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Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
Unit 1: Earliest forms of plant life The kingdom Plantae constitutes large and varied groups of organisms. There are more than 300,000 species of catalogued plants. Of these, more than 260,000 are seed plants. Mosses, ferns, conifers, and flowering plants are all members of the plant kingdom. Most biologists also consider green algae to be plants, although others exclude all algae from the plant kingdom. The reason for this disagreement stems from the fact that only green algae, the Charophytes , share common characteristics with land plants (such as using chlorophyll a and b plus carotene in the same proportion as plants). These characteristics are absent in other types of algae.
The earliest environment formation of first cell It appears that life first emerged at least 3.8 billion years ago, approximately 750 million years after Earth was formed. At the time life arose, the atmosphere of Earth is thought to have contained little or no free oxygen, instead consisting principally of C02 and N2 in addition to smaller amounts of gases such as H2, H2S, and CO. Such an atmosphere provides reducing conditions in which organic molecules, given a source of energy such as sunlight or electrical discharge, can form spontaneously. The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s , when Stanley Miller (then a graduate student) showed that the discharge of electric sparks into a mixture of H2, CH4, and NH3, in the presence of water, leads to the formation of a variety of organic molecules, including several amino acids. Although Miller's experiments did not precisely reproduce the conditions of primitive Earth, they clearly demonstrated the plausibility of the spontaneous synthesis of organic molecules, providing the basic materials from which the first living organisms arose. The next step in evolution was the formation of macromolecules. The monomeric building blocks of macromolecules have been demonstrated to polymerize spontaneously under plausible prebiotic conditions. Heating dry mixtures of amino acids, for example, results in their polymerization to form polypeptides. But the critical characteristic of the macromolecule from which life evolved must have been the ability to replicate itself. Only a macromolecule capable of directing the synthesis of new copies of itself would have been capable of reproduction and further evolution. Of the two major classes of informational macromolecules in present-day cells (nucleic acids and proteins), only the nucleic acids are capable of directing their own self-replication. Nucleic acids can serve as templates for their own synthesis as a result of specific base pairing between complementary nucleotides. A critical step in understanding molecular evolution was thus reached in the early 1980s , when it was discovered in the laboratories of Sid Altman and Tom Cech that RNA is capable of catalyzing a number of
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
chemical reactions, including the polymerization of nucleotides. Further studies have extended the known catalytic activities of RNA, including the description of RNA molecules that direct the synthesis of a new RNA strand from an RNA template. RNA is thus uniquely able to both serve as a template for and to catalyze its own replication. Consequently, RNA is generally believed to have been the initial genetic system, and an early stage of chemical evolution is thought to have been based on self-replicating RNA molecules—a period of evolution known as the RNA world. Ordered interactions between RNA and amino acids then evolved into the present- day genetic code, and DNA eventually replaced RNA as the genetic material. The first cell is presumed to have arisen by the enclosure of self- replicating RNA in a membrane composed of phospholipids. Phospholipids are the basic components of all present-day biological membranes, including the plasma membranes of both prokaryotic and eukaryotic cells. The key characteristic of the phospholipids that form membranes is that they are amphipathic molecules, meaning that one portion of the molecule is soluble in water and another portion is not. Phospholipids have long, water-insoluble (hydrophobic) hydrocarbon chains joined to water-soluble (hydrophilic) head groups that contain phosphate. When placed in water, phospholipids spontaneously aggregate into a bilayer with their phosphate-containing head groups on the outside in contact with water and their hydrocarbon tails in the interior in contact with each other. Such a phospholipid bilayer forms a stable barrier between two aqueous compartments—for example, separating the interior of the cell from its external environment. The enclosure of self-replicating RNA and associated molecules in a phospholipid membrane would thus have maintained them as a unit, capable of self-reproduction and further evolution. RNA-directed protein synthesis may already have evolved by this time, in which case the first cell would have consisted of self-replicating RNA and its encoded proteins.
First prokaryotes; evolution of eukaryotes The Origin of Eukaryotes is a critical step in the evolution of eukaryotic cells was the acquisition of membrane-enclosed subcellular organelles, allowing the development of the complexity characteristic of these cells. The organelles of eukaryotes are thought to have arisen by endosymbiosis —one cell living inside another. In particular, eukaryotic organelles are thought to have evolved from prokaryotic cells living inside the ancestors of eukaryotes. The hypothesis that eukaryotic cells evolved by endosymbiosis is particularly well supported by studies of mitochondria and chloroplasts, which are thought to have evolved from eubacteria living in larger cells. Both mitochondria and chloroplasts are similar to bacteria in size, and like bacteria, they reproduce by dividing in two. Most important, both
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
Unit 2: Evolutionary trends: green algae to land plants; Land plants evolved from a group of green algae, perhaps as early as 850 mya, but algae-like plants might have evolved as early as 1 billion years ago.
1. The closest living relatives of land plants are the charophytes,
specifically Charales; assuming that the habit of the Charales has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools.
2. 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.
3. 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. 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,[15]^ but 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.
4. Evidence of the earliest land plants occurs much later at about 470Ma, in
lower middle Ordovician rocks from Saudi Arabia and Gondwana 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.
5. Their walls contain sporopollenin – further evidence of an embryophytic
affinity. It could be that atmospheric 'poisoning' prevented eukaryotes from colonising the land prior to this, or it could simply have taken a great time for the necessary complexity to evolve.
6. 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. There were also microbial mats.
7. 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
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
putting their metabolism "on hold" until more wat er arrives, as in the liverwort genus Targionia. Tracheophyte s resist desiccation, by controlling the rate of water loss.
8. 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 CO2 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, and moved away from a gametophyte dominated life cycle. 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 850-630 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. 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% wildfires became possible, evident from charcoal in the fossil record. Apart from a controversial gap in the Late Devonian, charcoal is 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 is prone to preservation. It is also robust, so can withstand pressure and display exquisite, sometimes sub-cellular, detail.
Non-vascular to vascular plants All multicellular plants have a life cycle comprising two generations or phases. The gametophyte phase has a single set of chromosomes (denoted 1 n ), and produces gametes (sperm and eggs). The sporophyte phase has paired
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
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. The tissue of sporophytes and gametophytes of vascular plants such as Rhynia preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis. By contrast, with the exception of Psilotum modern vascular plants 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 , a rootless vascular plant known from Devonian fossils in the Rhynie chert[37]^ 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 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.
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
Gymnosperm to angiosperms; Evolution of Gymnosperms Seed ferns gave rise to the gymnosperms during the Devonian Period, allowing them to adapt to dry conditions. LEARNING OBJECTIVES Explain how and why gymnosperms became the dominant plant group during the Permian period KEY TAKEAWAYS Key Points Seed ferns were the first seed plants, protecting their reproductive parts in structures called cupules. Seed ferns gave rise to the gymnosperms during the Paleozoic Era, about 390 million years ago. Gymnosperms include the gingkoes and conifers and inhabit many ecosystems, such as the taiga and the alpine forests, because they are well adapted for cold weather. True seed plants became more numerous and diverse during the Carboniferous period around 319 million years ago; an explosion that appears to be due to a whole genome duplication event. Key Terms cupule : any small structure shaped like a cup gymnosperm : any plant, such as a conifer, whose seeds are not enclosed in an ovary mutualism : any interaction between two species that benefits both Evolution of Gymnosperms The fossil plant Elkinsia polymorpha , a “seed fern” from the Devonian period (about 400 million years ago) is considered the earliest seed plant known to date. Seed ferns produced their seeds along their branches without specialized structures. What makes them the first true seed plants is that they developed structures called cupules to enclose and protect the ovule (the female gametophyte and associated tissues) which develops into a seed upon fertilization. Seed plants resembling modern tree ferns became more numerous and diverse in the coal swamps of the Carboniferous period. This appears to have been the result of a whole genome duplication event around 319 million years ago. Fossil records indicate the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era, during the middle Devonian period about 390 million years ago. Following the wet Mississippian and Pennsylvanian periods, which were dominated by giant fern trees, the Permian period was dry. This gave a reproductive edge to seed plants, which are better adapted to survive dry spells. The Ginkgoales, a group of gymnosperms with only one surviving species, the Gingko biloba, were the first gymnosperms to appear during the lower Jurassic. Gymnosperms expanded in the Mesozoic era (about 240 million years ago), supplanting ferns in the landscape, and reaching their greatest diversity during this time. It has been suggested that during the mid-Mesozoic era, pollination of
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
evidence indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse, flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants. Fossil evidence of angiosperms : This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. A large number of pollinating insects also appeared during this same time. Although several hypotheses have been offered to explain this sudden profusion and variety of flowering plants, none have garnered the consensus of paleobotanists (scientists who study ancient plants). New data in comparative genomics and paleobotany have, however, shed some light on the evolution of angiosperms. Rather than being derived from gymnosperms, angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were basal angiosperms related to tropical grasses. Both views draw support from cladistic studies. The so-called woody magnoliid hypothesis (which proposes that the early ancestors of angiosperms were shrubs) also offers molecular biological evidence. The most primitive living angiosperm is considered to be Amborella trichopoda , a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree. A few other angiosperm groups, known as basal angiosperms, are viewed as primitive because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more primitive because they share morphological traits with both monocots and eudicots. Flowers and Fruits as an Evolutionary Adaptation Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations. Flowering plants are the most diverse phylum on Earth after insects; flowers come in a bewildering array of sizes, shapes, colors, smells, and arrangements. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution.
Coevolution of flowers and pollinators : Many flowers have coevolved with particular pollinators, such that the flower is uniquely structured for the mouthparts of the pollinator. It often has features considered attractive to its particular pollinator. Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop from an ovary; such structures are “false fruits.” Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, walnut shells and avocados are all examples of fruit. As with pollen and seeds, fruits also act as agents of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances. The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.
Evolution of plants using C4 and CAM photosynthetic pathway. Photosynthesis is not quite as simple as adding water to CO2 to produce sugars and oxygen. A complex chemical pathway is involved, facilitated along the way by a range of enzymes and co-enzymes. The enzyme RuBisCO is responsible for "fixing" CO2 – that is, it attaches it to a carbon-based molecule to form a sugar, which can be used by the plant, releasing an oxygen molecule along the way. However, the enzyme is notoriously inefficient, and just as effectively will also fix oxygen instead of CO2 in a process called photorespiration. This is energetically costly as the plant has to use energy to turn the products of photorespiration back into a form that can react with CO2. Concentrating carbon The C 4 metabolic pathway is a valuable recent evolutionary innovation in plants, involving a complex set of adaptive changes to physiology and gene expression patterns.[21]^ About 7600 species of plants use C 4 carbon fixation, which represents about 3% of all terrestrial species of plants. All these 7600 species are angiosperms. C 4 plants evolved carbon concentrating mechanisms. These work by increasing the concentration of CO2 around RuBisCO, thereby facilitating photosynthesis and decreasing photorespiration. The process of concentrating CO2 around RuBisCO requires more energy than allowing gases to diffuse, but under certain conditions – i.e. warm temperatures (>25 °C), low CO2 concentrations, or high oxygen concentrations – pays off in terms of the decreased loss of sugars through photorespiration.
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
in its fixation. Because C 4 metabolism involves a further chemical step, this effect is accentuated. Plant material can be analysed to deduce the ratio of the heavier 13 C to 12 C. This ratio is denoted δ^13 C. C 3 plants are on average around 14‰ (parts per thousand) lighter than the atmospheric ratio, while C 4 plants are about 28‰ lighter. The δ^13 C of CAM plants depends on the percentage of carbon fixed at night relative to what is fixed in the day, being closer to C 3 plants if they fix most carbon in the day and closer to C 4 plants if they fix all their carbon at night. It is troublesome procuring original fossil material in sufficient quantity to analyse the grass itself, but fortunately there is a good proxy: horses. Horses were globally widespread in the period of interest, and browsed almost exclusively on grasses. There's an old phrase in isotope palæontology, "you are what you eat (plus a little bit)" – this refers to the fact that organisms reflect the isotopic composition of whatever they eat, plus a small adjustment factor. There is a good record of horse teeth throughout the globe, and their δ^13 C has been measured. The record shows a sharp negative inflection around 6 to 7 million years ago, during the Messinian, and this is interpreted as the rise of C 4 plants on a global scale.[33] When is C 4 an advantage? [edit] While C 4 enhances the efficiency of RuBisCO, the concentration of carbon is highly energy intensive. This means that C 4 plants only have an advantage over C 3 organisms in certain conditions: namely, high temperatures and low rainfall. C 4 plants also need high levels of sunlight to thrive.[36]^ Models suggest that, without wildfires removing shade-casting trees and shrubs, there would be no space for C 4 plants.[37]^ But, wildfires have occurred for 400 million years – why did C 4 take so long to arise, and then appear independently so many times? The Carboniferous period (~ 300 million years ago) had notoriously high oxygen levels – almost enough to allow spontaneous combustion[38]^ – and very low CO 2 , but there is no C 4 isotopic signature to be found. And there doesn't seem to be a sudden trigger for the Miocene rise.[ citation needed ] During the Miocene, the atmosphere and climate were relatively stable. If anything, CO 2 increased gradually from 14 to 9 million years ago before settling down to concentrations similar to the Holocene.[39]^ This suggests that it did not have a key role in invoking C 4 evolution.[32]^ Grasses themselves (the group which would give rise to the most occurrences of C 4 ) had probably been around for 60 million years or more, so had had plenty of time to evolve C 4 ,[40][41]^ which, in any case, is present in a diverse range of groups and thus evolved independently. There is a strong signal of climate change in South Asia;[32]^ increasing aridity – hence increasing fire frequency and intensity – may have led to an increase in the importance of grasslands.[42]^ However, this is difficult to reconcile with the North American record.[32]^ It is possible that the signal is entirely biological, forced by the fire- and grazer-[43]^ driven acceleration of grass evolution – which, both by increasing weathering and incorporating more carbon into sediments, reduced atmospheric CO2 levels.[43]^ Finally, there is evidence that the onset of C 4 from 9 to 7 million years ago is a biased signal, which only holds true for North
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
America, from where most samples originate; emerging evidence suggests that grasslands evolved to a dominant state at least 15Ma earlier in South America.[
Unit 3: Phylogeny of plants: The archetypes of plants The word Archetype comes from the Greek word archetypos. It is composed of two main elements: arche meaning first, original – and typos meaning model or type. It refers to the primal essence of something, it’s essential foundational principle, even before something becomes manifest in an individualized and / or materialized form. Plant Archetypes: a systematic study approach The understanding of the Archetype of each plant happens as we connect with its unique expression. The plant is observed in its dynamic change along its cycles from seed, to bud, to whole plant, to flower (when in that category), to fruit and back to seed. How it relates to the ecosystem it is part, its native origins in the wild.We observe it on many levels, including its presence in myth and lore and its traditional uses by humans as food, medicine and ritual. When growing healthy, we look at how it relates to the soil and the cosmos and expresses planetary forces, and how it interacts with other plants, its pollinators and other living beings. The full range of its gesture conveys an Archetypal expression. By observing how it has come into manifestation and evolved over time, we learn from its evolutionary teaching and we can deliberately connect with it for meditation, inspiration and for flower essence therapy. The language of Plants: a Vocabulary of Archetypal Expressions, Gestures, Qualities and Passages Plants offer us a whole vocabulary of Archetypal Qualities. Oak trees are strong, enduring and life harboring, while the gentle Calendula flower is luminous and benevolent in its regenerative healing presence. Our lives on earth are possible because Flowering Plants are here. Their prolific existence changed the atmosphere of this planet and made Earth hospitable to complex forms of life. Every breath we take confirms our interdependence and deep relationship with plants. We descend from Flowering Plants , they are our Ancestors. A Solar, Masculine, Radiant plant Archetype A typical experience of a Plant Archetype can be felt when connecting with the presence of the Sunflower, clearly embodying Fire Element qualities and Solar Archetypal forces. This inspiring video, a production of the Flower Essence Society, presents a beautiful integration of images, animation and a narrated poem, Sunflowers, by Mary Oliver. Observing the Sunflower, we can feel how this plant clearly imparts potent and vivid Solar qualities. And in the combination of poem, visuals and narrative, we experience an example of how good art has always drawn its power from nature’s Archetypal principles. The Sunflower belongs to the Asteraceae family, and is composed of many small flowers integrated in a central Mandala
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
with this ability: many protists can photosynthesize too, as can several important groups of bacteria. Algae in Plant Evolution Photosynthetic protists (commonly called algae) are a diverse group of organisms and are divided into several phyla. Many are unicellular, including most euglenoids (phylum Euglenophyta) and dinoflagellates (Dinophyta), and some diatoms (Bacillariophyta) and green algae (Chlorophyta). These, along with the cyanobacteria (often misleadingly called blue-green algae), form the phytoplankton of aquatic ecosystems. Others, including all brown algae (Phaeophyta), most red algae (Rhodophyta), and many green algae are multicellular. The large marine forms of these phyla are usually called seaweeds. Plants are thought to have evolved from a class of freshwater green algae called the charophytes. Two particular groups of charophyte, the Coleochaetales and the Charales, resemble the earliest land plants (bryophytes) in a variety of ways, including the structure of their chloroplasts and sperm cells, and the way their cells divide during mitosis. Bryophytes Since bryophytes are land plants, they need to support themselves in air. However, because they lack lignified vascular tissues, this support must be provided largely by the turgor pressure of their cells. Consequently, they cannot grow to be very tall, and most bryophytes are small and rather inconspicuous. An additional important feature of their lifestyle is their reproductive system. The male gametes , produced by reproductive structures called antheridia, are free- swimming sperm cells that need water to transport them to the female gametes, which are enclosed within structures called archegonia. Because of the need for water, bryophytes are especially common in wet habitats such as bogs, streambanks, and in moist forests. However, they are not restricted to these habitats, and some mosses thrive in deserts, above the treeline, and in the Arctic tundra. Among the living bryophytes, liverworts are probably most closely related to the earliest land plants, since unlike hornworts, mosses, and all vascular plants they do not possess stomata. Indeed, the fact that stomata first appeared in hornworts and mosses is evidence that vascular plants evolved from one of these two groups. Vascular plants appear to be more closely related to mosses than to hornworts, because some mosses possess food-conducting cells (leptoids) and water- conducting cells (hydroids) that resemble the phloem and xylem of vascular plants. Early Vascular Plants The first detailed vascular plant fossils appear in rocks from middle Silurian, about 425 million years ago. The oldest of these, including a plant called Aglaophyton, appear to have possessed conducting cells similar to the hydroids of mosses. These ancient plants, which are sometimes called prototracheophytes, may have been an evolutionary link between the bryophytes and the true tracheophytes. Early vascular plants possessed two features that made them especially well adapted to life on land. First, their vascular tissues transported sugars, nutrients, and water far more efficiently than the conducting
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
cells of mosses. Second, they evolved the ability to synthesize lignin , which made the cell walls of their vascular tissues rigid and supportive. Taken together, these features allowed them to grow much larger than their bryophyte ancestors and considerably reduced their dependence on moist habitats. There are three major groups of tracheophytes: seedless vascular plants, gymnosperms, and angiosperms. Since the first appearance of tracheophytes in the Silurian, the fossil record shows three major evolutionary transitions, in each of which a group of plants that were predominant before the transition is largely replaced by a different group that becomes predominant afterward. The first such transition occurred in the late Devonian, approximately 375 million years ago. Prior to this time the most common plants were simple, seedless vascular plants in various phyla, several of which are now extinct. However, one phylum from this time, the Psilophyta, still has two living genera, including a greenhouse weed called Psilotum. From the late Devonian until the end of the Carboniferous period (290 million years ago) larger, more complex seedless plants were predominant. The main phyla were the Lycophyta, the Sphenophyta, and the Pterophyta. All three groups contain living relatives, including club mosses (Lycopodiaceae) in the Lycophyta, Equisetum (the only living genus of sphenophytes), and ferns, which are pterophytes. Only the ferns, which have about 11,000 living species, are common today, but in the Carboniferous these three phyla comprised a large fraction of the vegetation on the planet. Many grew to the size of trees and dominated the tropical and subtropical swamps that covered much of the globe at this time. The second major transition was the decline of the lycophytes, sphenophytes, and pterophytes at the end of the Carboniferous and their replacement by gymnosperms in the early Permian. Gymnosperms dominated the vegetation of the land for the next 200 million years until they themselves began to decline and were replaced by angiosperms in the middle of the Cretaceous. Although one group of gymnosperms (the conifers) is still abundant, the angiosperms have been the most diverse and widespread group of plants on Earth for the last 100 million years. Gymnosperms The gymnosperms probably evolved from an extinct phylum of seedless vascular plants, the progymnosperms, that appeared about 380 million years ago. The fossils of these plants, some of which were large trees, appear to form a link between the trimerophytes (another extinct phylum of seedless vascular plants) and true gymnosperms. Progymnosperms reproduced by means of spores like the former, but their vascular tissues were very similar to those of living conifers. The oldest true gymnosperms, which produce seeds rather than spores, first appeared about 365 million years ago. The evolution of seeds, with their hard, resilient coats, was almost certainly a key factor in the success of the group. A second factor was the evolution of pollen grains to protect and transport the male gametes. As a consequence of this, gymnosperms, unlike seedless vascular plants, were no longer dependent on water for successful fertilization and could broadcast their male gametes on the wind.
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
more complex flowers that were very similar to living magnolias. Magnoliids, probably those with small, inconspicuous flowers, gave rise to the two main groups of angiosperms, monocots and eudicots , although a few angiosperm families, including the water lilies, may have evolved earlier. These plants possessed a number of adaptations that were probably crucial to their eventual success. Their vascular tissues were particularly efficient, their embryos were enclosed in a protective seed coat, their leaves were resistant to desiccation , and they were pollinated by insects, rather than by the wind. This last feature made pollen transfer much more efficient and was almost certainly a key innovation in the diversification of the group, as coevolution of plants and their pollinators, particularly bees, gave rise to increasing specialization of both flowers and insects. The orchid family contains some of the most specialized insect-pollinated flowers of all and has more species (at least 24,000) than any other plant family. Other groups of angiosperms re-evolved the ability to be pollinated by wind. One of these groups —the grasses—appeared about 50 million years ago, diversified rapidly, and became the dominant plants over many regions of the planet. They still thrive and are crucial to human well-being. Approximately 54 percent of the food eaten by people is provided by grain (seed) from cultivated varieties of just three grasses: rice, wheat, and corn. Phylogeny of flowering plants: Basal flowering plants and Eumagnoliids; Monocots; Eudicots; The basal angiosperms are the flowering plants which diverged from the lineage leading to most flowering plants. In particular, the most basal angiosperms were called the ANITA grade which is made up of Amborella (a single species of shrub from New Caledonia), Nymphaeales (water lilies, together with some other aquatic plants) and Austrobaileyales (woody aromatic plants including star anise).[1] ANITA stands for A mborella , N ymphaeales and I lliciales, T rimeniaceae- A ustrobaileya .[2]^ Some authors have shortened this to ANA -grade for the three orders, A mborellales, N ymphaeales, and A ustrobaileyales, since the order I lliciales was reduced to the family Illiciaceae and placed, along with the family T rimeniaceae, within the A ustrobaileyales. The basal angiosperms are only a few hundred species, compared with hundreds of thousands of species of eudicots, monocots, and magnoliids. They diverged from the ancestral angiosperm lineage before the five groups comprising the mesangiosperms diverged from each other.
Credits : Alokesh Das, Dept. of Botany, Rampurhat DSE
Phylogeny The exact relationships between Amborella, Nymphaeales and Austrobaileyales are not yet clear. Although most studies show that Amborella and Nymphaeales are more basal than Austrobaileyales, and all three are more basal than the mesangiosperms, there is significant molecular evidence in favor of two different trees, one in which Amborella is sister to the rest of the angiosperms, and one in which a clade of Amborella and Nymphaeales is in this position.[3]^ A 2014 paper says that it presents "the most convincing evidence to date that Amborella plus Nymphaeales together represent the earliest diverging lineage of extant angiosperms". Angiospe rmae Amborella Nymphaeale s Austrobail eyales Mesangios permae Angiospe rmae Amborella Nymphae ales Austrobail eyales Mesangios permae Older terms[edit] Amborella Paleodicots (sometimes spelled "palaeodicots") is an informal name used by botanists (Spichiger & Savolainen 1997,[5]^ Leitch et al. 1998 [6]) to refer to angiosperms which are not monocots or eudicots. The paleodicots correspond to Magnoliidae sensu Cronquist 1981 (minus Ranunculales and Papaverales) and to Magnoliidae sensu Takhtajan 1980 (Spichiger & Savolainen 1997). Some of the paleodicots share apparently plesiomorphic characters with monocots, e.g., scattered vascular bundles, trimerous flowers, and non-tricolpate pollen. The "paleodicots" are not a monophyletic group and the term has not been widely adopted. The APG II system does not recognize a group called "paleodicots" but assigns these early-diverging dicots to several orders and unplaced families: Amborellaceae, Nymphaeaceae (including Cabombaceae), Austrobaileyale s, Ceratophyllales, Chloranthaceae, and the magnoliid clade (orders Canellales, Piperales, Laurales, and Magnoliales). Subsequent research has added Hydatellaceae to the paleodicots. The term paleoherb is another older term for flowering plants which are neither eudicots nor monocots. EUDICOTS
