origin and evolution of angiosperms pdf

Origin and evolution of angiosperms pdf

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Origin of angiosperms and the puzzle of the Jurassic gap

Flowering Plants

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Dr Sergei V. An obituary note appeared in the International Organization of Palaeobotany Newsletter, 33 July Gamoheterotopy the transfer of characters from one sex to another is very common in animals, and it is quite possible that it also played an important role in the evolution of plant fructifications.

Origin of angiosperms and the puzzle of the Jurassic gap

The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats , through multicellular marine and freshwater green algae , terrestrial bryophytes , lycopods and ferns , to the complex gymnosperms and angiosperms of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones, e.

Also by late Devonian, Elkinsia , an early seed fern , had evolved seeds. Most plant groups were relatively unscathed by the Permo-Triassic extinction event , although the structures of communities changed. Land plants evolved from a group of green algae , perhaps as early as mya, [8] but algae-like plants might have evolved as early as 1 billion years ago.

It would only very briefly have had paired chromosomes the diploid condition when the egg and sperm first fused to form a zygote that would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes the haploid condition. Co-operative interactions with fungi may have helped early plants adapt to the stresses of the terrestrial realm.

Plants were not the first photosynthesisers on land. Evidence of the earliest land plants occurs much later at about Ma, in lower middle Ordovician rocks from Saudi Arabia [16] and Gondwana [17] in the form of spores with decay-resistant walls.

These spores, known as cryptospores , were produced either singly monads , in pairs dyads or groups of four tetrads , and their microstructure resembles that of modern liverwort spores, suggesting they share an equivalent grade of organisation. Trilete spores similar to those of vascular plants appear soon afterwards, in Upper Ordovician rocks about million years ago.

This resistance is closely associated with having a desiccation-resistant outer wall—a trait only of use when spores must survive out of water.

Indeed, even those embryophytes that have returned to the water lack a resistant wall, thus don't bear trilete marks. The earliest megafossils of land plants were thalloid organisms, which dwelt in fluvial wetlands and are found to have covered most of an early Silurian flood plain. They could only survive when the land was waterlogged.

Once plants had reached the land, there were two approaches to dealing with desiccation. Modern bryophytes either avoid it or give in to it, restricting their ranges to moist settings, or drying out and putting their metabolism "on hold" until more water arrives, as in the liverwort genus Targionia.

Tracheophytes resist desiccation, by controlling the rate of water loss. They all bear a waterproof outer cuticle layer wherever they are exposed to air as do some bryophytes , to reduce water loss, but since a total covering would cut them off from CO 2 in the atmosphere tracheophytes use variable openings, the stomata , to regulate the rate of gas exchange.

Tracheophytes also developed vascular tissue to aid in the movement of water within the organisms see below , and moved away from a gametophyte dominated life cycle see below. Vascular tissue ultimately also facilitated upright growth without the support of water and paved the way for the evolution of larger plants on land. A snowball earth , from around mya, is believed to have been caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and increased the amount of oxygen in the atmosphere.

Charcoalification is an important taphonomic mode. Wildfire or burial in hot volcanic ash drives off the volatile compounds, leaving only a residue of pure carbon. This is not a viable food source for fungi, herbivores or detritovores, so is prone to preservation.

It is also robust, so can withstand pressure and display exquisite, sometimes sub-cellular, detail. All multicellular plants have a life cycle comprising two generations or phases.

The pattern in plant evolution has been a shift from homomorphy to heteromorphy. The algal ancestors of land plants were almost certainly haplobiontic , being haploid for all their life cycles, with a unicellular zygote providing the 2N stage.

All land plants i. It has been proposed that the basis for the emergence of the diploid phase of the life cycle as the dominant phase, is that diploidy allows masking of the expression of deleterious mutations through genetic complementation. As the diploid phase was becoming predominant, the masking effect likely allowed genome size , and hence information content, to increase without the constraint of having to improve accuracy of replication. The opportunity to increase information content at low cost is advantageous because it permits new adaptations to be encoded.

This view has been challenged, with evidence showing that selection is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms. The interpolation theory also known as the antithetic or intercalary theory [32] holds that the interpolation of a multicellular sporophyte phase between two successive gametophyte generations was an innovation caused by preceding meiosis in a freshly germinated zygote with one or more rounds of mitotic division, thereby producing some diploid multicellular tissue before finally meiosis produced spores.

This theory implies that the first sporophytes bore a very different and simpler morphology to the gametophyte they depended on. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on a gametophyte, as seen in some hornworts Anthoceros , and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes vascular plants.

The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides a possible route for the development of a self-sufficient sporophyte phase.

The alternative hypothesis, called the transformation theory or homologous theory , posits that the sporophyte might have appeared suddenly by delaying the occurrence of meiosis until a fully developed multicellular sporophyte had formed.

Since the same genetic material would be employed by both the haploid and diploid phases they would look the same. This explains the behaviour of some algae, such as Ulva lactuca , which produce alternating phases of identical sporophytes and gametophytes. Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, might have resulted in the simplification of the sexually active gametophyte, and elaboration of the sporophyte phase to better disperse the waterproof spores.

There is no evidence that early land plants of the Silurian and early Devonian had roots, although fossil evidence of rhizoids occurs for several species, such as Horneophyton. The earliest land plants did not have vascular systems for transport of water and nutrients either. Aglaophyton , a rootless vascular plant known from Devonian fossils in the Rhynie chert [36] was the first land plant discovered to have had a symbiotic relationship with fungi [37] which formed arbuscular mycorrhizas , literally "tree-like fungal roots", in a well-defined cylinder of cells ring in cross section in the cortex of its stems.

The fungi fed on the plant's sugars, in exchange for nutrients generated or extracted from the soil especially phosphate , to which the plant would otherwise have had no access. Like other rootless land plants of the Silurian and early Devonian Aglaophyton may have relied on arbuscular mycorrhizal fungi for acquisition of water and nutrients from the soil.

Evidence from DNA sequence analysis indicates that the arbuscular mycorrhizal mutualism arose in the common ancestor of these land plant groups during their transition to land [40] and it may even have been the critical step that enabled them to colonise the land.

To photosynthesise, plants must absorb CO 2 from the atmosphere. However, making the tissues available for CO 2 to enter allows water to evaporate, so this comes at a price. Early land plants transported water apoplastically , within the porous walls of their cells.

First, a waterproof outer covering or cuticle evolved that reduced water loss. This three-part system provided improved homoiohydry, the regulation of water content of the tissues, providing a particular advantage when water supply is not constant. As CO 2 was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant water acquisition and transport mechanisms evolved. By the end of the Carboniferous, when CO 2 concentrations had been reduced to something approaching that of today, around 17 times more water was lost per unit of CO 2 uptake.

Water can be wicked by capillary action along a fabric with small spaces. In narrow columns of water, such as those within the plant cell walls or in tracheids, when molecules evaporate from one end, they pull the molecules behind them along the channels.

Therefore, evaporation alone provides the driving force for water transport in plants. To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. As plants grew upwards, specialised water transport vascular tissues evolved, first in the form of simple hydroids of the type found in the setae of moss sporophytes. These simple elongated cells were dead and water-filled at maturity, providing a channel for water transport, but their thin, unreinforced walls would collapse under modest water tension, limiting the plant height.

This transition from poikilohydry to homoiohydry opened up new potential for colonisation. The early Devonian pretracheophytes Aglaophyton and Horneophyton have unreinforced water transport tubes with wall structures very similar to moss hydroids, but they grew alongside several species of tracheophytes , such as Rhynia gwynne-vaughanii that had xylem tracheids that were well reinforced by bands of lignin. The earliest macrofossils known to have xylem tracheids are small, mid-Silurian plants of the genus Cooksonia.

Tracheids have perforated end walls, which impose a great deal of resistance on water flow, [49] but may have had the advantage of isolating air embolisms caused by cavitation or freezing. Vessels first evolved during the dry, low CO 2 periods of the Late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in gnetophytes and angiosperms. An endodermis may have evolved in the earliest plant roots during the Devonian, but the first fossil evidence for such a structure is Carboniferous.

The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver. Leaves are the primary photosynthetic organs of a modern plant. The origin of leaves was almost certainly triggered by falling concentrations of atmospheric CO 2 during the Devonian period, increasing the efficiency with which carbon dioxide could be captured for photosynthesis.

Leaves certainly evolved more than once. Based on their structure, they are classified into two types: microphylls , which lack complex venation and may have originated as spiny outgrowths known as enations, and megaphylls , which are large and have complex venation that may have arisen from the modification of groups of branches. It has been proposed that these structures arose independently. All three steps happened multiple times in the evolution of today's leaves.

It is widely believed that the telome theory is well supported by fossil evidence. However, Wolfgang Hagemann questioned it for morphological and ecological reasons and proposed an alternative theory. Axes such as stems and roots evolved later as new organs.

Rolf Sattler proposed an overarching process-oriented view that leaves some limited room for both the telome theory and Hagemann's alternative and in addition takes into consideration the whole continuum between dorsiventral flat and radial cylindrical structures that can be found in fossil and living land plants. Thus, James [62] concluded that "it is now widely accepted that In fact, it is simply the timing of the KNOX gene expression! Before the evolution of leaves , plants had the photosynthetic apparatus on the stems.

Today's megaphyll leaves probably became commonplace some mya, about 40my after the simple leafless plants had colonized the land in the Early Devonian. This spread has been linked to the fall in the atmospheric carbon dioxide concentrations in the Late Paleozoic era associated with a rise in density of stomata on leaf surface.

Increasing the stomatal density allowed for a better-cooled leaf, thus making its spread feasible, but increased CO2 uptake at the expense of decreased water use efficiency. The rhyniophytes of the Rhynie chert consisted of nothing more than slender, unornamented axes. The early to middle Devonian trimerophytes may be considered leafy.

This group of vascular plants are recognisable by their masses of terminal sporangia, which adorn the ends of axes which may bifurcate or trifurcate. These are small, spiny outgrowths of the stem, lacking their own vascular supply.

The zosterophylls were already important in the late Silurian, much earlier than any rhyniophytes of comparable complexity. The first evidence of vascularised enations occurs in a fossil clubmoss known as Baragwanathia that had already appeared in the fossil record in the Late Silurian.

Lycopods bear distinctive microphylls , defined as leaves with a single vascular trace. Microphylls could grow to some size, those of Lepidodendrales reaching over a meter in length, but almost all just bear the one vascular bundle. An exception is the rare branching in some Selaginella species. The more familiar leaves, megaphylls , are thought to have originated four times independently, in the ferns, horsetails, progymnosperms and seed plants.

The cessation of further diversification can be attributed to developmental constraints, [68] but why did it take so long for leaves to evolve in the first place? When stomata open to allow water to evaporate from leaves it has a cooling effect, resulting from the loss of latent heat of evaporation.

It appears that the low stomatal density in the early Devonian meant that evaporation and evaporative cooling were limited, and that leaves would have overheated if they grew to any size.

The stomatal density could not increase, as the primitive steles and limited root systems would not be able to supply water quickly enough to match the rate of transpiration. Secondary evolution can also disguise the true evolutionary origin of some leaves.

Flowering Plants

The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats , through multicellular marine and freshwater green algae , terrestrial bryophytes , lycopods and ferns , to the complex gymnosperms and angiosperms of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones, e. Also by late Devonian, Elkinsia , an early seed fern , had evolved seeds. Most plant groups were relatively unscathed by the Permo-Triassic extinction event , although the structures of communities changed. Land plants evolved from a group of green algae , perhaps as early as mya, [8] but algae-like plants might have evolved as early as 1 billion years ago. It would only very briefly have had paired chromosomes the diploid condition when the egg and sperm first fused to form a zygote that would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes the haploid condition. Co-operative interactions with fungi may have helped early plants adapt to the stresses of the terrestrial realm.

One of the world's leading evolutionary biologists here reexamines the evolutionary history of flowering plants. This important book is the first to interpret the phylogeny of flowering plants in the light of modern knowledge about genetics, developmental biology, and ecology. Stebbins is concerned with the evolution of genera, families, and other higher taxa; his analysis is based upon a unified theory that identifies the same fundamental processes at work in the origin of both species and the broader taxonomic categories. He shows, however, that subspecific evolution depends primarily on the natural selection of vegetative characters, whereas the emphasis at the transspecific level is on reproductive characters. Major changes in evolutionary direction are interpreted as resulting from an interaction of environmental change and the inherent capacity of the organism to alter preferentially in some ways and not in others.

Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. The major diversification of flowering plants angiosperms in the Early Cretaceous, between about and 90 million years ago, initiated fundamental changes in terrestrial ecosystems and set in motion processes that generated most of the extant plant diversity.

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Angiosperm , any of about , species of flowering plants, the largest and most diverse group within the kingdom Plantae. Angiosperms represent approximately 80 percent of all the known green plants now living. The angiosperms are vascular seed plants in which the ovule egg is fertilized and develops into a seed in an enclosed hollow ovary.

Angiosperms, which evolved in the Cretaceous period, are a diverse group of plants which protect their seeds within an ovary called a fruit. Undisputed fossil records place the massive appearance and diversification of angiosperms in the middle to late Mesozoic era. Fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous, about million years ago, and were rapidly diversifying by the Middle Cretaceous, about million years ago. Earlier traces of angiosperms are scarce.

Evolutionary history of plants

Angiosperm , any of about , species of flowering plants, the largest and most diverse group within the kingdom Plantae. Angiosperms represent approximately 80 percent of all the known green plants now living. The angiosperms are vascular seed plants in which the ovule egg is fertilized and develops into a seed in an enclosed hollow ovary.

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Он знал, что задумал Чатрукьян. Отключение ТРАНСТЕКСТА было логичным шагом в случае возникновения чрезвычайной ситуации, а ведь тот был уверен, что в машину проник вирус. К несчастью, это был самый надежный способ собрать в шифровалке всех сотрудников Отдела обеспечения системной безопасности. После таких экстренных действий на главном коммутаторе раздавался сигнал общей тревоги. Проверку шифровалки службой безопасности Хейл допустить не. Он выбежал из помещения Третьего узла и направился к люку.

The origin and early diversification of angiosperms

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Не лезь не в свое. - Ну. Беккер кивнул. Уже в дверях он грустно улыбнулся: - Вы все же поосторожнее. ГЛАВА 67 - Сьюзан? - Тяжело дыша, Хейл приблизил к ней свое лицо. Он сидел у нее на животе, раскинув ноги в стороны.

Пальцы Соши стремительно забегали по клавишам. - Так посылал свои распоряжения Цезарь! - сказала Сьюзан.  - Количество букв всегда составляло совершенный квадрат. - Готово! - крикнула Соши. Все посмотрели на вновь организованный текст, выстроенный в горизонтальную линию. - По-прежнему чепуха, - с отвращением скривился Джабба.  - Смотрите.

Лейтенант глубоко затянулся. - Долгая история. Чутье подсказывало Беккеру, что это открытие не сулит ему ничего хорошего. - Все равно расскажите. ГЛАВА 15 Сьюзан Флетчер расположилась за компьютерным терминалом Третьего узла. Этот узел представлял собой звуконепроницаемую уединенную камеру, расположенную неподалеку от главного зала. Двухдюймовое искривленное стекло односторонней видимости открывало перед криптографами панораму зала, не позволяя увидеть камеру снаружи.

Angiosperm

Она это заслужила, подумал он и принял решение: Сьюзан придется его выслушать. Он надеялся, что не совершает ошибку. - Сьюзан, - начал он, - этого не должно было случиться.

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