Lower Plants

Lower Plants: The Cryptogams

 Cryptogams are plants that reproduce by spores, a term once widely used but now considered obsolete in taxonomy. These "spore plants," also referred to as "lower plants," include well-known groups such as algae, lichens, mosses, and ferns. Historically, cryptogams were classified as a group within the plant kingdom. In Carolus Linnaeus' system, he placed cryptogams in the class Cryptogamia, which comprised plants with hidden reproductive organs. He further divided them into four orders: Algae, Musci (mosses), Filices (ferns), and Fungi.


Today, not all cryptogams are considered part of the plant kingdom. Fungi, for instance, are now classified as a separate kingdom, more closely related to animals than plants. Some algae are also classified with bacteria. As a result, "Cryptogamae" no longer represents a scientifically valid group, as it is polyphyletic from an evolutionary perspective. Despite this, cryptogams remain under the study of botanists, and their nomenclature is regulated by the International Code of Botanical Nomenclature.


Ferns are one of the most recognizable groups of cryptogams, with around 12,000 species. Unlike mosses, ferns have xylem and phloem, making them vascular plants, and possess roots, stems, and leaves. However, they do not produce seeds or flowers. The largest group of ferns is the leptosporangiate ferns, but other groups like horsetails, whisk ferns, marattioid ferns, and ophioglossoid ferns are also classified as ferns. They belong to the broader group of pteridophytes, which encompasses ferns and other seedless vascular plants.


Ferns first appeared around 360 million years ago during the Carboniferous period, though many modern families and species emerged much later, about 145 million years ago in the late Cretaceous period.


Life Cycle

Ferns differ from seed plants in their reproduction, as they lack flowers and seeds. Like other vascular plants, ferns have a life cycle characterized by alternation of generations, consisting of a diploid sporophytic phase and a haploid gametophytic phase. A key difference from gymnosperms and angiosperms is that the fern gametophyte is free-living. The fern life cycle proceeds as follows:



Ecology

Ferns inhabit a wide range of environments, from moist, shady forests to deserts, mountains, and even water bodies. They often thrive in marginal habitats where conditions limit the success of flowering plants. Some ferns, such as the bracken fern in the British highlands and the mosquito fern in tropical lakes, can become invasive and form large colonies. Ferns typically grow in four primary habitats: moist forests, rock crevices, acid wetlands, and as epiphytes on tropical trees.


Many ferns rely on relationships with mycorrhizal fungi, and some have specific pH requirements. For example, the climbing fern (Lygodium) grows in highly acidic soils, while the bulblet bladder fern (Cystopteris bulbifera) is found on limestone. Fern spores, rich in nutrients, are a food source for animals such as the European woodmouse and the bullfinch.


Structure

The sporophytes of ferns consist of stems, leaves, and roots, similar to seed plants. Fern stems are often underground rhizomes but can also be above-ground stolons or semi-woody trunks. The leaves, called fronds, unroll in a spiral pattern known as circinate vernation. Fern leaves can be divided into three types:


Trophophylls: Photosynthetic leaves that do not produce spores.

Sporophylls: Leaves that produce spores, analogous to the reproductive organs of seed plants.

Brophophylls: Leaves that produce an unusually large number of spores.

Fern roots are fibrous and function similarly to the roots of seed plants, absorbing water and nutrients from the soil.


The gametophytes of ferns, on the other hand, are quite different from those of seed plants. They typically consist of a small, heart- or kidney-shaped prothallus, which produces gametes. The prothallus has:


Structure and Classification of Lower Plants

Ferns first appeared in the fossil record during the early Carboniferous period. By the Triassic period, evidence of ferns related to modern families began to emerge, and a significant evolutionary diversification, or "fern radiation," occurred in the late Cretaceous, when many modern fern families first appeared.


One challenge in fern classification is the existence of cryptic species. These are species that appear morphologically similar but are genetically distinct, preventing fertile interbreeding. An example is Asplenium trichomanes, or the maidenhair spleenwort, which is actually a species complex of genetically distinct diploid and tetraploid groups. These groups prefer different habitats and have slight morphological differences, but are often difficult to distinguish. Such cryptic species have sometimes been reclassified as separate species, increasing the overall number of fern species, and many more cryptic species are likely yet to be discovered.


Traditionally, ferns were classified under the class Filices, but modern taxonomy assigns them to their own division in the plant kingdom, Pteridophyta (or Filicophyta). This group is also referred to as Polypodiophyta or Polypodiopsida when treated as a subdivision of vascular plants (Tracheophyta), though Polypodiopsida sometimes specifically refers to leptosporangiate ferns. The term "pteridophyte" has historically been used to describe all seedless vascular plants, including ferns and their allies, which can lead to confusion. The study of ferns and their allies is known as pteridology, and those who specialize in this field are called pteridologists.


Ferns are traditionally divided into three groups: two eusporangiate fern families, Ophioglossaceae and Marattiaceae, and the leptosporangiate ferns. Marattiaceae is a primitive group of tropical ferns with fleshy rhizomes, and they are considered closely related to leptosporangiate ferns. Other groups once considered fern allies include the club mosses, spike mosses, and quillworts in the Lycopodiophyta, the whisk ferns in Psilotaceae, and horsetails in the Equisetaceae.


Recent genetic research has revealed that Lycopodiophyta are more distantly related to other vascular plants, having diverged early in the evolution of vascular plants. However, both whisk ferns and horsetails are now understood to be as much "true" ferns as the Ophioglossoids and Marattiaceae. Whisk ferns and Ophioglossoids form a clear evolutionary clade, while horsetails and Marattiaceae likely form another. Recent morphological studies, such as the structure of sperm and root peculiarities, support the inclusion of Equisetaceae (horsetails) within the fern group. However, debate continues over the exact placement of Equisetum species.


To address these classification challenges, some propose treating only leptosporangiate ferns as "true" ferns, while considering the other groups as fern allies. In practice, numerous classification systems have been suggested for ferns and their allies, leading to little consensus.


Modern classifications divide living ferns into four classes:



The last group, Polypodiopsida, includes the majority of plants commonly recognized as ferns. Current research supports earlier morphological ideas that the family Osmundaceae diverged early in the evolution of leptosporangiate ferns and is intermediate between eusporangiate and leptosporangiate ferns.

Colonisation of Land by Plants

Land plants are believed to have evolved from chlorophyte algae around 510 million years ago, with their closest living relatives being charophytes, particularly the Charales. If Charales have retained the characteristics of their common ancestor, land plants likely evolved from a branched, filamentous, haplontic alga that inhabited shallow freshwater, possibly at the edges of seasonal ponds. Early interactions with fungi may have played a crucial role in helping these early plants adapt to the challenges of terrestrial life.


However, plants were not the first photosynthetic organisms to colonise the land. Evidence from weathering rates suggests that land-based organisms existed around 1,200 million years ago, and microbial fossils in freshwater lake deposits date back to 1,000 million years. These early organisms were likely simple, forming little more than an algal scum and did not significantly affect the atmosphere until around 850 million years ago, when they became more widespread.


The first clear evidence of land plants comes from spore tetrads found in Mid-Ordovician (early Llanvirn) rocks, dating to around 470 million years ago. Spore tetrads are groups of four connected spores produced by meiosis, found in all land plants and some algae. The structure of these early spores closely resembles that of modern liverworts, suggesting that early land plants shared a similar level of complexity. It is possible that toxic atmospheric conditions initially prevented eukaryotes from colonising the land, or that the required complexity for land adaptation simply took time to evolve.


A key trait in early land plants was the development of desiccation-resistant spore walls, which allowed spores to survive out of water. This trait is absent in algae, even in species with relatively strong spore walls, as their spores typically disperse before acquiring this resistance. The earliest megafossils of land plants, dating from the Silurian, indicate that these thalloid organisms were restricted to waterlogged environments, such as wetlands and floodplains.


As plants adapted to life on land, they evolved different strategies to cope with desiccation. Bryophytes adopted two approaches: either avoiding desiccation by remaining in moist environments or drying out and suspending their metabolism until water became available. In contrast, tracheophytes developed the ability to resist desiccation. They evolved a waterproof cuticle to reduce water loss and developed stomata—small openings that allowed gas exchange with the atmosphere. Tracheophytes also evolved vascular tissues to transport water internally and shifted from a gametophyte-dominated life cycle to one dominated by the sporophyte phase.


As land plants spread, they contributed to a significant rise in atmospheric oxygen. Once oxygen levels reached 13%, wildfires became possible, as evidenced by the presence of charcoalified plant fossils from the early Silurian. These fossils represent the remnants of plants that were carbonised by fire, a taphonomic process that leaves behind pure carbon, which is more likely to be preserved due to its resistance to decomposition.


All multicellular plants undergo a life cycle that alternates between two distinct phases: the gametophyte, which is haploid and produces gametes, and the sporophyte, which is diploid (2n) and produces spores. In the evolutionary history of plants, the dominant trend has been a reduction in the gametophytic phase and an increase in sporophyte dominance. The ancestors of land plants were likely haplobiontic, spending most of their life cycle in the haploid phase, while land plants evolved a diplobiontic life cycle, with both haploid and diploid phases being multicellular.


There are two primary theories regarding the evolution of the diplobiontic life cycle. The interpolation theory suggests that the sporophyte phase was a novel development, resulting from the mitotic division of a zygote, leading to spore formation through meiosis. This theory aligns well with what is observed in bryophytes, where the simple sporophyte remains dependent on the gametophyte. Over time, the sporophyte evolved greater complexity, eventually becoming independent, as seen in vascular plants. The transformation theory, on the other hand, posits that the sporophyte phase emerged suddenly due to a delay in meiosis after zygote germination. This would result in similar morphologies between sporophytes and gametophytes, as seen in certain algae. Evidence from fossilised tissues in the Rhynie chert, which show similar levels of complexity in both phases, supports this theory.


Leaves, in their modern form, are an adaptation that enhances photosynthesis by capturing more sunlight. The first leafy structures likely emerged as protective spiny outgrowths. Early vascular plants, such as the rhyniophytes from the Rhynie chert, had simple, unornamented stems, while more recognisable leafy structures appeared in the Devonian with trimerophytes and zosterophyllophytes. Some of these early plants, such as Asteroxylon and Baragwanathia, developed simple vascularised leaves known as microphylls, which had a single vein.


Lycopods, a group that still exists today in the form of quillworts and club mosses, were early developers of microphylls. These leaves could grow quite large, with some, like the Lepidodendrales, reaching over a metre in length. In contrast, more complex leaves, or megaphylls, evolved independently at least four times, in ferns, horsetails, progymnosperms, and seed plants. Megaphylls likely developed from branches that either overlapped or fused together, eventually forming leaf-like structures. This process, known as the teleome theory, explains how dichotomising branches could evolve into leaves.


The evolution of leaves occurred during the Devonian, around 50 million years after plants first colonised land. It is hypothesised that the slow development of leaves was due to high atmospheric CO2 levels during the early Devonian, which would have caused leaves to overheat without adequate stomatal density to regulate water loss. As CO2 levels dropped during the Devonian, stomatal density increased, allowing leaves to cool more efficiently through transpiration, enabling their growth and diversification.


Secondary loss of leaves has also occurred in some plant groups, such as cacti and the whisk fern Psilotum, showing that leaves are not always advantageous. Certain ferns and horsetails have simplified their leaves over time, with structures that resemble microphylls despite their evolutionary origins as more complex megaphylls.


Deciduous trees, for example, shed their leaves not in response to shorter daylight but to cope with winter weather conditions, such as strong winds and heavy snow. Leaf loss has evolved independently several times in different plant groups, including the ginkgo, conifers, and angiosperms, and may also have arisen as a defence against insect predation. Rather than investing energy in leaf repair, it may have been more efficient for plants to shed their leaves during the winter or dry season.

Evolution of Trees

In the early Devonian period, the landscape lacked vegetation taller than waist height. This limitation was due to the absence of a robust vascular system, which is essential for plants to grow taller. However, there was constant evolutionary pressure for plants to increase their height. Taller plants had several advantages, such as the ability to capture more sunlight for photosynthesis by shading their competitors, and the benefit of enhanced spore dispersal, as spores released from greater heights could travel further. This is evident from Prototaxites, a giant fungus from the late Silurian that is thought to have reached heights of up to eight metres.


To achieve tree-like structures, early plants had to develop woody tissues that would provide both structural support and water transport. To understand this, it's important to explore how vascular systems work. Plants that underwent secondary growth developed a vascular cambium, a ring of cells that produces xylem and phloem. Xylem cells, which are dead and lignified, accumulate over time, forming wood.


The earliest plants to develop secondary growth and woody structures were the ferns. By the mid-Devonian, a fern species called Wattieza had already reached heights of 8 metres, exhibiting a tree-like form. Soon after, other plant groups followed. One of these was Archaeopteris, a late Devonian precursor to gymnosperms that evolved from trimerophytes and grew up to 30 metres tall. These progymnosperms were the first plants to develop true wood, produced by a bifacial cambium. The first example of this is found in the mid-Devonian plant Rellimia. True wood is thought to have evolved only once, leading to the formation of the lignophyte clade.


Forests of Archaeopteris were later joined by lycopods, particularly the lepidodendrales, which could grow up to 50 metres tall with trunks 2 metres in diameter. These lycopods dominated the late Devonian and Carboniferous coal deposits. Unlike modern trees, lepidodendrales exhibited determinate growth: after accumulating nutrients, they would grow rapidly to a set height, branch, disperse their spores, and die. Their rapid growth was supported by a type of "cheap" wood, with a large central cavity, and their wood was generated by a unifacial vascular cambium, meaning their trunks could not increase in width over time.


Next to appear was the horsetail Calamites, which thrived during the Carboniferous. Unlike the modern horsetail Equisetum, Calamites developed wood through a unifacial vascular cambium and could grow over 10 metres tall, branching multiple times.


Although these early trees resembled those of today in form, modern tree groups had not yet evolved. The dominant tree groups today include gymnosperms, such as conifers, and angiosperms, which include flowering and fruit-bearing trees. Initially, it was believed that angiosperms evolved from within the gymnosperms, but recent molecular evidence suggests that the two groups form distinct clades. While the molecular data has not yet been fully reconciled with the morphological evidence, it is becoming accepted that both gymnosperms and angiosperms may have originated from pteridosperms, possibly as early as the Permian period.


Angiosperms, and their ancestors, played only a minor role until their rapid diversification in the Cretaceous period. They initially emerged as small, moisture-loving plants in the understory of forests and have diversified ever since, becoming the dominant group in non-boreal forests today.


Roots are vital to plants for two main reasons: they provide anchorage and absorb water and nutrients from the soil. The development of roots allowed plants to grow taller and faster. Roots also had a significant global impact by disturbing the soil and promoting its acidification, enabling deeper weathering of the soil. This process may have contributed to the drawdown of atmospheric carbon dioxide, with major implications for the Earth's climate, potentially leading to mass extinction events.


But how and when did roots first evolve? While impressions of root-like structures have been found in fossil soils from the late Silurian, early plants lacked true roots. Many had tendrils that sprawled along or beneath the ground, and some even had non-photosynthetic subterranean branches that lacked stomata. True roots, as organs differentiated from stems, did not evolve until later. Fossil evidence of early root-like structures is rare, leaving much about their evolutionary origins unclear.


Rhizoids, small structures performing similar functions to roots, likely evolved very early, possibly even before plants colonised land. They are found in algae such as Characeae, a sister group to land plants. Rhizoids may have evolved multiple times independently, as similar structures are seen in lichens and even some animals. More advanced root-like structures appear in fossils from the early Devonian, such as the rhyniophytes, which had fine rhizoids. Trimerophytes and herbaceous lycopods also developed root-like structures that penetrated the soil, but these were not true roots.


By the mid-Devonian, most plant groups had independently developed some form of rooting system. As roots became larger and more complex, they enabled the growth of larger trees, promoting deeper soil weathering and opening up new habitats for fungi and animals. The roots of lycopod trees reached depths of around 20 cm during the Eifelian and Givetian stages of the Devonian, while progymnosperms rooted up to 1 metre deep in the Frasnian stage. By the Famennian period, gymnosperms and zygopterid ferns had also formed shallow rooting systems.


Lycopods like Isoetes provide evidence that roots evolved independently in at least two plant groups: lycophytes and other vascular plants. As roots became more advanced, plants were able to penetrate deeper into the soil and tap into the water table, allowing them to grow even taller.


The evolution of flowers, seen only in angiosperms, was a later development. Flowers are modified leaves, and their origin has long been debated. Early fossils from the Permian and Triassic, such as Glossopteris and Caytonia, show flower-like structures, but these are not true flowers. The Bennettitales, a group of seed plants related to cycads, also bore flower-like organs, but these evolved independently from true flowering plants. True flowers first appeared in the early Cretaceous, and by the end of the period, angiosperms had become dominant over conifers.


Angiosperms likely originated in dark, damp environments and remained constrained to such habitats throughout the Cretaceous. However, their flexibility allowed for rapid diversification, and by the end of the Cretaceous, flowering trees had become the dominant group in many ecosystems.


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