Plant pathology is the scientific study of diseases in plants caused by pathogens and environmental conditions. Organisms that cause infectious disease include fungi, bacteria, viroids, virus-like organisms, protozoa and parasitic plants. Not included are ectoparasites like insects, vertebrate, or other pests that affect plant health by eating of plant tissues. Plant pathology involves the study of pathogen identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics, management of plant diseases. Control of plant diseases is crucial to the reliable production of food, it provides significant problems in agricultural use of land, water and other inputs. Plants in both natural and cultivated populations carry inherent disease resistance, but there are numerous examples of devastating plant disease impacts such as Irish potato famine and chestnut blight, as well as recurrent severe plant diseases like rice blast, soybean cyst nematode, citrus canker.
However, disease control is reasonably successful for most crops. Disease control is achieved by use of plants that have been bred for good resistance to many diseases, by plant cultivation approaches such as crop rotation, use of pathogen-free seed, appropriate planting date and plant density, control of field moisture, pesticide use. Across large regions and many crop species, it is estimated that diseases reduce plant yields by 10% every year in more developed settings, but yield loss to diseases exceeds 20% in less developed settings. Continuing advances in the science of plant pathology are needed to improve disease control, to keep up with changes in disease pressure caused by the ongoing evolution and movement of plant pathogens and by changes in agricultural practices. Plant diseases cause major economic losses for farmers worldwide; the Food and Agriculture Organization estimates indeed that pests and diseases are responsible for about 25% of crop loss. To solve this issue, new methods are needed to detect diseases and pests early, such as novel sensors that detect plant odours and spectroscopy and biophotonics that are able to diagnose plant health and metabolism.
Most phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes. The fungi reproduce both sexually and asexually via the production of other structures. Spores may be spread long distances by air or water. Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their life cycle in the soil; these are facultative saprotrophs. Fungal diseases may be controlled through the use of other agriculture practices. However, new races of fungi evolve that are resistant to various fungicides. Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. Significant fungal plant pathogens include: Fusarium spp. Thielaviopsis spp. Verticillium spp. Magnaporthe grisea Sclerotinia sclerotiorum Ustilago spp. smut of barley Rhizoctonia spp. Phakospora pachyrhizi Puccinia spp. Armillaria spp; the oomycetes are fungus-like organisms.
They include some of the most destructive plant pathogens including the genus Phytophthora, which includes the causal agents of potato late blight and sudden oak death. Particular species of oomycetes are responsible for root rot. Despite not being related to the fungi, the oomycetes have developed similar infection strategies. Oomycetes are capable of using effector proteins to turn off a plant's defenses in its infection process. Plant pathologists group them with fungal pathogens. Significant oomycete plant pathogens include: Pythium spp. Phytophthora spp. including the potato blight of the Great Irish Famine Some slime molds in Phytomyxea cause important diseases, including club root in cabbage and its relatives and powdery scab in potatoes. These are caused by species of Spongospora, respectively. Most bacteria that are associated with plants are saprotrophic and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease. Bacterial diseases are much more prevalent in tropical regions of the world.
Most plant pathogenic bacteria are rod-shaped. In order to be able to colonize the plant they have specific pathogenicity factors. Five main types of bacterial pathogenicity factors are known: uses of cell wall–degrading enzymes, effector proteins and exopolysaccharides. Pathogens such as Erwinia species use cell wall–degrading enzymes to cause soft rot. Agrobacterium species change the level of auxins to cause tumours with phytohormones. Exopolysaccharides are produced by bacteria and block xylem vessels leading to the death of the plant. Bacteria control the production of pathogenicity factors via quorum sensing. Significant bacterial plant pathogens: Burkholderia Proteobacteria Xanthomonas spp. Pseudomonas spp. Pseudomonas syringae pv. tomato causes tomato plants to produce less fruit, it "continues to adapt to the tomato by minimizing its recognition by the tomato immune system." Phytoplasma and Spiroplasma are genera of bacteria that lack cell walls and are related to the mycoplasmas, which are human pathogens.
Together they are referred to
Plant ecology is a subdiscipline of ecology which studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, the interactions among and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands. A global overview of the Earth's major vegetation types is provided by O. W. Archibold, he recognizes 11 major vegetation types: tropical forests, tropical savannas, arid regions, Mediterranean ecosystems, temperate forest ecosystems, temperate grasslands, coniferous forests, terrestrial wetlands, freshwater ecosystems and coastal/marine systems. This breadth of topics shows the complexity of plant ecology, since it includes plants from floating single-celled algae up to large canopy forming trees. One feature that defines plants is photosynthesis.
Photosynthesis is the process of a chemical reactions to create glucose and oxgyen, vital for plant life. One of the most important aspects of plant ecology is the role plants have played in creating the oxygenated atmosphere of earth, an event that occurred some 2 billion years ago, it can be dated by the deposition of banded iron formations, distinctive sedimentary rocks with large amounts of iron oxide. At the same time, plants began removing carbon dioxide from the atmosphere, thereby initiating the process of controlling Earth's climate. A long term trend of the Earth has been toward increasing oxygen and decreasing carbon dioxide, many other events in the Earth's history, like the first movement of life onto land, are tied to this sequence of events. One of the early classic books on plant ecology was written by J. E. Weaver and F. E. Clements, it talks broadly about plant communities, the importance of forces like competition and processes like succession. Plant ecology can be divided by levels of organization including plant ecophysiology, plant population ecology, community ecology, ecosystem ecology, landscape ecology and biosphere ecology.
The study of plants and vegetation is complicated by their form. First, most plants are rooted in the soil, which makes it difficult to observe and measure nutrient uptake and species interactions. Second, plants reproduce vegetatively, asexually, in a way that makes it difficult to distinguish individual plants. Indeed, the concept of an individual is doubtful, since a tree may be regarded as a large collection of linked meristems. Hence, plant ecology and animal ecology have different styles of approach to problems that involve processes like reproduction and mutualism; some plant ecologists have placed considerable emphasis upon trying to treat plant populations as if they were animal populations, focusing on population ecology. Many other ecologists believe that while it is useful to draw upon population ecology to solve certain scientific problems, plants demand that ecologists work with multiple perspectives, appropriate to the problem, the scale and the situation. Plant ecology has its origin in the application of plant physiology to the questions raised by plant geographers.
Carl Ludwig Willdenow was one of the first to note that similar climates produced similar types of vegetation when they were located in different parts of the world. Willdenow's student, Alexander von Humboldt, used physiognomy to describe vegetation types and observed that the distribution vegetation types was based on environmental factors. Plant geographers who built upon Humboldt's work included Joakim Frederik Schouw, A. P. de Candolle, August Grisebach and Anton Kerner von Marilaun. Schouw's work, published in 1822, linked plant distributions to environmental factors and established the practice of naming plant associations by adding the suffix -etum to the name of the dominant species. Working from herbarium collections, De Candolle searched for general rules of plant distribution and settled on using temperature as well. Grisebach's two-volume work, Die Vegetation der Erde nach Ihrer Klimatischen Anordnung, published in 1872, saw plant geography reach its "ultimate form" as a descriptive field.
Starting in the 1870s, Swiss botanist Simon Schwendener, together with his students and colleagues, established the link between plant morphology and physiological adaptations, laying the groundwork for the first ecology textbooks, Eugenius Warming's Plantesamfund and Andreas Schimper's 1898 Pflanzengeographie auf Physiologischer Grundlage. Warming incorporated plant morphology, physiology taxonomy and biogeography into plant geography to create the field of plant ecology. Although more morphological than physiological, Schimper's has been considered the beginning of plant physiological ecology. Plant ecology was built around static ideas of plant distribution. Henry Chandler Cowles' studies of plant succession on the Lake Michigan sand dunes and Frederic Clements' 1916 monograph on the subject established it as a key element of plant ecology. Plant ecology developed within the wider discipline of ecology over the twentieth century. Inspired by Warming's Plantesamfund, Arthur Tansley set out to map British plant communities.
In 1904 he teamed up with William Gardner Smith and others involved in vegetation mapping to establish the Central Committee for the Survey and Study of British Vegetation shortened to British Vegetation Committee. In 1913, the British Vegetation Committee organised the British Ecological
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough and sometimes rigid, it provides the cell with both structural support and protection, acts as a filtering mechanism. Cell walls are present in most prokaryotes, in algae and fungi but in other eukaryotes including animals. A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters; the composition of cell walls varies between species and may depend on cell type and developmental stage. The primary cell wall of land plants is composed of the polysaccharides cellulose and pectin. Other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls. Algae possess cell walls made of glycoproteins and polysaccharides such as carrageenan and agar that are absent from land plants. In bacteria, the cell wall is composed of peptidoglycan; the cell walls of archaea have various compositions, may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides.
Fungi possess cell walls made of the N-acetylglucosamine polymer chitin. Unusually, diatoms have a cell wall composed of biogenic silica. A plant cell wall was first observed and named by Robert Hooke in 1665. However, "the dead excrusion product of the living protoplast" was forgotten, for three centuries, being the subject of scientific interest as a resource for industrial processing or in relation to animal or human health. In 1804, Karl Rudolphi and J. H. F. Link proved. Before, it had been thought that fluid passed between them this way; the mode of formation of the cell wall was controversial in the 19th century. Hugo von Mohl advocated the idea. Carl Nägeli believed that the growth of the wall in thickness and in area was due to a process termed intussusception; each theory was improved in the following decades: the apposition theory by Eduard Strasburger, the intussusception theory by Julius Wiesner. In 1930, Ernst Münch coined the term apoplast in order to separate the "living" symplast from the "dead" plant region, the latter of which included the cell wall.
By the 1980s, some authors suggested replacing the term "cell wall" as it was used for plants, with the more precise term "extracellular matrix", as used for animal cells, but others preferred the older term. Cell walls serve similar purposes in those organisms, they may give cells offering protection against mechanical stress. In multicellular organisms, they permit the organism to hold a definite shape. Cell walls limit the entry of large molecules that may be toxic to the cell, they further permit the creation of stable osmotic environments by preventing osmotic lysis and helping to retain water. Their composition and form may change during the cell cycle and depend on growth conditions. In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerable tensile strength; the apparent rigidity of primary plant tissues is enabled by cell walls, but is not due to the walls' stiffness. Hydraulic turgor pressure creates this rigidity, along with the wall structure.
The flexibility of the cell walls is seen when plants wilt, so that the stems and leaves begin to droop, or in seaweeds that bend in water currents. As John Howland explains Think of the cell wall as a wicker basket in which a balloon has been inflated so that it exerts pressure from the inside; such a basket is rigid and resistant to mechanical damage. Thus does the prokaryote cell gain strength from a flexible plasma membrane pressing against a rigid cell wall; the apparent rigidity of the cell wall thus results from inflation of the cell contained within. This inflation is a result of the passive uptake of water. In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed by suberin in cork cell walls; these compounds are rigid and waterproof. Both wood and bark cells of trees have secondary walls. Other parts of plants such as the leaf stalk may acquire similar reinforcement to resist the strain of physical forces.
The primary cell wall of most plant cells is permeable to small molecules including small proteins, with size exclusion estimated to be 30-60 kDa. The pH is an important factor governing the transport of molecules through cell walls. Cell walls evolved independently including within the photosynthetic eukaryotes. In these lineages, the cell wall is related to the evolution of multicellularity, terrestrialization and vascularization; the walls of plant cells must have sufficient tensile strength to withstand internal osmotic pressures of several times atmospheric pressure that result from the difference in solute concentration between the cell interior and external solutions. Plant cell walls vary from 0.1 to several µm in thickness. Up to three strata or layers may be found in plant cell walls: The primary cell wall a thin and extensible layer formed while the cell is growing; the secondary cell wall, a thick layer formed inside the primary cell wall after the cell is grown. It is not found in all cell types.
Some cells, such as the conducting cells in xylem, possess a secondary wall containing lignin, which strengthens and waterproofs the wall. The middle lamella, a layer rich in pectins; this outermost layer
History of botany
The history of botany examines the human effort to understand life on Earth by tracing the historical development of the discipline of botany—that part of natural science dealing with organisms traditionally treated as plants. Rudimentary botanical science began with empirically-based plant lore passed from generation to generation in the oral traditions of paleolithic hunter-gatherers; the first written records of plants were made in the Neolithic Revolution about 10,000 years ago as writing was developed in the settled agricultural communities where plants and animals were first domesticated. The first writings that show human curiosity about plants themselves, rather than the uses that could be made of them, appears in the teachings of Aristotle's student Theophrastus at the Lyceum in ancient Athens in about 350 BC. In Europe, this early botanical science was soon overshadowed by a medieval preoccupation with the medicinal properties of plants that lasted more than 1000 years. During this time, the medicinal works of classical antiquity were reproduced in manuscripts and books called herbals.
In China and the Arab world, the Greco-Roman work on medicinal plants was extended. In Europe the Renaissance of the 14th–17th centuries heralded a scientific revival during which botany emerged from natural history as an independent science, distinct from medicine and agriculture. Herbals were replaced by floras: books; the invention of the microscope stimulated the study of plant anatomy, the first designed experiments in plant physiology were performed. With the expansion of trade and exploration beyond Europe, the many new plants being discovered were subjected to an rigorous process of naming and classification. Progressively more sophisticated scientific technology has aided the development of contemporary botanical offshoots in the plant sciences, ranging from the applied fields of economic botany, to the detailed examination of the structure and function of plants and their interaction with the environment over many scales from the large-scale global significance of vegetation and plant communities through to the small scale of subjects like cell theory, molecular biology and plant biochemistry.
Botany and zoology are the core disciplines of biology whose history is associated with the natural sciences chemistry and geology. A distinction can be made between botanical science in a pure sense, as the study of plants themselves, botany as applied science, which studies the human use of plants. Early natural history divided pure botany into three main streams morphology-classification and physiology – that is, external form, internal structure, functional operation; the most obvious topics in applied botany are horticulture and agriculture although there are many others like weed science, plant pathology, pharmacognosy, economic botany and ethnobotany which lie outside modern courses in botany. Since the origin of botanical science there has been a progressive increase in the scope of the subject as technology has opened up new techniques and areas of study. Modern molecular systematics, for example, entails the principles and techniques of taxonomy, molecular biology, computer science and more.
Within botany there are a number of sub-disciplines that focus on particular plant groups, each with their own range of related studies. Included here are: phycology, pteridology and palaeobotany and their histories are treated elsewhere. To this list can be added mycology, the study of fungi, which were once treated as plants, but are now ranked as a unique kingdom. Nomadic hunter-gatherer societies passed on, by oral tradition, what they knew about the different kinds of plants that they used for food, poisons, for ceremonies and rituals etc; the uses of plants by these pre-literate societies influenced the way the plants were named and classified—their uses were embedded in folk-taxonomies, the way they were grouped according to use in everyday communication. The nomadic life-style was drastically changed when settled communities were established in about twelve centres around the world during the Neolithic Revolution which extended from about 10,000 to 2500 years ago depending on the region.
With these communities came the development of the technology and skills needed for the domestication of plants and animals and the emergence of the written word provided evidence for the passing of systematic knowledge and culture from one generation to the next. During the Neolithic Revolution plant knowledge increased most through the use of plants for food and medicine. All of today's staple foods were domesticated in prehistoric times as a gradual process of selection of higher-yielding varieties took place unknowingly, over hundreds to thousands of years. Legumes were cultivated on all continents but cereals made up most of the regular diet: rice in East Asia and barley in the Middle east, maize in Central and South America. By Greco-Roman times popular food plants of today, including grapes, apples and olives, were being listed as named varieties in early manuscripts. Botanical authority William Stearn has observed that "cultivated plants are mankind's most vital and precious heritage from remote antiquity".
It is from the Neolithic, in about 3000 BC, that we glimpse the first known illustrations of plants and read descript
In vascular plants, the root is the organ of a plant that lies below the surface of the soil. Roots can be aerial or aerating, that is, growing up above the ground or above water. Furthermore, a stem occurring below ground is not exceptional either. Therefore, the root is best defined as the non-nodes bearing parts of the plant's body. However, important internal structural differences between stems and roots exist; the fossil record of roots—or rather, infilled voids where roots rotted after death—spans back to the late Silurian, about 430 million years ago. Their identification is difficult, because casts and molds of roots are so similar in appearance to animal burrows, they can be discriminated using a range of features. The first root that comes from a plant is called the radicle. A root's four major functions are: absorption of inorganic nutrients. In response to the concentration of nutrients, roots synthesise cytokinin, which acts as a signal as to how fast the shoots can grow. Roots function in storage of food and nutrients.
The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizae, a large range of other organisms including bacteria closely associate with roots. When dissected, the arrangement of the cells in a root is root hair, epiblem, endodermis, pericycle and, the vascular tissue in the centre of a root to transport the water absorbed by the root to other places of the plant; the most striking characteristic of roots is that, roots have an endogenous origin, i.e. it originates and develops from an inner layer of the mother axis. Whereas Stem-branching and leaves are exogenous, i.e. start to develop from the cortex, an outer layer. In its simplest form, the term root architecture refers to the spatial configuration of a plant’s root system; this system can be complex and is dependent upon multiple factors such as the species of the plant itself, the composition of the soil and the availability of nutrients. The configuration of root systems serves to structurally support the plant, compete with other plants and for uptake of nutrients from the soil.
Roots grow to specific conditions. For example, a root system that has developed in dry soil may not be as efficient in flooded soil, yet plants are able to adapt to other changes in the environment, such as seasonal changes. Root architecture plays the important role of providing a secure supply of nutrients and water as well as anchorage and support; the main terms used to classify the architecture of a root system are: Branch magnitude: the number of links. Topology: the pattern of branching, including:Herringbone: alternate lateral branching off a parent root Dichotomous: opposite, forked branches Radial: whorl of branches around a rootLink length: the distance between branches. Root angle: the radial angle of a lateral root’s base around the parent root’s circumference, the angle of a lateral root from its parent root, the angle an entire system spreads. Link radius: the diameter of a root. All components of the root architecture are regulated through a complex interaction between genetic responses and responses due to environmental stimuli.
These developmental stimuli are categorised as intrinsic, the genetic and nutritional influences, or extrinsic, the environmental influences and are interpreted by signal transduction pathways. The extrinsic factors that affect root architecture include gravity, light exposure and oxygen, as well as the availability or lack of nitrogen, sulphur and sodium chloride; the main hormones and respective pathways responsible for root architecture development include: Auxin – Auxin promotes root initiation, root emergence and primary root elongation. Cytokinins – Cytokinins regulate root apical meristem size and promote lateral root elongation. Gibberellins -- Together with ethylene they promote elongation. Together with auxin they promote root elongation. Gibberellins inhibit lateral root primordia initiation. Ethylene – Ethylene promotes crown root formation. Early root growth is one of the functions of the apical meristem located near the tip of the root; the meristem cells more or less continuously divide, producing more meristem, root cap cells, undifferentiated root cells.
The latter become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. These cells differentiate and mature into specialized cells of the root tissues. Growth from apical meristems is known as primary growth. Secondary growth encompasses all growth in diameter, a major component of woody plant tissues and many nonwoody plants. For example, storage roots of sweet potato are not woody. Secondary growth occurs at the lateral meristems, namely the vascular cork cambium; the former forms secondary phloem, while the latter forms the periderm. In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root; the vascular cambium forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, those on the outside forming secondary phloem cells. As
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material, a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves, it conveys water and nutrients between the leaves, other growing tissues, the roots. Wood may refer to other plant materials with comparable properties, to material engineered from wood, or wood chips or fiber. Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons and paper. More it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate.
As of 2005, the growing stock of forests worldwide was about 434 billion cubic meters, 47% of, commercial. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 1991 3.5 billion cubic meters of wood were harvested. Dominant uses were for building construction. A 2011 discovery in the Canadian province of New Brunswick yielded the earliest known plants to have grown wood 395 to 400 million years ago. Wood can be dated by carbon dating and in some species by dendrochronology to determine when a wooden object was created. People have used wood for thousands of years for many purposes, including as a fuel or as a construction material for making houses, weapons, packaging and paper. Known constructions using wood date back ten thousand years. Buildings like the European Neolithic long house were made of wood. Recent use of wood has been enhanced by the addition of bronze into construction; the year-to-year variation in tree-ring widths and isotopic abundances gives clues to the prevailing climate at the time a tree was cut.
Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, roots. This process is known as secondary growth; these cells go on to form thickened secondary cell walls, composed of cellulose and lignin. Where the differences between the four seasons are distinct, e.g. New Zealand, growth can occur in a discrete annual or seasonal pattern, leading to growth rings. If the distinctiveness between seasons is annual, these growth rings are referred to as annual rings. Where there is little seasonal difference growth rings are to be indistinct or absent. If the bark of the tree has been removed in a particular area, the rings will be deformed as the plant overgrows the scar. If there are differences within a growth ring the part of a growth ring nearest the center of the tree, formed early in the growing season when growth is rapid, is composed of wider elements.
It is lighter in color than that near the outer portion of the ring, is known as earlywood or springwood. The outer portion formed in the season is known as the latewood or summerwood. However, there are major differences, depending on the kind of wood; as a tree grows, lower branches die, their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot. The dead branch may not be attached to the trunk wood except at its base, can drop out after the tree has been sawn into boards. Knots affect the technical properties of the wood reducing the local strength and increasing the tendency for splitting along the wood grain, but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a circular "solid" piece of wood around which the grain of the rest of the wood "flows". Within a knot, the direction of the wood is up to 90 degrees different from the grain direction of the regular wood. In the tree a knot is either the base of a dormant bud.
A knot is conical in shape with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud. In grading lumber and structural timber, knots are classified according to their form, size and the firmness with which they are held in place; this firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow. Knots materially affect cracking and warping, ease in working, cleavability of timber, they are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression; the extent to which knots affect the strength of a beam depends upon their position, size and condition. A knot on the upper side is compressed. If there is a season check
A sporangium is an enclosure in which spores are formed. It can be multicellular. All plants and many other lineages form sporangia at some point in their life cycle. Sporangia can produce spores by mitosis, but in nearly all land plants and many fungi, sporangia are the site of meiosis and produce genetically distinct haploid spores. In some phyla of fungi, the sporangium plays a role in asexual reproduction, may play an indirect role in sexual reproduction; the sporangium contains haploid nuclei and cytoplasm. Spores are formed in the sporangiophore by encasing each haploid nucleus and cytoplasm in a tough outer membrane. During asexual reproduction, these spores are dispersed via germinate into haploid hyphae. Although sexual reproduction in fungi varies between phyla, for some fungi the sporangium plays an indirect role in sexual reproduction. For Zygomycota, sexual reproduction occurs when the haploid hyphae from two individuals join to form a zygosporangium in response to unfavorable conditions.
The haploid nuclei within the zygosporangium fuse into diploid nuclei. When conditions improve the zygosporangium germinates, undergoes meiosis and produces a sporangium, which releases spores. In mosses and hornworts, an unbranched sporophyte produces a single sporangium, which may be quite complex morphologically. Most non-vascular plants, as well as many lycophytes and most ferns, are homosporous; some bryophytes, most lycophytes, some ferns are heterosporous. These plants produce microspores and megaspores, which give rise to gametophytes that are functionally male or female, respectively. In some cases, both kinds of spores are produced in the same sporangium, may develop together as part of a spore tetrad. However, in most heterosporous plants there are two kinds of sporangia, termed microsporangia and megasporangia. A few ferns and some lycophytes are heterosporous with two kinds of sporangia, as are all the seed plants. Sporangia can associated with leaves. In ferns, sporangia are found on the abaxial surface of the leaf and are densely aggregated into clusters called sori.
Sori may be covered by a structure called an indusium. Some ferns have their sporangia scattered along reduced leaf segments or along the margin of the leaf. Lycophytes, in contrast, bear their sporangia on the adaxial surface of leaves or laterally on stems. Leaves that bear sporangia are called sporophylls. If the plant is heterosporous, the sporangia-bearing leaves are distinguished as either microsporophylls or megasporophylls. In seed plants, sporangia are located within strobili or flowers. Cycads form their microsporangia on microsporophylls. Megasporangia are formed within ovules, which are borne on megasporophylls, which are aggregated into strobili on separate plants. Conifers bear their microsporangia on microsporophylls aggregated into papery pollen strobili, the ovules, are located on modified stem axes forming compound ovuliferous cone scales. Flowering plants contain microsporangia in the anthers of stamens and megasporangia inside ovules inside ovaries. In all seed plants, spores are produced by meiosis and develop into gametophytes while still inside the sporangium.
The microspores become microgametophytes. The megaspores become megagametophytes. Categorized based on developmental sequence and leptosporangia are differentiated in the vascular plants. In a leptosporangium, found only in ferns, development involves a single initial cell that becomes the stalk and spores within the sporangium. There are around 64 spores in a leptosporangium. In a eusporangium, characteristic of all other vascular plants and some primitive ferns, the initials are in a layer. A eusporangium is larger, its wall is multi-layered. Although the wall may be stretched and damaged, resulting in only one cell-layer remaining. A cluster of sporangia that have become fused in development is called a synangium; this structure is most prominent in Psilotum and Marattiaceae such as Christensenia and Marattia. A columella is a sterile structure that supports the sporangium. In fungi, the columella, which may be branched or unbranched, may be of host origin. Secotium species have a simple, unbranched columella, while in Gymnoglossum species, the columella is branched.
In some Geastrum species, the columella appears as an extension of the stalk into the spore mass. Microsporangium Archegonium Antheridium Spore formation