Lower Plants
Cryptogams
The name cryptogam is used fairly widely as a phrase of convenience, although regarded as an obsolete taxonomic term. A cryptogam is a plant that reproduces by spores. Other names such as "lower plants" and "spore plants" are also occasionally used. The best known groups of cryptogams are algae, lichens, mosses and ferns.
At one time, the cryptogams were formally recognised as a group within the plant kingdom. In his system for classification of all known plants and animals, Carolus Linnaeus (1707–1778) divided the plant kingdom into 25 classes, one of which was the Cryptogamia. This included all plants with concealed reproductive organs. He divided the Cryptogamia into four orders: Algae, Musci, Filices or ferns, and Fungi.
Currently, not all cryptogams are treated as part of the plant kingdom; the fungi, in particular, are regarded as a separate kingdom, more closely related to animals than plants, while some algae are now regarded as allied with the bacteria. Therefore, in contemporary plant systematics "Cryptogamae" is not a name of a scientifically coherent group, but is cladistically polyphyletic. However, all organisms known as cryptogams belong to the field traditionally studied by botanists and the names of all cryptogams are regulated by the International Code of Botanical Nomenclature.
A fern is any one of a group of about 12,000 species of plants. Unlike mosses, they have xylem and phloem making them vascular plants. They have stems, leaves, and roots like other vascular plants. Ferns do not have either seeds or flowers.
By far the largest groups of ferns are the leptosporangiate ferns, but ferns as defined here also called monilophytes which include: horsetails, whisk ferns, marattioid ferns, and ophioglossoid ferns. The term pteridophyte also refers to ferns and possibly other seedless vascular plants; see classification section below.
Ferns first appear in the fossil record 360 million years ago in the Carboniferous but many of the current families and species did not appear until roughly 145 million years ago in the late Cretaceous.
Life Cycle
Ferns are vascular plants differing from lycophytes by having true leaves. They differ from seed plants in their mode of reproduction lacking flowers and seeds. Like all other vascular plants, they have a life cycle referred to as alternation of generations, characterized by a diploid sporophytic and a haploid gametophytic phase. Unlike the gymnosperms and angiosperms, the ferns' gametophyte is a free-living organism.
Life cycle of a typical fern:
A sporophyte (diploid) phase produces haploid spores by meiosis.
A spore grows by mitosis into a gametophyte, which typically consists of a photosynthetic prothallus.
The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis.
A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus.
The fertilized egg is now a diploid zygote and grows by mitosis into a sporophyte.
Ecology
The stereotypic image of ferns growing in moist shady woodland nooks is far from being a complete picture of the habitats where ferns can be found growing. Fern species live in a wide variety of habitats, from remote mountain elevations, to dry desert rock faces, to bodies of water or in open fields. Ferns in general may be thought of as largely being specialists in marginal habitats, often succeeding in places where various environmental factors limit the success of flowering plants. Some ferns are among the world's most serious weed species, including the bracken fern growing in the British highlands, or the mosquito fern growing in tropical lakes, both species forming large aggressively spreading colonies. There are four particular types of habitats that ferns are found in: moist, shady forests; crevices in rock faces, especially when sheltered from the full sun; acid wetlands including bogs and swamps; and tropical trees, where many species are epiphytes.
Many ferns depend on associations with mycorrhizal fungi. Many ferns only grow within specific pH ranges; for instance, the climbing fern (Lygodium) of eastern North America will only grow in moist, intensely acid soils, while the bulblet bladder fern (Cystopteris bulbifera), with an overlapping range, is only found on limestone.
The spores are rich in lipids, protein and calories and some vertebrates so eat these. The European woodmouse (Apodemus sylvaticus) has been found to eat the spores of Culcita macrocarpa and the bullfinch (Pyrrhula murina) and the short-tailed bat (Mystaina tuberculata) also eat fern spores.
Like the sporophytes of seed plants, those of ferns consist of:
Stems: Most often an underground creeping rhizome, but sometimes an above-ground creeping stolon, or an above-ground erect semi-woody truck reaching up to 20 m in a few species.
Leaf: The green, photosynthetic part of the plant. In ferns, it is often referred to as a frond, but this is because of the historical division between people who study ferns and people who study seed plants, rather than because of differences in structure. New leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead. This uncurling of the leaf is termed circinate vernation. Leaves are divided into three types:
Trophophyll: A leaf that does not produce spores, instead only producing sugars by photosynthesis. Analogous to the typical green leaves of seed plants.
Sporophyll: A leaf that produces spores. These leaves are analogous to the scales of pine cones or to stamens and pistil in gymnosperms and angiosperms, respectively. Unlike the seed plants, however, the sporophylls of ferns are typically not very specialized, looking similar to trophophylls and producing sugars by photosynthesis as the trophophylls do.
Brophophyll: A leaf that produces abnormally large amounts of spores. Their leaves are also larger than the other leaves but bear a resemblance to trophophylls.
Roots: The underground non-photosynthetic structures that take up water and nutrients from soil. They are always fibrous and are structurally very similar to the roots of seed plants.
The gametophytes of ferns, however, are very different from those of seed plants. They typically consist of:
Prothallus: A green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad. The prothallus produces gametes by means of:
Antheridia: Small spherical structures that produce flagellate sperm.
Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck.
Rhizoids: root-like structures that consist of single greatly elongated cells, water and mineral salts are absorbed over the whole structure. Rhizoids anchor the prothallus to the soil.
One difference between sporophytes and gametophytes might be summed up by the saying that "Nothing eats ferns, but everything eats gametophytes." This is an over-simplification, but it is true that gametophytes are often difficult to find in the field because they are far more likely to be food than are the sporophytes.
Structure
Classification of lower plants and how they evolved
Ferns first appear in the fossil record in the early-Carboniferous period. By the Triassic, the first evidence of ferns related to several modern families appeared. The great fern radiation occurred in the late-Cretaceous, when many modern families of ferns first appeared.
One problem with fern classification is the problem of cryptic species. A cryptic species is a species that is morphologically similar to another species, but differs genetically in ways that prevent fertile interbreeding. A good example of this is the currently designated species Asplenium trichomanes, the maidenhair spleenwort. This is actually a species complex that includes distinct diploid and tetraploid races. There are minor but unclear morphological differences between the two groups, which prefer distinctly differing habitats. In many cases such as this, the species complexes have been separated into separate species, thus raising the number of overall fern species. Possibly many more cryptic species are yet to be discovered and designated.
Ferns have traditionally been grouped in the Class Filices, but modern classifications assign them their own phylum or division in the plant kingdom called Pteridophyta, also known as Filicophyta. The group is also referred to as Polypodiophyta or Polypodiopsida when treated as a subdivision of tracheophyta (vascular plants), although Polypodiopsida sometimes refers to only the leptosporangiate ferns). The term pteridophyte has traditionally been used to describe all seedless vascular plants, making it synonymous with "ferns and fern allies". This can be confusing since members of the fern phylum Pteridophyta are also sometimes referred to as pteridophytes. The study of ferns and other pteridophytes is called pteridology, and one who studies ferns and other pteridophytes is called a pteridologist.
Traditionally, three discrete groups of plants have been considered ferns: two groups of eusporangiate ferns families Ophioglossaceae and Marattiaceae and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with a large, fleshy rhizome, and are now thought to be a sibling taxon to the main group of ferns, the leptosporangiate ferns. Several other groups of plants were considered fern allies: the club mosses, spike mosses, and quillworts in the Lycopodiophyta, the whisk ferns in Psilotaceae, and the horsetails in the Equisetaceae. More recent genetic studies have shown that the Lycopodiophyta are more distantly related to other vascular plants, having radiated evolutionarily at the base of the vascular plant clade, while both the whisk ferns and horsetails are as much “true” ferns as are the Ophioglossoids and Marattiaceae. In fact, the whisk ferns and Ophioglossoids are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade. Molecular data which remain poorly constrained for many parts of the plants' phylogeny have been supplemented by recent morphological observations supporting the inclusion of Equisetaceae within the ferns, notably relating to the construction of their sperm, and peculiarities of their roots. However, there are still differences of opinion about the placement of the Equisetum species.
One possible means of treating this situation is to consider only the leptosporangiate ferns as 'true' ferns, while considering the other three groups as fern allies. In practice, numerous classification schemes have been proposed for ferns and fern allies, and there has been little consensus among them.
This classification divides extant ferns into four classes:
Psilotopsida (whisk ferns and ophioglossoid ferns), about 92 species
Equisetopsida (horsetails), about 15 species
Marattiopsida, about 150 species
Polypodiopsida (leptosporangiate ferns), about 9000 species
The last group includes most plants familiarly known as ferns. Modern research supports older ideas based on morphology that the Osmundaceae diverged early in the evolutionary history of the leptosporangiate ferns; in certain ways this family is intermediate between the eusporangiate ferns and the leptosporangiate ferns.
Evolutionary importance
The evolution of plants occurred through increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns and gymnosperms to the complex angiosperms of today. While the simple plants continue to thrive, especially in the environments in which they evolved, each new grade of organisation has eventually become more "successful" than its predecessors by most measures. Further, most cladistic analyses, where they agree, suggest that each "more complex" group arose from the most complex group at the time.
Evidence suggests that an algal scum formed on the land 1,200 million years ago, but it was not until the Ordovician period, around 450 million years ago, that land plants appeared. These began to diversify in the late Silurian period around 420 million years ago, and the fruits of their diversification are displayed in remarkable detail in an early Devonian fossil assemblage known as the Rhynie chert. This chert preserved early plants in cellular detail, petrified in volcanic springs. By the middle of the Devonian period most of the features recognised in plants today are present, including roots, leaves and secondary wood, and by late Devonian times seeds had evolved. Late Devonian plants had thereby reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued after the Devonian period. 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 evolution of flowering plants in the Triassic (200 million years ago), which exploded in the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.
Colonisation of the land
Land plants evolved from chlorophyte algae perhaps as early as 510 million years ago; their closest living relatives are the charophytes, specifically Charales. Assuming that the Charales' habit has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous, haplontic alga, dwelling in shallow fresh water perhaps at the edge of seasonally desiccating pools. 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, though: consideration of weathering rates suggests that organisms 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 the carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around 850 million years ago. These organisms were probably small and simple, forming little more than an algal scum.
The first evidence of plants on land comes from spore tetrads attributed to land plants from the Mid-Ordovician early Llanvirn, 470 million years ago. Spore tetrads consist of four identical, connected spores, produced when a single cell undergoes meiosis. Spore tetrads are borne by all land plants, and some algae. The microstructure of the earliest spores resembles that of modern liverwort spores suggesting they share an equivalent grade of organisation. 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.
shape, reflecting the points at which each cell was squashed up against its neighbours However, in order for this to happen, the spore walls must 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 have to survive out of water. Indeed, even those embryophytes that have returned to 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 it is, the spores disperse before they are squashed enough to develop the mark, or don't fit into a tetrahedral tetrad.
The earliest mega fossils 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 desiccation. The bryophytes 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. Tracheophytes resist desiccation. They all bear a waterproof outer cuticle layer wherever they are exposed to air, to reduce water loss – but since a total covering would cut them off from CO2 in the atmosphere, they rapidly evolved stomata small openings to allow 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.
The establishment of a land-based flora permitted the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire. This is first recorded in the early Silurian fossil record by charcoalified plant fossils. Apart from a controversial gap in the Late Devonian, charcoal is present ever since.
Charcoalification is an important taphonomic mode. Wildfire drives off the volatile compounds, leaving only a shell of pure carbon. This is not a viable food source for herbivores or detritovores, so is prone to preservation; it is also robust, so can withstand pressure and display exquisite, sometimes sub-cellular, detail.
All muilticelluar plants have a life cycle comprising two phases often confusingly referred to as generation. One is termed the gametophyte has a single set of chromosomes, and produces gametes. The other is termed the sporophyte, has paired chromosomes (denoted 2n), and produces spores. The two phases may be identical, or phenomenally different.
The overwhelming pattern in plant evolution is for a reduction of the gametophytic phase, and the increase in sporophyte dominance. The algal ancestors to land plants were almost certainly haplobiontic, being haploid for all their life cycles, with a unicellular zygote providing the 2n stage. All land plants are diplobiontic – that is, both the haploid and diploid stages are muilticelluar.
There are two competing theories to explain the appearance of a diplobiontic lifecycle.
The interpolation theory holds that the sporophyte phase was a fundamentally new invention, caused by the mitotic division of a freshly germinated zygote, continuing until meiosis produces spores. This theory implies that the first sporophytes would bear a very different morphology to the gametophyte, on which they would have been dependent. This seems to fit well with what we know of the bryophytes, in which a vegetative thalloid gametophyte is parasitised by simple sporophytes, which often comprise no more than a 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 a gametophyte, as we see in some hornworts, and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes. 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 a possible route for the development of a self-sufficient sporophyte phase.
The alternative hypothesis is termed the transformation theory. This posits that the sporophyte appeared suddenly by a delay in the occurrence of meiosis after the zygote germinated. Since the same genetic material would be employed, the haploid and diploid phases would look the same. This explains the behaviour of some algae, which produce alternating phases of identical sporophytes and gametophytes. Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, would result 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 preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis
Leaves today are, in almost all instances, an adaptation to increase the amount of sunlight that can be captured for photosynthesis. Leaves certainly evolved more than once, and probably originated as spiny outgrowths to protect early plants from herbivory.
The rhyniophytes of the Rhynie chert comprised nothing more than slender, unornamented axes. The early to middle Devonian trimerophytes, therefore, are the first evidence we have of anything that could 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 the stem, lacking their own vascular supply.
Around the same time, the zosterophyllophytes were becoming important. This group is recognisable by their kidney-shaped sporangia, which grew on short lateral branches close to the main axes. They sometimes branched in a distinctive H-shape. The majority of this group bore pronounced spines on their axes. However, none of these had a vascular trace, and the first evidence of vascularised enations occurs in the Rhynie genus Asteroxylon. The spines of Asteroxylon had a primitive vascular supply – at the very least, leaf traces could be seen departing from the central protostele towards each individual leaf. A fossil known as Baragwanathia appears in the fossil record slightly earlier, 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 leaves developed by outgrowths of the protostele connecting with existing enations, but it is also possible that microphylls evolved by a branching axis forming webbing.
Asteroxylon and Baragwanathia are widely regarded as primitive lycopods. The lycopods are still extant today, familiar as the quillwort Isoetes and the club mosses. Lycopods bear distinctive microphylls – leaves with a single vascular tr. Microphylls could grow to some size – the Lepidodendrales boasted microphylls over a meter in length but almost all just bear the one vascular bundle.
The more familiar leaves megaphylls are thought to have separate origins indeed, they appeared four times independently, in the ferns, horsetails, progymnosperms, and seed plants. They appear to have originated from dichotomising branches, which first overlapped or “overtopped” one another, and eventually developed webbing and evolved into gradually more leaf-like structures. So megaphylls, by this teleome theory, are composed of a group of webbed branches hence the leaf gap left where 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, but why did it take so long for leaves to evolve in the first place? Plants had been on the 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 incorporates observations that atmospheric CO2 was declining rapidly during this time falling by around 90% during the Devonian. This corresponded with an increase in stomata density by 100 times. Stomata allow water to evaporate from leaves, which causes them to curve. It appears that the low stomata density in the early Devonian meant that evaporation was limited, and leaves would overheat if they grew to any size. The stomata 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. Clearly, leaves are not always beneficial, as illustrated by the frequent occurrence of secondary loss of leaves, famously exemplified by cacti and the whisk fern Psilotum.
Secondary evolution can also disguise the true evolutionary origin of some leaves. Some genera of ferns display complex leaves which are attached to the pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap. Further, horsetail leaves bear only a single vein, and appear for all the world to be microphyllous; however, in the light of the fossil record and molecular evidence, we conclude that their forbears bore leaves with complex venation, and the current state is a result of secondary simplification.
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 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 ginkgo-ales, pinophyta and 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 the winter or dry season than to continue investing resources in their repair.
Evolution of the Leaf
Trilete spores, the progeny of spore tetrads, appear soon afterwards, in the Late Ordovician. Depending exactly when the tetrad splits, each of the four spores may bear a trilete mark.
Evolution of trees
The early Devonian landscape was devoid of vegetation taller than waist height. Without the evolution of a robust vascular system taller heights could not be obtained. There was however, a constant evolutionary pressure to attain greater height. The most obvious advantage is the harvesting of more sunlight for photosynthesis – by overshadowing competitors but a further advantage is present in spore distribution, as spores can be blown greater distances if they start higher. This may be demonstrated by Prototaxites, thought to be a late Silurian fungus reaching eight metres in height.
In order to attain arborescence, early plants needed to develop woody tissue that would act as both support and water transport. To understand wood, we must know a little of vascular behaviour. The stele of plants undergoing secondary growth is surrounded by the vascular cambium, a ring of cells which produces more xylem and phloem. Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood.
The first plants to develop this secondary growth, and a woody habit, were apparently the ferns, and as early as the middle Devonian one species, Wattieza, had already reached heights of 8 m and a tree-like habit.
Other clades did not take long to develop a tree-like stature; the late Devonian Archaeopteris, a precursor to gymnosperms which evolved from the trimerophytes, reached 30 m in height. These progymnosperms were the first plants to develop true wood, grown from a bifacial cambium, of which the first appearance is in the mid Devonian Rellimia. True wood is only thought to have evolved once, giving rise to the concept of a lignophyte clade.
These Archaeopteris forests were soon supplemented by lycopods, in the form of lepidodendrales, which topped 50 m in height and 2 m across at the base. These lycopods rose to dominate late Devonian and Carboniferous coal deposits. Lepidodendrales differ from modern trees in exhibiting determinate growth: after building up a reserve of nutrients at a low height, the plants would bolt to a genetically determined height, branch at that level, spread their spores and die. They consisted of “cheap” wood to allow their rapid growth, with at least half of their stems comprising a pith-filled cavity. Their wood was also generated by a unifacial vascular cambium it did not produce new phloem, meaning that the trunks could not grow wider over time.
The horsetail Calamites was next on the scene, appearing in the Carboniferous. Unlike the modern horsetail Equisetum, Calamites had a unifacial vascular cambium, allowing them to develop wood and grow to heights in excess of 10 m. They also branched multiple times.
While the form of early trees was similar to that of todays, the groups containing all modern trees had yet to evolve.
The dominant groups today are the gymnosperms, which include the coniferous trees, and the angiosperms, which contain all fruiting and flowering trees. It was long thought that the angiosperms arose from within the gymnosperms, but recent molecular evidence suggests that their living representatives form two distinct groups. It must be noted that the molecular data has yet to be fully reconciled with morphological data, but it is becoming accepted that the morphological support for paraphyly is not especially strong. This would lead to the conclusion that both groups arose from within the pteridosperms, probably as early as the Permian.
The angiosperms and their ancestors played a very small role until they diversified during the Cretaceous. They started out as small, damp-loving organisms in the understory, and have been diversifying ever since the mid Cretaceous, to become the dominant member of non-boreal forests today.
Roots are important to plants for two main reasons: Firstly, they provide anchorage to the substrate; more importantly, they provide a source of water and nutrients from the soil. Roots allowed plants to grow taller and faster.
The onset of roots also had effects 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, promoting the draw-down of carbon dioxide with huge implications for climate. These effects may have been so profound they led to a mass extinction.
But how and when did roots evolve in the first place? 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 tendrils which sprawled along or beneath the ground, with upright axes or thalli dotted here and there, and some even had non-photosynthetic subterranean branches which lacked stomata. The distinction between root and specialised branch is developmental; true roots follow a different developmental trajectory to stems. Further, roots differ in their branching pattern, and in possession of a root cap. So while Silu-Devonian plants such as Rhynia and Horneophyton possessed 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, and our understanding of their evolutionary origin is sparse.
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 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 animal have root like structures.
More advanced structures are common in 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 centimeters into the soil. However, none of these fossils display all the features borne by modern roots. Roots and root-like structures became increasingly more common and deeper penetrating during the Devonian period 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 period.
The rhizomorphs of the lycopods provide a slightly approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing 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 vascular system is indispensable to a rooted plant, as non-photosynthesizing roots need a supply of sugars, and a vascular system is required to transport water and nutrients from the roots to the rest of the plant. These plants are little more advanced than their Silurian forbears, without 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 mid-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 the aforementioned drawdown of CO2, but also opened up new habitats for colonisation by fungi and animals.
Roots today have developed to the physical limits. They penetrate many metres of soil to tap the water table. The narrowest roots are a mere 40 μm in diameter, and could not physically transport water if they were any narrower. The earliest fossil roots recovered, by contrast, narrowed from 3 mm to under 700 μm in diameter; of course, taphonomy is the ultimate control of what thickness we can see.
Flowers are modified leaves possessed only by the group known as the angiosperms, which are relatively late to appear in the fossil record. Colourful and/or pungent structures surround the cones of plants such as cycads and gnetales, making a strict definition of the term flower elusive.
The flowering plants have long been assumed to have evolved from within the gymnosperms; according to the traditional morphological view, they are closely allied to the gnetales. However, as noted above, recent molecular evidence is at odds to this hypothesis, and further suggests that gnetales are more closely related to some gymnosperm groups than angiosperms, and that extant gymnosperms form a distinct clade to the angiosperms, the two clades diverging some 300 million years ago.
The relationship of stem groups to the angiosperms is of utmost importance in determining the evolution of flowers; stem groups provide an insight into the state of earlier "forks" on the path to the current state. If we identify an unrelated group as a stem group, then we will gain an incorrect image of the lineages' history. The traditional view that flowers arose by modification of a structure similar to that of the gnetales, for example, no longer bears weight in the light of the molecular data.
Convergence increases our chances of misidentifying stem groups. Since the protection of the mega gametophyte is evolutionarily desirable, it would be unsurprising if many separate groups stumbled upon protective encasements independently. Distinguishing ancestry in such a situation, especially where we usually only have fossils to go on, is tricky to say the least.
In flowers, this protection is offered by the carpel, an organ believed to represent an adapted leaf, recruited into a protective role, shielding the ovules. These ovules are further protected by a double-walled integument.
Penetration of these protective layers needs something more that a free-floating micro gametophyte. Angiosperms have pollen grains comprising just three cells. One cell is responsible for drilling down through the integuments, and creating a conduit for the two sperm cells to flow down. The mega gametophyte has just seven cells; of these, one fuses with a sperm cell, forming the nucleus of the egg itself, and another other joins with the other sperm, and dedicates itself to forming a nutrient-rich endosperm. The other cells take auxiliary roles. This process of "double fertilization" is unique and common to all angiosperms.
In the fossil record, there are three intriguing groups which bore flower-like structures. The first is the Permian pteridosperms Glossopteris, which already bore recurred leaves resembling carpels. The Triassic Caytonia is more flower-like still, with enclosed ovules – but only a single integument. Further details of their pollen and stamens set them apart from true flowering plants.
The Bennettitales bore remarkably flower-like organs, protected by whorls of bracts which may have played a similar role to the petals and sepals of true flowers; however, these flower-like structures evolved independently, as the Bennettitales are more closely related to cycads and ginkgos than to the angiosperms.
However, no true flowers are found in any groups save those extant today. Most morphological and molecular analyses place Amborella, the nymphaeales and Austrobaileyaceae in a basal clade dubbed "ANA". This clade appear to have diverged in the early Cretaceous, around 130 million years ago – around the same time as the earliest fossil angiosperm and just after the first angiosperm-like pollen, 136 million years ago. The magnoliids diverged soon after, and a rapid radiation had produced eudicots and monocots by 125 million years ago. By the end of the Cretaceous 65.5 million years ago, over 50% of today's angiosperm orders had evolved, and the clade accounted for 70% of global species. It was around this time that flowering trees became dominant over conifers.
The features of the basal "ANA" groups suggest that angiosperms originated in dark, damp, frequently disturbed areas. It appears that the angiosperms remained constrained to such habitats throughout the Cretaceous – occupying the niche of small herbs early in the successional series. This may have restricted their initial significance, but given them the flexibility that accounted for the rapidity of their later
Spore production and evolution of the early seeds
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoans. A chief difference between spores and seeds as dispersal units is that spores have very little stored food resources compared with seeds.
Spores are usually haploid and unicellular and are produced by meiosis in the sporangium by the sporophyte. Once conditions are favorable, the spore can develop into a new organism using mitotic division, producing a muilticelluar gametophyte, which eventually goes on to produce gametes.
Two gametes fuse to create a new sporophyte. This cycle is known as alternation of generations, but a better term is biological life cycle, as there may be more than one phase and so it cannot be a direct alternation. Haploid spores produced by mitosis are used by many fungi for asexual reproduction.
Many ferns, especially those adapted to dry conditions, produce diploid spores. This form of asexual reproduction is called apogamy. It is a form of apomixis.
Spores are the units of asexual reproduction, because a single spore develops into a new organism. By contrast, gametes are the units of sexual reproduction, as two gametes need to fuse to create a new organism.
Classification of Spores
Sporangiospores: spores produced by a sporangium in many fungi such as zygomycetes.
Zygospores: spores produced by a zygosporangium, characteristic of zygomycetes.
Ascospores: spores produced by an ascus, characteristic of ascomycetes.
Basidiospores: spores produced by a basidium, characteristic of basidiomycetes.
Aeciospores: spores produced by an aecium in some fungi such as rusts or smuts.
Urediospores: spores produced by a uredinium in some fungi such as rusts or smuts.
Teliospores: spores produced by a telium in some fungi such as rusts or smuts.
Oospores: spores produced by an oogonium, characteristic of oomycetes.
Carpospores: spores produced by a carposporophyte, characteristic of red algae.
Tetraspores: spores produced by a tetrasporophyte, characteristic of red algae.
Under high magnification, spores can be categorized as either monolete spores or trilete spores. In monolete spores, there is a single line on the spore indicating the axis on which the mother spore was split into four along a vertical axis. In trilete spores, all four spores share a common origin and are in contact with each other, so when they separate, each spore shows three lines radiating from a center pole.
Vascular plant spores are always haploid. Vascular plants are either homosporous or heterosporous. Plants that are homosporous produce spores of the same size and type. Heterosporous plants, such as spike mosses, quillworts, and some aquatic ferns produce spores of two different sizes: the larger spore in effect functioning as a female spore and the smaller functioning as a male.
Life Cycle of the moss
Most kinds of plants have two sets of chromosomes in their vegetative cells and are said to be diploid, i.e. each chromosome has a partner that contains the same, or similar, genetic information. By contrast, mosses and other bryophytes have only a single set of chromosomes and so are haploid. There are periods in the moss life cycle when they do have a double set of paired chromosomes, but this happens only during the sporophyte stage.
The life of a moss starts from a haploid spore. The spore germinates to produce a protonema, which is either a mass of thread-like filaments or thalloid. Moss protonemata typically look like a thin green felt, and may grow on damp soil, tree bark, rocks, concrete, or almost any other reasonably stable surface. This is a transitory stage in the life of a moss, but from the protonema grows the gametophore that is structurally differentiated into stems and leaves. A single mat of protonemata may develop several gametophore shoots, resulting in a clump of moss.
From the tips of the gametophore stems or branches develop the sex organs of the mosses. The female organs are known as archegonia and are protected by a group of modified leaves known as the perichaetum. The archegonia are small flask-shaped clumps of cells with an open neck down which the male sperm swim. The male organs are known as antheridia and are enclosed by modified leaves called the perigonium. The surrounding leaves in some mosses form a splash cup, allowing the sperm contained in the cup to be splashed to neighboring stalks by falling water droplets.
Mosses can be either dioicous or monoicous. In dioicous mosses, male and female sex organs are borne on different gametophyte plants. In monoicous mosses, both are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilization occurs, leading to the production of a diploid sporophyte. The sperm of mosses is biflagellate, i.e. they have two flagellae that aid in propulsion. Since the sperm must swim to the archegonium, fertilization cannot occur without water. After fertilization, the immature sporophyte pushes its way out of the archegonial venter. It takes about a quarter to half a year for the sporophyte to mature. The sporophyte body comprises a long stalk, called a seta, and a capsule capped by a cap called the operculum. The capsule and operculum are in turn sheathed by a haploid calyptra which is the remains of the archegonial venter. The calyptra usually falls off when the capsule is mature. Within the capsule, spore-producing cells undergo meiosis to form haploid spores, upon which the cycle can start again. The mouth of the capsule is usually ringed by a set of teeth called peristome. This may be absent in some mosses.
In some mosses, for example Ulota phyllantha, green vegetative structures called gemmae are produced on leaves or branches, which can break off and form new plants without the need to go through the cycle of fertilization. This is a means of asexual reproduction, and the genetically identical units can lead to the formation of clonal populations.