyte is dominant generation.

  • Water is required for fertilization.

  • Pollen and embryo sac are much reduced gametophytes.




  1. Development of fluid-transport system, the xylem and phloem.







  1. The ability to synthesize lignin.




  • Early land plants were small and probably stayed upright by means of turgor pressure.

  • Lignin adds rigidity to the cell wall and allows the plant to reach greater heights.



PHYLUM PSILOTOPHYTA
It includes two living genera, Psilotum and Tmesipteris, from tropical and subtropical regions of the world...


  • Sporophyte with a dichotomously branching aerial and subterranean stem system.

  • Photosynthesis takes place in the epidermis and cortex of the stem.

  • Vascular stele of the stem consists of a central cylinder of xylem surrounded by phloem; this type of stele is called protostele.

  • True roots lacking.

  • Underground stems with rhizoids and with a fungal association, an endomycorrhizal zygomycete.

  • Aerial stems lacking leaves but with scale-like or larger leaf-like structures (enations); enations are spirally arranged along the length of the stem.

  • Gametophytes bisexual, subterranean, resembling a small piece of rhizome.

  • Gametophyte with a symbiotic fungus. Some have a vascular tissue.

  • The sperm is multiflagellated and requires water to swim to the archegonium.

  • Homosporous

  • Sporangia, a 2 or 3-chambered synangium, borne at the apex of small side-branches (appear to be arranged along the sides of the major stems of the plant--sporophyte)

  • Sporophyte remains attached to the gametophyte in the early stages by a foot and derives nourishment from the gametophyte.


Fossil plants
Fossil plants similar to whisk ferns have been found in the Silurian geological formations, ~440 to 400 million years ago.
There are three phyla of extinct seedless vascular plants: Rhyniophyta, Zosterophyllophyta and Trimerophytophyta.
The genera Cooksonia, Rhynia, Zosterophyllum and Trimerophyton are members of these phyla.
The earliest known go back about 425 million years ago and most went extinct by the end of the Devonian about 370 million years ago.
For the most part they were relatively simple plants 18 in to 36 inches tall. They had the following characteristics:
1.Naked photosynthetic stems

2.Terminal sporangia (some lateral)

3.No roots or leaves

4.They were all homosporous

5.They had protosteles
Cooksonia is the oldest known land plant and had xylem.

Zosterophyllum had globose or reniform sporangia borne on the upper part of short stalks.



PHYLUM LYCOPHYTA.
There are about 15 genera of lycophytes and approximately 1000 living species.
This evolutionary line extends back into the Devonian but was most prevalent in the wet swamps of the Carboniferous period.
They eventually split up into two evolutionary lines.


  • The first were very large woody trees that did not survive in the drier climate at the end of and after the Carboniferous age. In the Carboniferous some lycophytes were forest-forming trees more than 35 meters tall.

  • The second and the surviving group of Lycopods are the small and herbaceous trees.

Lycophyta remains became the largest coal deposits of all geologic time.


Lycophyta are characterized by


  • microphyllous leaves,

  • a special spore producing body called a strobilus,

  • the presence of true vascular stems, roots and leaves.

There are three prominent orders of Lycophyta:




  • Lycopodiales, or club mosses;

  • Selaginellales, or spike mosses;

  • Isoetales, or quillworts.


Family Lycopodiaceae
A family of about 400 species, mostly tropical.
The taxonomic boundaries of the genera are not well understood

  • Sporophyte with true leaves, stems and roots.

  • Dichotomous branching pattern.

  • Stems and roots are protostelic or siphonostelic. Leaf gaps absent.

  • Leaves are spirally arranged, sometimes opposite or whorled.

  • Sporophylls, fertile microphylls, grouped into strobili (strobilus, cluster of overlapping sporophylls).

  • One sporangium per sporophyll, near the base and on the adaxial side.

  • Homosporous.

  • Gametophyte bisexual, either green or subterranean, depending on the genus.

  • Gametangia may require 6 to 15 years to mature.

  • Self-fertilization is rare.

  • Biflagellated sperm requires water to reach the archegonium.



Family Selaginellaceae
There are about 700 species in this family, most of them tropical.


  • Plants herbaceous, annual or perennial, sometimes remaining green over winter.

  • Stems leafy, branching dichotomously, regularly or irregularly forked or branched.

  • Protostele held in place by trabeculae.

  • Rhizophores (modified leafless shoots producing roots) present or absent, geotropic, borne on stems at branch forks, throughout, or confined to base of stems.

  • Leaves on 1 plant dimorphic or monomorphic, small, with adaxial ligule near base, single-veined, rarely veins forked.

  • Strobili sometimes ill-defined, terminal, cylindrical, quadrangular, or flattened.

  • Heterosporous.

  • Sporophylls (fertile leaves) monomorphic or adjacently different, slightly or highly differentiated from vegetative (sterile) leaves: microsoporophylls and megasporophylls.

  • Sporophylls with ligule.

  • Sporangia short-stalked, solitary in axil of sporophylls, opening by distal slits.

  • Microsporangia contain microspores that will produce microgametophytes.

  • Megasporangia produce megaspores that will give rise to megagametophytes.

  • Spores of 2 types (plants heterosporous), megaspores (1--2--4), large; microspores numerous (hundreds), very small.

  • Gametophytes unisexual.

  • Gametophytes develop inside the spore wall: endosporic development.




  • Microgametophyte lacks chlorophyll.

  • At maturity, it consists of a single prothallial cell and an antheridium.

  • Antheridium produces many of biflagellated sperms.

  • Microspore wall ruptures to liberate the sperms.




  • Megagametophyte multicellular.

  • Megagametophyte protrudes through a rupture in the spore wall.

  • Archegonia develop in the exposed area.




  • Sperms require water to swim to the archegonia.

  • The suspensor develops and pushes the developing embryo deep into the female gametophyte.


Family Isoetaceae
A family of one genus, Isoetes (ca. 150 spp.), found world-wide, especially in temperate areas.
A second genus, Stylites, is sometimes recognized.
Some authors put fossil representatives in the genus Isoetites, which is known from as early as the upper Triassic.


  • Plants tufted, grass-like, perennial, evergreen aquatics to ephemeral terrestrials.

  • Rootstock brown, cormlike, lobed.

  • Roots arising along central groove separating each rootstock lobe, simple or dichotomously branched, containing eccentric vascular strand and surrounding lacuna.

  • Leaves linear, simple, spirally or distichously arranged, dilated toward base, tapering to apex, containing 4 transversely septate longitudinal lacunae, a central collateral vascular strand, and frequently several peripheral fibrous bundles.

  • Ligule inserted above sporangium on each sporophyll.

  • Heterosporous (megaspores and microspores not alike).

  • Megasporophylls and microsporophylls usually borne in alternating cycles; hardened scales and phyllopodia occasionally surround leaves.

  • Sporangia solitary, adaxial, embedded in basal cavity of leaf, velum (thin flap extending downward over sporangium) partly to completely covering adaxial surface of sporangium.

  • Megasporangium with several to hundreds of megaspores.

  • Microsporangium with thousands of microspores.

  • Megagametophytes white, endosporic, exposed when megaspore opens along proximal ridges; archegonia 1 to several, indicated by quartets of brownish neck cells.

  • Microgametophytes 9-celled, endosporic, antheridium releasing 4 multitailed spermatozoids.



PHYLUM EQUISETOPHYTA
This group extends back to the Devonian and reached their maximum development in the Carboniferous (380-280 m.y.a.).
A family of one extant genus, Equisetum (ca. 25 species), of nearly worldwide distribution in damp habitats such as riverbanks, lakeshores, and marshes. Michigan is a center of diversity for the genus with nine native species.


  • The sporophyte of Equisetum is differentiated into an underground rhizome that bears adventitious roots and an upright, photosynthetic stem with whorls of microphylls.

  • Tough perennial herbs with jointed, ridged aerial stems with distinct nodes.

  • Stems rough, accumulating silica and metals, and complex anatomically.

  • The aerial stems contain a large central pith region, which in mature plants is hollow.

  • Surrounding the pith cavity are discrete bundles of vascular tissue; this arrangement of conducting tissue is known as a eustele.

  • The bundles contain both xylem and phloem, and are marked by the presence of large canals known as carinal canals (under the ridges), which also function in water conduction.

  • External to the vascular bundles is another set of canals, the vallecular canals or cortical canals (under the valleys).

  • These canals line up with the depressions between the ribs on the surface of the stem. Most fossil sphenophytes had very similar stem morphology.

  • Leaves are small, whorled, non-photosynthetic, fused together to various degrees and adpressed to the stem.

  • Sporangia borne on peltate sphorangiophores aggregated into a strobilus.

  • Strobili consist of tightly packed appendages called sporangiophores.

  • Homosporous; spores with elaters.

  • Gametophytes green, epigeal, bisexual or unisexual, male gametophytes smaller than female; with rhizoids.

  • Archegonia develop before antheridia to increase the probability of cross-fertilization.

  • Sperms are multiflagellated and require water to swim to the egg.

  • Several eggs on a single gametophyte may be fertilized.



PHYLUM POLYPODIOPHYTA
The fossil record of ferns extends back into the Carboniferous
There are 11,000 species of ferns in the world. Most species are tropical.


  • There are about 550 species of ferns in U.S.A. and Canada.

  • Costa Rica has over 1,100 species.

  • Philippines has over 950 species.




  • Sporophyte is differentiated into true roots, stem (rhizome) and leaves.

  • The stele varies from a protostele to a complex dictyostele.

  • The xylem is composed of tracheids.

  • Leaves or fronds develop in curled position at the apex of the rhizome (stem) and uncoil as they mature, a condition called circinate vernation.

  • Leaves usually differentiated into stipe (petiole) and blade with a central rachis or vein.

  • Leaves may be simple or compound with pinnae.

  • Leaf gaps are conspicuous.

  • Most ferns are homosporous; a few aquatic genera are heterosporous.

  • Sporangia normally develop on the abaxial (dorsal, lower side) or on the margin of the leaf.

  • Sporangia develop in clusters called sori (sing. sorus) and are often protected by a flap of tissue called indusium.

  • The annulus of the sporangium helps to catapult the spores away from the mother plant.

  • Gametophytes (prothallus) are inconspicuous and arise directly from spores.

  • Gametophytes are bisexual in homosporous ferns and unisexual in heterosporous ferns.

  • Sperms are flagellated and coiled.

Sporangial development is important in the understanding of the evolutionary relationship of vascular plants.




  1. The eusporangiate sporangium has the sporangial wall developing from several superficial cells and the sporogenous tissue develops from internal cells of the sporophyll. The sporangial wall is several cells thick. The inner wall cells may be compressed and stretched so that the sporangial wall may appear to be made of a single layer of cells at maturity. The sporangia are large and contain a large number of spores.



  1. The leptosporangiate sporangium develops entirely from the periclinal division of a single superficial cell. By a precise pattern of division a stalk and a globose capsule are formed. A nutritive tapetum develops inside the capsule. The inner mass of cells develops into spore mother cells that undergo meiosis. The remnant of the tapetum is deposited around the spores and gives them their characteristic ornamentation. The number of spores produced is a multiple of 2 (between 16 and 512 in homosporous species).




  1. Eusporangiate sporangium is characteristic of all vascular plants except the leptosporangiate ferns.

There are five orders of ferns often recognized by pteridologists: Ophioglossales, Marattiales, Filicales, Marsileales and Salviniales.


Fossil ferns.
Ferns and relatives are surviving members of the earliest lineage of vascular plants that originated over 400 million years ago. These plants reproduced by means of spores and did not produce flowers, fruits or seeds. Coal deposits represent the remains of these plants that grew during the Carboniferous period.
Tracheophytes split into lycophytes and euphyllophytes about 400 mya in the middle Devonian.


  • Tracheophytes are plants with vascular tissue.

  • Euphyllophytes are divided into two groups: spermatophytes (seed plants) and monilophytes (ferns, horsetails and whisk ferns).

Monilophytes split in two clades by the Late Devonian, ~364 mya:




  1. ophioglossoid-whisk fern branch.

  2. marattioid-horsetail-leptosporangiate branch.

The relationship of the marattioid ferns, horsetails and leptosporangiate ferns remains unclear.




  1. Horsetails appeared in the Late Devonian (374-350 mya).

  2. The earliest known occurrence of leptosporangiate ferns is in the Early Carboniferous (~354 mya).

  3. Marattioid ferns appeared in the Middle Carboniferous (~323 mya). The Marattiales is first recognized in the Pennsylvanian (~323 million years ago) and continues to the Recent, where 90 species in six extant genera inhabit tropical areas around the world.

The ophioglossoid-whiskfern lineage split in the Late Carboniferous (~306 mya).




  • Psilotum and Tmesipteris diverged in the Late Cretaceous (~88 mya).

  • Botrychium and Ophioglossum diverged in the Middle Jurassic (~162 mya).


Marattioid ferns and horsetails diverged from each other by the end of the Devonian (~354 mya).


  • Extant horsetail diversified in the Tertiary (~38 mya).


Leptosporangiate ferns separated from the parent marattioid-horsetail group during the Carboniferous, and by the end of the Permian (~248 mya) the osmundaceous, filmy, gleichenioid and schizaeoid ferns, as well as the core of leptosporangiate ferns had appeared.


      • The polypods-tree fern lineage differentiate in the Late Triassic (~211 mya), but the oldest fossils of these lineages date from the Middle Jurassic.




        • This radiation of polypods parallels that of angiosperms.




      • The two living families of heterosporous ferns (Azollaceae and Marsileaceae) diverged in the Middle Jurassic (~173 mya). The oldest fossils are from the Early Cretaceous.


Uses of ferns
Ferns are used as ornamentals. Other uses are very secondary, e. g. basketry, food, weaving.
Azolla is an important fertilizer of rice paddies in southeast Asia.


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