Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the arborescent horsetails, progymnosperms; this occurred as part of the process of evolution of the timing of sex differentiation. Heterospory developed due to natural selection pressures that encouraged an increase in propagule size; this may first have led to an increase in spore size and resulted in the species producing larger megaspores as well as smaller microspores. Heterospory evolved from homospory many times, but the species in which it first appeared are now extinct. Heterosporic plants that produce seeds are their most widespread descendants. Seed plants constitute the largest subsection of heterosporic plants. Microspores are haploid spores that in endosporic species contain the male gametophyte, carried to the megaspores by wind, water currents or animal vectors.
Microspores are nearly all nonflagellated, are therefore not capable of active movement. The morphology of the microspore consists of an outer double walled structures surrounding the dense cytoplasm and central nucleus. Megaspores contain the female gametophytes in heterosporic plant species, they develop archegonia that produce egg cells that are fertilized by sperm of the male gametophyte originating from the microspore. This results in the formation of a fertilized diploid zygote, that develops into the sporophyte embryo. While heterosporous plants produce fewer megaspores, they are larger than their male counterparts. In exosporic species, the smaller spores germinate into free-living male gametophytes and the larger spores germinate into free-living female gametophytes. In endosporic species, the gametophytes of both sexes are highly reduced and contained within the spore wall; the microspores of both exosporic and endosporic species are free-sporing, distributed by wind, water or animal vectors, but in endosporic species the megaspores and the megagametophyte contained within are retained and nurtured by the sporophyte phase.
Endosporic species are thus dioecious, a condition that promotes outcrossing. Some exosporic species produce micro- and megaspores in the same sporangium, a condition known as homoangy, while in others the micro- and megaspores are produced in separate sporangia; these may both be borne on the same monoecious sporophyte or on different sporophytes in dioicous species. Heterospory was a key event in the evolution of surviving plants; the retention of megaspores and the dispersal of microspores allow for both dispersal and establishment reproductive strategies. This adaptive ability of heterospory increases reproductive success as any type of environment favors having these two strategies. Heterospory stops self-fertilization from occurring in a gametophyte, but does not stop two gametophytes that originated from the same sporophyte from mating; this specific type of self-fertilization is termed as sporophytic selfing, it occurs most among angiosperms. While heterospory stops extreme inbreeding from occurring, it does not prevent inbreeding altogether as sporophytic selfing can still occur.
A complete model for the origin of heterospory, known as the Haig-Westoby model, establishes a connection between minimum spore size and successful reproduction of bisexual gametophytes. For the female function, as minimum spore size increases so does the chance for successful reproduction. For the male function, reproductive success does not change as the minimum spore size increases
A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separate from the other eukaryotic life kingdoms of plants and animals. A characteristic that places fungi in a different kingdom from plants and some protists is chitin in their cell walls. Similar to animals, fungi are heterotrophs. Fungi do not photosynthesize. Growth is their means of mobility, except for spores, which may travel through the water. Fungi are the principal decomposers in ecological systems; these and other differences place fungi in a single group of related organisms, named the Eumycota, which share a common ancestor, an interpretation, strongly supported by molecular phylogenetics. This fungal group oomycetes; the discipline of biology devoted to the study of fungi is known as mycology. In the past, mycology was regarded as a branch of botany, although it is now known fungi are genetically more related to animals than to plants.
Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and parasites, they may become noticeable when fruiting, either as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment, they have long been used in the form of mushrooms and truffles. Since the 1940s, fungi have been used for the production of antibiotics, more various enzymes produced by fungi are used industrially and in detergents. Fungi are used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans; the fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies.
Fungi can break down manufactured materials and buildings, become significant pathogens of humans and other animals. Losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and local economies; the fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, estimated at 2.2 million to 3.8 million species. Of these, only about 120,000 have been described, with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. Since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christian Hendrik Persoon, Elias Magnus Fries, fungi have been classified according to their morphology or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits.
Phylogenetic studies published in the last decade have helped reshape the classification within Kingdom Fungi, divided into one subkingdom, seven phyla, ten subphyla. The English word fungus is directly adopted from the Latin fungus, used in the writings of Horace and Pliny; this in turn is derived from the Greek word sphongos, which refers to the macroscopic structures and morphology of mushrooms and molds. The word mycology is derived from the Greek logos, it denotes the scientific study of fungi. The Latin adjectival form of "mycology" appeared as early as 1796 in a book on the subject by Christiaan Hendrik Persoon; the word appeared in English as early as 1824 in a book by Robert Kaye Greville. In 1836 the English naturalist Miles Joseph Berkeley's publication The English Flora of Sir James Edward Smith, Vol. 5. Refers to mycology as the study of fungi. A group of all the fungi present in a particular area or geographic region is known as mycobiota, e.g. "the mycobiota of Ireland". Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: both fungi and plants are immobile, have similarities in general morphology and growth habitat.
Like plants, fungi grow in soil and, in the case of mushrooms, form conspicuous fruit bodies, which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago; some morphological and genetic features are shared with other organisms, while others are unique to the fungi separating them from the other kingdoms: Shared features: With other euka
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts and the green algae, but excludes the red and brown algae. Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria, their chloroplasts contain b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs; the scientific study of plants is known as a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships; these are not yet settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, they are no longer classified as plants as defined here; the Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common, they undergo closed mitosis without centrioles, have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. Two additional groups, the Rhodophyta and Glaucophyta have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour.
These groups differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event; this is the broadest modern definition of the term'plant'. In contrast, most other algae not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in the broadest modern definition of the plant kingdom, although they were in the past; the green plants or Viridiplantae were traditionally divided into the green algae (including
Selaginella is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses. This family is distinguished from Lycopodiaceae by having scale-leaves bearing a ligule and by having spores of two types, they are sometimes included in an informal paraphyletic group called the "fern allies". S. moellendorffii is an important model organism. Its genome has been sequenced by the United States Department of Energy's Joint Genome Institute; the name Selaginella was erected by Palisot de Beauvois for the species Selaginella selaginoides, which turns out to be a clade, sister to all other Selaginellas, so any definitive subdivision of the species leaves two taxa in Selaginella, with the hundreds of other species in new or resurrected genera. Selaginella occurs in the tropical regions of the world, with a handful of species to be found in the arctic-alpine zones of both hemispheres. Selaginella species are creeping or ascendant plants with simple, scale-like leaves on branching stems from which roots arise.
The stems are aerial, horizontally creeping on the substratum, sub erect. The vascular steles are polystelic protosteles. Stem section shows the presence of more than two protosteles; each stele is made up of exarch xylem in centre. The steles are connected with the cortex by means of many tube-like structures called trabeculae, which are modified endodermal cells with casparian strips on their lateral walls; the stems contain no pith. Unusually for the lycopods, which have microphylls with a single unbranched vein, the microphylls of Selaginella species contain a branched vascular trace. In Selaginella, each microphyll and sporophyll has a small scale-like outgrowth called a ligule at the base of the upper surface; the plants are heterosporous with spores of two different size classes, known as megaspores and microspores. Under dry conditions, some species of Selaginella can survive dehydration. In this state, they may roll up into brown balls and be uprooted, but can rehydrate under moist conditions, become green again and resume growth.
This phenomenon is known as poikilohydry, poikilohydric plants such as Selaginella bryopteris are sometimes referred to as resurrection plants. Some scientists still place the Selaginellales in the class Lycopodiopsida; some modern authors recognize three generic divisions of Selaginella: Selaginella, Bryodesma Sojak 1992, Lycopodioides Boehm 1760. Lycopodioides would include the North American species S. apoda and S. eclipes, while Bryodesma would include S. rupestris. Stachygynandrum is sometimes used to include the bulk of species; the first major attempt to define and subdivide the group was by Palisot de Beauvois in 1803-1805. He established the genus Selaginella as a monotypic genus, placed the bulk of species in Stachygynandrum. Gymnogynum was another monotypic genus, but that name is superseded by his own earlier name of Didiclis; this turns out, today, to be a group of around 45-50 species known as the Articulatae, since his genus Didiclis/Gymnogynum was based on Selaginella plumosa. He described the genus Diplostachyum to include a group of species similar to Selaginella apoda.
Spring inflated the genus Selaginella to hold all selaginelloid species four decades later. Phylogenetic studies by Korall & Kenrick determined that the Euselaginella group, comprising the type species, Selaginella selaginoides and a related Hawaiian species, Selaginella deflexa, is a basal and anciently diverging sister to all other Selaginella species. Beyond this, their study split the remainder of species into two broad groups, one including the Bryodesma species, the Articulatae, section Ericetorum Jermy and others, the other centered on the broad Stachygynandrum group. In the Manual of Pteridology, the following classification was used by Walton & Alston: genus: Selaginella subgenus: Euselaginella group: selaginoides group: pygmaea group: uliginosa group: rupestris subgenus: Stachygynandrum series: Decumbentes series: Ascendentes series: Sarmentosae series: Caulescentes series: Circinatae series: Articulatae subgenus: Homostachys subgenus: HeterostachysHowever, this is now known to be paraphyletic in most of its groupings.
Two recent classifications, employing modern methods of phylogenetic analysis, are as follows: genus: Selaginella subgenus: Selaginella clade: "Rhizophoric clade" clade A subgenus Rupestrae subgenus Lepidophyllae subgenus Gymnogynum subgenus Exaltatae subgenus Ericetorum clade B subgenus Stachygynandrum genus: Selaginella subgenus: Selaginella Type: Selaginella selaginoides P. Beauv. Ex Mart. & Schrank subgenus: Boreoselaginella Type: Selaginella sanguinolenta Spring subgenus: Ericetorum Type: Selaginella uliginosa Spring section: Lyallia Type: Selaginella uliginosa Spring section: Myosurus Type: Selaginella myosurus Alston section: Megalosporarum Type: Selaginella exaltata Spring section: Articulatae Type: Selaginella kraussiana A. Braun section: Homoeophyllae Type: Selaginella rupestris Spring section: Lepidophyllae Type: Selaginella lepidophylla Spring subgenus: Pulviniella Type: Selaginella pulvinata Maxim subgenus: Heterostachys Type: Selaginella heterostachys Baker section: Oligomacrosporangiatae Type: Selaginella uncinata Spring section: Auriculatae Type: Selaginella douglasii
A sporophyll is a leaf that bears sporangia. Both microphylls and megaphylls can be sporophylls. In heterosporous plants, sporophylls bear either megasporangia and thus are called megasporophylls, or microsporangia and are called microsporophylls; the overlap of the prefixes and roots makes these terms a confusing subset of botanical nomenclature. Sporophylls vary in appearance and structure, may or may not look similar to sterile leaves. Plants that produce sporophylls include: Alaria esculenta a brown alga shows sporophylls attached near the base of the alga. Lycophytes, where sporophylls may be aggregated into strobili or distributed singly among sterile leaves. Sporangia are borne on the adaxial surface of the sporophyll. In heterosporous members and microsporophylls may be intermixed or separated in a variety of patterns. Ferns, which may produce sporophylls that are similar to sterile fronds or that appear different from sterile fronds; these may be non-photosynthetic and lack typical pinnae Cycads produce strobili, both pollen-producing and seed-producing, that are composed of sporophylls.
Ginkgo produces microsporophylls aggregated into a pollen strobilus. Ovules are not born on sporophylls. Conifers, like cycads, produce microsporophylls, aggregated into pollen strobili. However, unlike these other groups, ovules are produced on cone scales, which are modified shoots rather than sporophylls; some plants do not produce sporophylls. Sporangia are produced directly on stems. Psilotum has been interpreted as producing sporangia on the terminus of a stem. Equisetum always produce strobili, but the structures bearing sporangia have been interpreted as modified stems; the sporangia, despite being recurved are interpreted as terminal. Gnetophytes produce both compound seed strobili. C. Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment. Washington, DC
Isoetes known as the quillworts, is a genus of plants in the family Isoetaceae. They are the only genus in Isoetaceae. There are 192 recognized species, with a cosmopolitan distribution but with the individual species scarce to rare; some botanists split the genus, separating two South American species into the genus Stylites, although molecular data place these species among other species of Isoetes, so Stylites does not warrant taxonomic recognition. The name of the genus may be spelled Isoëtes; the diaeresis indicate that the e are to be pronounced in two distinct syllables. Including this in print is optional. Quillworts are aquatic or semi-aquatic in clear ponds and slow-moving streams, though several grow on wet ground that dries out in the summer; the Quillworts are spore producing plants and rely on water dispersion. Quillwort have different way to spread their spores based on the environment. Quillwort leaves are quill-like, with a minute ligule at the base of the upper surface. Arising from a central corm.
Each leaf is narrow, 2–20 centimetres long and 0.5–3.0 mm wide. Stomata are absent, yet the leaves have a thick cuticle which prevents CO2 uptake, a task, performed by their hollow roots instead, which absorbs CO2 from the sediment. Isoetes andicola is unusual in being the only known terrestrial vascular plant that take up all its CO2 through the roots. Only 4% of total biomass, the tips of the leaves, is chlorophyllous; the roots broaden to a swollen base up to 5 mm wide where they attach in clusters to a bulb-like, underground rhizome characteristic of most quillwort species, though a few form spreading mats. This swollen base contains male and female sporangia, protected by a thin, transparent covering, used diagnostically to help identify quillwort species, they are heterosporous. Quillwort species are difficult to distinguish by general appearance; the best way to identify them is by examining their megaspores under a microscope. Moreover, texture, spore size, velum provide features that will distinguish Isoëtes taxa.
Compared to other genera, Isoetes is poorly known. After studies with cytology, scanning electron microscopy, chromatography, species are difficult to identify and their phylogeny is disputed. Vegetative characters used to distinguish other genera, such as leaf length, color, or shape are variable and depend on habitat. Most classification systems for Isoetes rely on spore characteristics, which make species identification nearly impossible without microscopy. Selected species Many species, such as the Louisiana quillwort and the mat-forming quillwort, are endangered species. Several species of Isoetes are called Merlin's grass I. lacustris, but the endangered species I. tegetiformans and I. virginica. Fossilised specimens of I. beestonii have been found in rocks dating to the latest Permian. Quillworts are considered to be the closest extant relatives of the fossil tree Lepidodendron, with which they share some unusual features including the development of both wood and bark, a modified shoot system acting as roots, bipolar growth, an upright stance.
Checklist of World Ferns, Family Isoetaceae, genus Isoetes. Distribution and classification list for world isoetes
A sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It develops from the zygote produced when a haploid egg cell is fertilized by a haploid sperm and each sporophyte cell therefore has a double set of chromosomes, one set from each parent. All land plants, most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploid gametophyte phase. In the seed plants, flowering plants, the sporophyte phase is more prominent than the gametophyte, is the familiar green plant with its roots, stem and cones or flowers. In flowering plants the gametophytes are reduced in size, are represented by the germinated pollen and the embryo sac; the sporophyte produces spores by meiosis, a process known as "reduction division" that reduces the number of chromosomes in each spore mother cell by half. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes.
The mature gametophyte produces female gametes by mitosis. The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte; this cycle is known as alternation of alternation of phases. Bryophytes have a dominant gametophyte phase on which the adult sporophyte is dependent for nutrition; the embryo sporophyte develops by cell division of the zygote within the female sex organ or archegonium, in its early development is therefore nurtured by the gametophyte. Because this embryo-nurturing feature of the life cycle is common to all land plants they are known collectively as the embryophytes. Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar. An independent sporophyte is the dominant form in all clubmosses, ferns and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, fewer in number, than the male microspores.
During the Devonian period several plant groups independently evolved heterospory and subsequently the habit of endospory, in which the gametophytes develop in miniaturized form inside the spore wall. By contrast in exosporous plants, including modern ferns, the gametophytes break the spore wall open on germination and develop outside it; the megagametophytes of endosporic plants such as the seed ferns developed within the sporangia of the parent sporophyte, producing a miniature multicellular female gametophyte complete with female sex organs, or archegonia. The oocytes were fertilized in the archegonia by free-swimming flagellate sperm produced by windborne miniaturized male gametophytes in the form of pre-pollen; the resulting zygote developed into the next sporophyte generation while still retained within the pre-ovule, the single large female meiospore or megaspore contained in the modified sporangium or nucellus of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms today.
P. Kenrick & P. R. Crane The origin and early evolution of plants on land. Nature 389, 33-39. T. N. Taylor, H. Kerp and H. Hass Life history biology of early land plants: Deciphering the gametophyte phase. Proceedings of the National Academy of Sciences 102, 5892-5897. P. R. Bell & A. R. Helmsley Green plants, their Origin and Diversity. Cambridge University Press ISBN 0-521-64673-1