98-735: The Caytoniales (Figs. 1-2) are an extinct order of seed plants known from fossils collected throughout the Mesozoic Era , around 252 to 66 million years ago . They are regarded as seed ferns because they are seed -bearing plants with fern -like leaves. Although at one time considered angiosperms because of their berry-like cupules, that hypothesis was later disproven. Nevertheless, some authorities consider them likely ancestors or close relatives of angiosperms. The origin of angiosperms remains unclear, and they cannot be linked with any known seed plants groups with certainty. The first fossils identified in this order were discovered in
196-406: A clade of gymnosperms , with the gnetophytes in or near the conifers. For example, one common proposed set of relationships is known as the gne-pine hypothesis and looks like: (flowering plants) [REDACTED] Cycads [REDACTED] Ginkgo [REDACTED] Pinaceae (the pine family) [REDACTED] Gnetophytes [REDACTED] other conifers [REDACTED] However,
294-495: A phanerogam (taxon Phanerogamae ) or a phaenogam (taxon Phaenogamae ), is any plant that produces seeds . It is a category of embryophyte (i.e. land plant) that includes most of the familiar land plants, including the flowering plants and the gymnosperms , but not ferns , mosses , or algae . The term phanerogam or phanerogamae is derived from the Greek φανερός ( phanerós ), meaning "visible", in contrast to
392-591: A better-cooled leaf, thus making its spread feasible, but increased CO 2 uptake at the expense of decreased water use efficiency. The rhyniophytes of the Rhynie chert consisted only of 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. Some organisms, such as Psilophyton , bore enations . These are small, spiny outgrowths of
490-411: A channel for water transport, but their thin, unreinforced walls would collapse under modest water tension, limiting the plant height. Xylem tracheids , wider cells with lignin -reinforced cell walls that were more resistant to collapse under the tension caused by water stress, occur in more than one plant group by mid-Silurian, and may have a single evolutionary origin, possibly within
588-507: A condition known as heteromorphy . 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.e. embryophytes ) are diplobiontic – that is, both the haploid and diploid stages are multicellular. Two trends are apparent: bryophytes ( liverworts , mosses and hornworts ) have developed
686-543: A continuous spectrum. 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, which they retain albeit leaves have largely assumed that job. Today's megaphyll leaves probably became commonplace some 360mya, about 40my after the simple leafless plants had colonized the land in the Early Devonian . This spread has been linked to
784-543: A controversial gap in the Late Devonian, charcoal has been present ever since. 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 it is prone to preservation. It is also robust and can withstand pressure, displaying exquisite, sometimes sub-cellular, detail in remains. In addition to
882-458: A dedicated root system; however, the flat-lying axes can be clearly seen to have growths similar to the rhizoids of bryophytes today. By the Middle to Late Devonian, most groups of plants had independently developed a rooting system of some nature. As roots became larger, they could support larger trees, and the soil was weathered to a greater depth. This deeper weathering had effects not only on
980-411: A film of surface moisture, enabling them to grow to much greater size but as a result of their increased independence from their surroundings, most vascular plants lost their ability to survive desiccation - a costly trait to lose. In early land plants, support was mainly provided by turgor pressure, particularly of the outer layer of cells known as the sterome tracheids, and not by the xylem, which
1078-474: 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). This theory may be supported by observations that smaller Cooksonia individuals must have been supported by a gametophyte generation. The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides
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#17327760360911176-564: A great deal of resistance on water flow, 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. Vessel members are open tubes with no end walls, and are arranged end to end to operate as if they were one continuous vessel. Vessels allowed
1274-437: A group of freshwater green algae , perhaps as early as 850 mya, but algae-like plants might have evolved as early as 1 billion years ago. The closest living relatives of land plants are the charophytes , specifically Charales ; if modern Charales are similar to the distant ancestors they share with land plants, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at
1372-509: A group still extant today, represented by the quillworts , the spikemosses and the club mosses . 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
1470-431: A life cycle comprising two generations or phases. The gametophyte phase has a single set of chromosomes (denoted 1n ) and produces gametes (sperm and eggs). The sporophyte phase has paired chromosomes (denoted 2n ) and produces spores. The gametophyte and sporophyte phases may be homomorphic, appearing identical in some algae, such as Ulva lactuca , but are very different in all modern land plants,
1568-564: A mass extinction . While there are traces of root-like impressions in fossil soils in the Late Silurian, body fossils show the earliest plants to be devoid of roots. Many had prostrate branches that sprawled along the ground, with upright axes or thalli dotted here and there, and some even had non-photosynthetic subterranean branches which lacked stomata. Roots have a root cap , unlike specialised branches. So while Siluro-Devonian plants such as Rhynia and Horneophyton possessed
1666-524: A micropylar canal, that allowed pollen to pass into the pollen chamber. The outer layers of the cupules were fleshy and fruit-like; it is possible this was to aid in animal dispersal. The cupules are 4-5mm in diameter and about 3 mm long (Fig 1-2), and resemble a blueberry. The extra protection of the reproductive organs gave rise to the idea that Caytoniales were predecessors to angiosperms , which have completely enclosed seeds. The pollen grains were small, between 25 and 30 μm in diameter. The size of
1764-585: 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 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
1862-404: A palmate manner. The individual leaflets are up to 6 cm in length. The leaflets have anastomosing veins, like those of some ferns, but lacking orders of venation found in angiosperm leaves. Caytonia was first described by Hamshaw Thomas in 1925. His close examination of the cupules led him to believe this was one of the earliest examples of angiosperms. He mistakenly thought the entire ovule
1960-426: 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
2058-470: A rootless vascular plant known from Devonian fossils in the Rhynie chert was the first land plant discovered to have had a symbiotic relationship with fungi 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
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#17327760360912156-423: A structure is Carboniferous. The endodermis in the roots surrounds the water transport tissue and regulates ion exchange between the groundwater and the tissues and prevents unwanted pathogens etc. from entering the water transport system. 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
2254-502: A system to guide the pollen to the seed. Runcaria was followed shortly after by plants with a more condensed cupule, such as Spermasporites and Moresnetia . Seed-bearing plants had diversified substantially by the Famennian , the last stage of the Devonian. Examples include Elkinsia , Xenotheca , Archaeosperma , " Hydrasperma ", Aglosperma , and Warsteinia . Some of these Devonian seeds are now classified within
2352-451: A three-dimensional branching system of radially symmetrical axes (telomes), according to Hagemann's alternative the opposite is proposed: the most primitive land plants that gave rise to vascular plants were flat, thalloid, leaf-like, without axes, somewhat like a liverwort or fern prothallus. 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
2450-577: A wide range of complexity, from the earliest algal mats of unicellular archaeplastids evolved through endosymbiosis , through multicellular marine and freshwater green algae , to spore -bearing terrestrial bryophytes , lycopods and ferns , and eventually to the complex seed -bearing gymnosperms and angiosperms ( flowering plants) 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; for example,
2548-422: Is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the mutlilobed integument . It is suspected that the extension was involved in anemophilous (wind) pollination . Runcaria sheds new light on the sequence of character acquisition leading to the seed. Runcaria has all of the qualities of seed plants except for a solid seed coat and
2646-481: Is the flowering plants , also known as angiosperms or magnoliophytes, the largest and most diverse group of spermatophytes: In addition to the five living taxa listed above, the fossil record contains evidence of many extinct taxa of seed plants, among those: By the Triassic period, seed ferns had declined in ecological importance, and representatives of modern gymnosperm groups were abundant and dominant through
2744-513: The Characeae , an algal sister group to land plants. That said, rhizoids probably evolved more than once; the rhizines of lichens , for example, perform a similar role. Even some animals ( Lamellibrachia ) have root-like structures. Rhizoids are clearly visible in the Rhynie chert fossils, and were present in most of the earliest vascular plants, and on this basis seem to have presaged true plant roots. More advanced structures are common in
2842-641: The Devonian , with lycopod trees forming roots around 20 cm long during the Eifelian and Givetian. These were joined by progymnosperms, which rooted up to about a metre deep, during the ensuing Frasnian stage. True gymnosperms and zygopterid ferns also formed shallow rooting systems during the Famennian. The rhizophores of the lycopods provide a slightly different approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing
2940-1114: The Middle Jurassic Gristhorpe bed of the Cloughton Formation in Cayton Bay, Yorkshire , with the name of the bay giving the name to the group. They have since been found in Mesozoic rocks all over world. It is likely that Caytoniales flourished in wetland areas, because they are often found with other moisture-loving plants such as horsetails in waterlogged paleosols. The first fossil Caytoniales were preserved as compressions in shale with excellent preservation of cuticles allowing study of cellular histology. The woody nature of associated stalks and preserved short shoots are evidence that Caytoniales were seasonally deciduous , shrubs and trees. Caytoniales had fertile branches with seed-bearing cupules . The ovules were located inside fleshy cupules with tough outer cuticle . Individual ovules had an apical tube called
3038-625: The Rhynie chert is of similar complexity, which is taken to support this hypothesis. By contrast, modern vascular plants, with the exception of Psilotum , have heteromorphic sporophytes and gametophytes in which the gametophytes rarely have any vascular tissue. 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 ,
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3136-466: The Zygnematophyceae may reflect further adaptations to a cold loving life style. The establishment of a land-based flora increased the rate of accumulation of oxygen in the atmosphere, as the land plants produced oxygen as a waste product. When this concentration rose above 13%, around 0.45 billion years ago, wildfires became possible, evident from charcoal in the fossil record. Apart from
3234-511: The carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around 850 million years ago . These organisms, although phylogenetically diverse, were probably small and simple, forming little more than an algal scum. Since lichens initiate the first step in primary ecological succession in contemporary contexts, one hypothesis has been that lichens came on land first and facilitated colonization by plants; however, both molecular phylogenies and
3332-462: The zosterophylls by mid-Devonian. Overall transport rate also depends on the overall cross-sectional area of the xylem bundle itself, and some mid-Devonian plants, such as the Trimerophytes, had much larger steles than their early ancestors. While wider tracheids provided higher rates of water transport, they increased the risk of cavitation, the formation of air bubbles resulting from
3430-477: The CO 2 to the chloroplasts. 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. The high CO 2 concentrations of the Silurian and early Devonian, when plants were first colonising land, meant that they used water relatively efficiently. As CO 2 was withdrawn from the atmosphere by plants, more water
3528-463: The Cretaceous and Paleogene . The latest major group of plants to evolve were the grasses , which became important in the mid-Paleogene, from around 40 million years ago . The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO 2 and warm, dry conditions of the tropics over the last 10 million years . Land plants evolved from
3626-485: The Devonian. This required an increase in stomatal density by 100 times to maintain the rate of photosynthesis. 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
3724-619: The Rhynie chert, and many other fossils of comparable early Devonian age bear structures that look like, and acted like, roots. The rhyniophytes bore fine rhizoids, and the trimerophytes and herbaceous lycopods of the chert bore root-like structure penetrating a few centimetres into the soil. However, none of these fossils display all the features borne by modern roots, with the exception of Asteroxylon , which has recently been recognized as bearing roots that evolved independently from those of extant vascular plants. Roots and root-like structures became increasingly common and deeper penetrating during
3822-442: The ability to control the inevitable water loss that accompanied CO 2 acquisition. First, a waterproof outer covering or cuticle evolved that reduced water loss. Secondly, variable apertures, the stomata that could open and close to regulate the amount of water lost by evaporation during CO 2 uptake and thirdly intercellular space between photosynthetic parenchyma cells that allowed improved internal distribution of
3920-439: The advent of charcoal in the rock record, the terrestrialization of plants has made significant contributions to changes in geology and landscapes. The Ordovician and Silurian show a 1.4 times greater proportion of mudrock in the geologic record than the previous 90% of earth's history and this increase in mudrock is considered to be a result of land plants retaining muds in a terrestrial setting. All multicellular plants have
4018-487: The angiosperm double integument, and the carpels formed from an elaboration of their stalk (Fig. 5). Other theories for the origin of angiosperms derive them from Glossopteridales (Fig.5), among other groups (see Evolutionary history of plants ). Spermatophyte A seed plant or spermatophyte ( lit. ' seed plant ' ; from Ancient Greek σπέρματος ( spérmatos ) 'seed' and φυτόν (phytón) 'plant'), also known as
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4116-480: The appearance of a diplobiontic lifecycle. The interpolation theory (also known as the antithetic or intercalary theory) 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
4214-487: The arbuscular mycorrhizal mutualism arose in the common ancestor of these land plant groups during their transition to land and it may even have been the critical step that enabled them to colonise the land. Appearing as they did before these plants had evolved roots, mycorrhizal fungi would have assisted plants in the acquisition of water and mineral nutrients such as phosphorus , in exchange for organic compounds which they could not synthesize themselves. Such fungi increase
4312-409: The ascendance of flowering plants over gymnosperms in terrestrial environments. There is evidence that cyanobacteria and multicellular thalloid eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian , around 850 million years ago . Evidence of
4410-526: The breakage of the water column under tension. Small pits in tracheid walls allow water to by-pass a defective tracheid while preventing air bubbles from passing through but at the cost of restricted flow rates. By the Carboniferous, Gymnosperms had developed bordered pits , valve-like structures that allow high-conductivity pits to seal when one side of a tracheid is depressurized. Tracheids have non-perforated end walls with pits, which impose
4508-448: The concentration of carbon dioxide and decreased the greenhouse effect in the atmosphere, leading to an icehouse climate . Based on molecular clock studies of the previous decade or so, a 2022 study observed that the estimated time for the origin of the multicellular streptophytes (all except the unicellular basal clade Mesostigmatophyceae ) fell in the cool Cryogenian while that of the subsequent separation of streptophytes fell in
4606-408: 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. There are two competing theories to explain
4704-415: 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
4802-698: The cuticle and interior cell organs. This allowed Harris to look closely at the ovules located inside. Upon close inspection of the ovule, whole pollen grains were found inside the micropylar canal. This is typical of a gymnosperm reproduction, not an angiosperm. Presumably pollination was at an early stage of cupule and ovule development, before full inflation of the cupules. While Thomas's original idea led many scientists to believe that Caytoniales may have been angiosperms, Harris's further research disproved this theory. The enclosure of ovules in Caytoniales has nevertheless been considered an early stage in evolution of
4900-472: The edge of seasonally desiccating pools. However, some recent evidence suggests that land plants might have originated from unicellular terrestrial charophytes similar to extant Klebsormidiophyceae . The alga would have had a haplontic life cycle . 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
4998-488: The emergence of embryophyte land plants first occurs in the middle Ordovician (~ 470 million years ago ), and by the middle of the Devonian (~ 390 million years ago ), many of the features recognised in land plants today were present, including roots and leaves. By the late Devonian (~ 370 million years ago ) some free-sporing plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees. Also by
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#17327760360915096-552: The end of the Cretaceous , when the angiosperms radiated. A whole genome duplication event in the ancestor of seed plants occurred about 319 million years ago . This gave rise to a series of evolutionary changes that resulted in the origin of modern seed plants. A middle Devonian (385-million-year-old) precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years. Runcaria , small and radially symmetrical,
5194-487: The expression of deleterious mutations through genetic complementation . Thus if one of the parental genomes in the diploid cells contains mutations leading to defects in one or more gene products , these deficiencies could be compensated for by the other parental genome (which nevertheless may have its own defects in other genes). As the diploid phase was becoming predominant, the masking effect likely allowed genome size , and hence information content, to increase without
5292-474: The fall in the atmospheric carbon dioxide concentrations in the Late Paleozoic era associated with a rise in density of stomata on leaf surface. This would have resulted in greater transpiration rates and gas exchange, but especially at high CO 2 concentrations, large leaves with fewer stomata would have heated to lethal temperatures in full sunlight. Increasing the stomatal density allowed for
5390-424: The ferns, horsetails, progymnosperms and seed plants. They appear to have originated by modifying dichotomising branches, which first overlapped (or "overtopped") one another, became flattened or planated and eventually developed "webbing" and evolved gradually into more leaf-like structures. Megaphylls, by Zimmerman's telome theory , are composed of a group of webbed branches and hence the "leaf gap" left where
5488-409: The first place. Plants had been on land for at least 50 million years before megaphylls became significant. However, small, rare mesophylls are known from the early Devonian genus Eophyllophyton – so development could not have been a barrier to their appearance. The best explanation so far is that atmospheric CO 2 was declining rapidly during this time – falling by around 90% during
5586-467: The first sporophytes bore a very different and simpler morphology to the gametophyte they depended on. This seems to fit well with what is known of the bryophytes, in which a vegetative thalloid gametophyte nurtures a simple sporophyte, which consists of little more than an unbranched sporangium on a stalk. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on
5684-731: The form of spores known as cryptospores . These spores have walls made of sporopollenin , an extremely decay-resistant material that means they are well-preserved by the fossil record. These spores 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. Their walls contain sporopollenin – further evidence of an embryophytic affinity. Trilete spores similar to those of vascular plants appear soon afterwards, in Upper Ordovician rocks about 455 million years ago. Depending exactly when
5782-445: The fossil record seem to contradict this. There are multiple potential reasons for why it took so long for land plants to emerge. It could be that atmospheric 'poisoning' prevented eukaryotes from colonising the land prior to the emergence of land plants, or it could simply have taken a great time for the necessary complexity to evolve. A major challenge to land adaptation would have been the absence of appropriate soil . Throughout
5880-401: The fossil record, soil is preserved, giving information on what early soils were like. Before land plants, the soil on land was poor in resources essential for life like nitrogen and phosphorus and had little capacity for holding water. Evidence of the earliest land plants occurs at about 470 million years ago , in lower middle Ordovician rocks from Saudi Arabia and Gondwana in
5978-408: The gametophyte as the dominant phase of the life cycle, with the sporophyte becoming almost entirely dependent on it; vascular plants have developed the sporophyte as the dominant phase, with the gametophytes being particularly reduced in the seed plants . It has been proposed as the basis for the emergence of the diploid phase of the life cycle as the dominant phase that diploidy allows masking of
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#17327760360916076-470: The genus Cooksonia . However, thickened bands on the walls of isolated tube fragments are apparent from the early Silurian onwards. Plants continued to innovate ways of reducing the resistance to flow within their cells, progressively increasing the efficiency of their water transport and to increase the resistance of the tracheids to collapse under tension. During the early Devonian, maximum tracheid diameter increased with time, but may have plateaued in
6174-865: The hornworts, uniting all tracheophytes. Alternatively, they may have evolved more than once. Much later, in the Cretaceous, tracheids were followed by vessels in flowering plants . As water transport mechanisms and waterproof cuticles evolved, plants could survive without being continually covered by a film of water. 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
6272-415: The land was waterlogged. There were also microbial mats. 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
6370-593: The late Devonian, Elkinsia , an early seed fern , had evolved seeds. Evolutionary innovation continued throughout the rest of the Phanerozoic eon and still continues today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event , although the structures of communities changed. This may have set the scene for the appearance of the flowering plants in the Triassic (~ 200 million years ago ), and their later diversification in
6468-452: The leaf's vascular bundle leaves that of the main branch resembles two axes splitting. In each of the four groups to evolve megaphylls, their leaves first evolved during the Late Devonian to Early Carboniferous, diversifying rapidly until the designs settled down in the mid Carboniferous. The cessation of further diversification can be attributed to developmental constraints, raising the question of why it took so long for leaves to evolve in
6566-443: The modification of groups of branches. It has been proposed that these structures arose independently. Megaphylls, according to Walter Zimmerman's telome theory, have evolved from plants that showed a three-dimensional branching architecture, through three transformations— overtopping , which led to the lateral position typical of leaves, planation , which involved formation of a planar architecture, webbing or fusion , which united
6664-427: The molecules behind them along the channels. Therefore, evaporation alone provides the driving force for water transport in plants. However, without specialized transport vessels, this cohesion-tension mechanism can cause negative pressures sufficient to collapse water conducting cells, limiting the transport water to no more than a few cm, and therefore limiting the size of the earliest plants. To be free from
6762-478: 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. Weathering rates suggest that organisms capable of photosynthesis were already living on the land 1,200 million years ago , and microbial fossils have been found in freshwater lake deposits from 1,000 million years ago , but
6860-540: The order Lyginopteridales . Seed-bearing plants are a clade within the vascular plants (tracheophytes). The spermatophytes were traditionally divided into angiosperms , or flowering plants, and gymnosperms , which includes the gnetophytes, cycads, ginkgo, and conifers. Older morphological studies believed in a close relationship between the gnetophytes and the angiosperms, in particular based on vessel elements . However, molecular studies (and some more recent morphological and fossil papers) have generally shown
6958-514: 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 global glaciation event called Snowball Earth , from around 720-635 mya in the Cryogenian period, is believed to have been at least partially caused by early photosynthetic organisms, which reduced
7056-399: The ovule, a defining trait in angiosperms. This theory was disproved 1933 by Thomas's student Tom Harris, who studied the same reproductive organs and found different results. "Most of the fruits were obtained by dissolving in hydrofluoric acid a single very small fragment of shale collected from Cape Stewart ," he wrote. The maceration of the fruits dissolves the main cell's wall and leaves
7154-401: The physiological equivalent of roots, roots – defined as organs differentiated from stems – did not arrive until later. Unfortunately, roots are rarely preserved in the fossil record. Rhizoids – small structures performing the same role as roots, usually a cell in diameter – probably evolved very early, perhaps even before plants colonised the land; they are recognised in
7252-434: The planar branches, thus leading to the formation of a proper leaf lamina. 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. Whereas according to the telome theory the most primitive land plants have
7350-575: The pollen grains supports the idea that they were wind-pollinated, and their bisaccate wings may have enabled entry into the seed by a pollination drop mechanism. In both respects they were like pollen of pine trees . They were produced in pollen sacs in coalesced groups of four, attached to branching structures. The pollen sacs hang off the structure in clusters, and are typically 2 cm in length. The most common and widespread part found fossilized are leaves of Sagenopteris (Fig. 3). These are compound leaves consisting of, usually, 4 leaflets arrayed in
7448-449: The primitive steles and limited root systems would not be able to supply water quickly enough to match the rate of transpiration. Clearly, leaves are not always beneficial, as illustrated by the frequent occurrence of secondary loss of leaves, exemplified by cacti and the "whisk fern" Psilotum . Secondary evolution can disguise the true evolutionary origin of some leaves. Some genera of ferns display complex leaves which are attached to
7546-479: The productivity even of simple plants such as liverworts. 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. Water is lost much faster than CO 2 is absorbed, so plants need to replace it. Early land plants transported water apoplastically , within the porous walls of their cells. Later, they evolved three anatomical features that provided
7644-405: The protostele connecting with existing enations The leaves of the Rhynie genus Asteroxylon , which was preserved in the Rhynie chert almost 20 million years later than Baragwanathia , had a primitive vascular supply – in the form of leaf traces departing from the central protostele towards each individual "leaf". Asteroxylon and Baragwanathia are widely regarded as primitive lycopods,
7742-469: The pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap. Deciduous trees deal with another disadvantage to having leaves. The popular belief that plants shed their leaves when the days get too short is misguided; evergreens prospered in the Arctic Circle during the most recent greenhouse earth . The generally accepted reason for shedding leaves during winter is to cope with
7840-403: 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
7938-560: The relationships between these groups should not be considered settled. Other classifications group all the seed plants in a single division , with classes for the five groups: A more modern classification ranks these groups as separate divisions (sometimes under the Superdivision Spermatophyta ): Unassigned extinct spermatophyte orders, some of which qualify as "seed ferns": Evolutionary history of plants The evolution of plants has resulted in
8036-447: The role of rootlets. A similar construction is observed in the extant lycopod Isoetes , and this appears to be evidence that roots evolved independently at least twice, in the lycophytes and other plants, a proposition supported by studies showing that roots are initiated and their growth promoted by different mechanisms in lycophytes and euphyllophytes. Early rooted plants are little more advanced than their Silurian forebears, without
8134-638: The same cross-sectional area of wood to transport much more water than tracheids. This allowed plants to fill more of their stems with structural fibres and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on. Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation. Once plants had evolved this level of control over water evaporation and water transport, they were truly homoiohydric , able to extract water from their environment through root-like organs rather than relying on
8232-507: 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. The tissue of sporophytes and gametophytes of vascular plants such as Rhynia preserved in
8330-629: 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. The fungi were of the phylum Glomeromycota , a group that probably first appeared 1 billion years ago and still forms arbuscular mycorrhizal associations today with all major land plant groups from bryophytes to pteridophytes, gymnosperms and angiosperms and with more than 80% of vascular plants. Evidence from DNA sequence analysis indicates that
8428-400: The stem, lacking their own vascular supply. The zosterophylls were already important in the late Silurian, much earlier than any rhyniophytes of comparable complexity. This group, recognisable by their kidney-shaped sporangia which grew on short lateral branches close to the main axes, sometimes branched in a distinctive H-shape. Many zosterophylls bore enations (small tissue outgrowths on
8526-457: The surface with variable morphologies) on their axes but none of these had a vascular trace. 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. In this organism, these leaf traces continue into the leaf to form their mid-vein. One theory, the "enation theory", holds that the microphyllous leaves of clubmosses developed by outgrowths of
8624-467: 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. This view is supported by research in molecular genetics. Thus, James (2009) concluded that "it is now widely accepted that... radiality [characteristic of axes such as stems] and dorsiventrality [characteristic of leaves] are but extremes of
8722-484: The term "cryptogam" or " cryptogamae " (from Ancient Greek κρυπτός (kruptós) 'hidden'), together with the suffix γαμέω ( gaméō ), meaning "to marry". These terms distinguish those plants with hidden sexual organs (cryptogamae) from those with visible ones (phanerogamae). The extant spermatophytes form five divisions, the first four of which are classified as gymnosperms , plants that have unenclosed, "naked seeds": The fifth extant division
8820-445: The tetrad splits, each of the four spores may bear a "trilete mark", a Y-shape, reflecting the points at which each cell squashed up against its neighbours. However, this requires that the spore walls be sturdy and resistant at an early stage. 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
8918-419: The warm Ediacaran , which they interpreted as an indication of selective pressure by the glacial period to the photosynthesizing organisms, a group of which succeeded in surviving in relatively warmer environments that remained habitable, subsequently flourishing in the later Ediacaran and Phanerozoic on land as embryophytes. The study also theorized that the unicellular morphology and other unique features of
9016-555: The water lack a resistant wall, thus don't bear trilete marks. A close examination of algal spores shows that none have trilete spores, either because their walls are not resistant enough, or, in those rare cases where they are, because the spores disperse before they are compressed enough to develop the mark or do not fit into a tetrahedral tetrad. 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
9114-403: The weather – the force of wind and weight of snow are much more comfortably weathered without leaves to increase surface area. Seasonal leaf loss has evolved independently several times and is exhibited in the ginkgoales , some pinophyta and certain angiosperms. Leaf loss may also have arisen as a response to pressure from insects; it may have been less costly to lose leaves entirely during
9212-427: The winter or dry season than to continue investing resources in their repair. The evolution of roots had consequences on a global scale. By disturbing the soil and promoting its acidification (by taking up nutrients such as nitrate and phosphate ), they enabled it to weather more deeply, injecting carbon compounds deeper into soils with huge implications for climate. These effects may have been so profound they led to
9310-440: Was enclosed in the cupule, unlike typical gymnosperms . He worked meticulously, collecting and cleaning specimens to get the best understanding. He spent weeks boiling fruits in different solutions to try to make them resemble their living states. He proposed that the fruits contained a stigma with a funnel-shaped opening in the center in which the pollen grains would get lodged. The entire pollen grain would not be able to enter into
9408-483: Was lost in its capture, and more elegant water acquisition and transport mechanisms evolved. Plants growing upwards into the air needed a system for transporting water from the soil to all the different parts of the above-soil plant, especially to photosynthesising parts. By the end of the Carboniferous , when CO 2 concentrations had been reduced to something approaching that of today, around 17 times more water
9506-411: Was lost per unit of CO 2 uptake. However, even in the "easy" early days, water was always at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation. 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
9604-473: Was too small, too weak and in too central a position to provide much structural support. Plants with secondary xylem that had appeared by mid-Devonian, such as the Trimerophytes and Progymnosperms had much larger vascular cross sections producing strong woody tissue. An endodermis may have evolved in the earliest plant roots during the Devonian, but the first fossil evidence for such
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