Botany

 The earliest land "plants"
d lFor 1500 million years photosynthetic organisms remained in the sea.WHY?- the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation Once atmospheric oxygen levels were high enough the ozone layer formed, meaning that it was possible for living things to venture onto the land.lThe seashore would have been enormously important in the colonisation of landlIn this zone algae would have been exposed to fresh water running off the land (and would have colonized the freshwater habitat before making the move to terrestrial existence).They would also be exposed to an alternating wet and desiccating environment.lAdaptations to survive drying out would have had strong survival value, and it is important to note that seaweeds are poikilohydric and able to withstand periods of desiccation.The earliest evidence for the appearance of land plants, in the form of fossilised spores, comes from the Ordovician period (510 - 439 million years ago), a time when the global climate was mild and extensive shallow seas surrounded the low-lying continental masses. (These spores were probably produced by submerged plants that raised their sporangia above the water - wind dispersal would offer a means of colonising other bodies of water.)

l However, DNA-derived dates suggest an even earlier colonisation of the land, around 700 million years ago.The earliest photosynthetic organisms on land would have resembled modern algae, cyanobacteria, and lichens, followed by bryophytes (liverworts & mosses, which evolved from the charophyte group of green algae Bryophytes are described as seedless, nonvascular plants. Their lack of -0.38in;">vascular tissue for transport of water and nutrients limits their size (most are between 2 and 20 cm high). Bryophytes don't have typical stems, leaves, or roots, but are anchored to the ground by rhizoids. They can grow in a wide range of environments and are poikilohydric: when the environment dries so does the plant, remaining dormant while dry but recovering rapidly when wetted. These features make them important pioneer species.

The earliest vascular land plants appeared about 425 million years ago in the late Early  Silurian, with their homoiohydric lifestyle.  The earliest known vascular plants come from the Silurian period. This form, Cooksonia, consisted of small naked, dichotomously bifurcating axes bearing terminal sporangia. These plants were only a few centimeters high.Baragwanathia is a much more robust plant consisting of up to 30 cm long axes with spirally arranged leaves and it has been described from the Upper Silurian of Australia; unfortunately, the quality of the compressions is rather poor and no organic material is preserved.

The Early Devonian is characterized by a very strong radiation of land plants.  Four  major groups can be distinguished: the Rhyniophytes, the Zosterophyllophytes, the Lycopods and the Trimerophytes. DissertationThe Rhyniophytes are definitely among the best known land plants.  These ca. 15-20 cm high plants consist of naked, dichotomously branching axes terminally bearing fusiform sporangia.  Rhyniophytes lacked true roots but had single-celled rhizoids for taking up water and nutrients.  The silicified material from the Rhynie chert shows many anatomical details.The equally sized Zosterophyllophytes are characterized by having dichotomously branching axes bearing laterally inserted sporangia; sporangia may be standing in strobili. Some Zosterophylls had small spine-like enations. The Trimerophytes have a more complex vascular system and terminally positioned sporangia standing in dense clusters.  Some trimerophytes had monopodial main axes; however, lateral axes still had dichotomous branching.  Some Trimerophytes could reach a considerable height; Pertica from the late Early Devionan may already have attained a height of ca. 3 metres. Although the oldest life forms which were found in marine sediments are as old as 3.5 billion years, continents remained uninhabited for many hundred millions years.

l The colonisation of the land by plants and animals did not occur until some 415 million years ago, at the end of the Silurian. Within the geologically relatively short time-span of only some 50 million years, following the first colonisation of the land by small land plants, the flora diversified rapidly and eventually gave rise to the oldest plants with a tree-like growth habit.The Zosterophyllophytes are considered to be the ancestral group that gave rise to the Lycopods, whereas all other groups of land plants are regarded to have been evolved from the Trimerophytes. The exquisite preservation of the Rhynie chert material enabled the recognition of the gametophyte stages of some of the earliest land plants.  Gametophytes have also been recognized in compression preservation.The earliest land plants were only few centimetres high, and consisted of rounded, naked, occasionally bifurcating axes with a central vascular strand. In order to prevent desiccation these axes were covered by a very resistant impenetrable outer layer, which is named the cuticle, and they had stomata for gas exchange. Sporangia, in which the spores necessary for the sexual reproduction were produced, were born terminally.As early as the Early Devonian up to two decimeters high, more complex plants developed which partly also had naked axes, but some had small spine-like protrusions, the so-called emergences. Real roots were still lacking; water and nutrients were taken up by so-called rhizoids.These are multicellular hair-like appendages which developed where the axes laid down on the humid substrate. lIn the lycophytes which have long and narrow, or small needle-shaped leaves the evolution of the leaf was already completed, whereas in other groups of plants a differentiation into larger axes and shorter strongly branched lateral axis systems had just begun.These latter lateral axis systems can be interpreted as the forerunners of fern-like fronds. Thus in the Middle Devonian all basic conditions for a tree-like growth habit and the formation of larger-scaled forests were met.

The earliest forests

As early as the Late Devonian the earliest tree-like plants appeared and formed forest-like stands.These plants include the earliest tree-like lycophytes which have been documented from several areas such as Spitsbergen and Ireland.Another well-known plant with a tree-like growth habit is Archaeopteris of which numerous, up to 10 m long silicified stems have been found in North America; their trunks basally may reach a diameter of up to 1.5 m.This plant had frond-like axis systems with fan-shaped leaflets and was heterosporous (having small micro- and much larger macrospores). lThe wood is anatomically remarkably similar to that of the primitive conifers. The formation of micro- and macrospores is the first evolutionary step towards the development of seeds. Because of this combination of characters Archaeopteris is generally regarded as the ancestor of the first gymnosperms.lThe first seed plants appeared towards the end of the Devonian. They belong to the pteridosperms or seed ferns, a rather heterogeneous, completely extinct group which was, however, very successful in the late Palaeozoic. These plants had fern-like fronds and did not reproduce with spores but with real seeds and pollen grains. With the appearance of the gymnosperms which are, because of their life cycle, not so strongly bound to humid environments as spore plants, the hitherto uninhabited hinterland could be colonised successfully. Nevertheless, many early forms still preferred humid habitats.Within a, in a geological perspective, relatively short time span of 50 million years a large diversity of plant groups had developed. Most of the presently still existing groups (lycophytes, sphenopsids, ferns and gymnosperms) had appeareded by the end of the Devonian. The earliest trees and forests are Late Devonian in age.During the Devonian floral provinces started to differentiate. This can be related to differences in geographical position and climate. This trend continued until the Permian. This contribution focuses on the so-called Euramerican floral province which consisted of large parts of Europe and North America which then were parts of the same continent

Plants, especially non-flowering ones, evolved rapidly during the Mesozoic Era. Some have survived with little change until the present, although many others were overtaken by the flowering plants which appeared toward the end of the era. Ginkgos first appeared 150 million years ago and became common in the Mesozoic Era. They are gymnosperms and produce seeds without a protective covering. One species, Ginkgo biloba, survives as a "living fossil" today.Certain characteristics enabled early plants to invade and become established on land. Internal vessels called vascular tissue circulated nutrients and water to all parts of the plant. An outer layer of waxy cuticle was developed to prevent drying out. The stomata located on the under surfaces of leaves open and close to allow a plant to breathe. Roots provide anchorage and nutrient uptake. Finally, spores and seeds ensure the continuation of the species.Four groups of plants dominated Triassic and Jurassic landscapes. The dominant understorey plants were ferns, which included a variety of foliage types. A middle storey of plants was quite diverse, including tree ferns, seed ferns, cycads, and cycadeoids.Cycads have a stem or trunk that commonly looks like a large pineapple and is composed of the coalesced bases of large leaves. The leaves break off as the plant grows, leaving a cluster of sturdy bases surrounding the stem. The leaves are large and palm-like, growing in a cluster at the tip of the stem.lFoliage of cycads and cycadeoids is abundant in Mesozoic strata. Because they are among the commonest compression-impression fossils found, the Mesozoic is also known as the "age of cycadophytes" (as well as the "age of reptiles”)

lThe extinct order Cycadeoidales is a mysterious group of Mesozoic gymnosperms that disappeared from the fossil record during the Cretaceous. Like the cycads many cycadeoids have trunk-like stems that are unbranched or sparsely branched and clothed in spirally-arranged persistent leaf bases. The fronds borne by plants of both orders are highly similar in appearance.lThe upper story of a Triassic forest was formed by a variety of conifers. These had distinctive patterns of cells in the wood that permit us to ally them with primitive living conifers called araucarians. The Norfolk Island pine is an example that is commonly grown as an indoor house plant.Although the fossil record of conifers and taxads is difficult to interpret, it is clear that they reached their maximum diversification during the Mesozoic.The ginkgos also appeared in Mesozoic landscapes. These gymnosperms are small to large and are slow-growing trees. Each individual is either male or female, bearing small reproductive structures of one sex or the other. The leaves are quite distinctive, having a fan shape with parallel veins and the outer margin split or entire. Ginkgos were very common during the Mesozoic all over the world.The major evolutionary innovation of plant communities during the late Mesozoic was the appearance and rapid radiation of the flowering plants, the angiosperms.By the beginning of the late Cretaceous period, angiosperms were abundant on a worldwide basis.The evolution of petals or distinctive smells and nectar to attract insects can be viewed as part of the selective process that led to the evolution of more complex flowers.The advent of flowering plants during the Cretaceous led directly to the tremendous increase in abundance and variety of living insects. Many species of flies, beetles, bees, wasps, and butterflies have life cycles that are controlled by, and dependent on, specific kinds of flowering plants.By the late Cretaceous Period, angiosperms had become the most diverse and floristically dominant group as evidenced by the composition of numerous macrofossils and pollen floras.

• The following lists some of the common plant genera of the Mesozoic Era:

Equisetites -- these small horsetails had ribbed stems, with spores present in terminal cones. Individual leaves were jointed, linear, or grasslike, with up to 30 per whorl. Scale leaves clothed the base of the leaf. The plant spread by underground stems and tubers. Equisetites lived in wetland conditions. Its tubers are often found in fossil soils. Typical height 20 ft.•Osmunda -- this is a fern with a short stem, large ordinary fronds, and specialized spore-bearing fronds. It is probable that fossil plants of this genus were to be found near water, often in warm, temperate wetland areas. Typical height 6-1/2 ft. All ferns require a moderately damp habitat for reproduction.Pachypteris -- a common constituent of many Jurassic floras, Pachypteris had small fronds with a lobed appearance. It was one of the last of the pteridosperms (seed ferns) and became extinct during the Cretaceous period. Pachypteris grew in salt marshes. Typical height 6-1/2 ft.Williamsonia -- this plant resembled a small tree with diamond-patterned bark and palmlike leaves. Its most interesting aspect was its star-shaped flowers. Williamsonia grew in tropical tree-fern forests. Typical height 10 ft.•Pityostrobus -- the form genus represents the cones of various pinelike trees common in the Jurassic and Cretaceous Periods. The cones were about three times longer than broad. Seeds were attached to the bract bases and fell out of the cone when mature. Pityostrobus grew in sub-tropical forests. Typical height 65 ft.Sequoia -- this genus includes very large trees. Small, very globular cones are a feature of these plants. The cones do not often disintegrate, even as fossils, but open to let the seeds fall out. Redwoods once formed extensive forests in sub-tropical regions of the world. Typical height 230 ft.Araucaria -- the leaves are small and toothlike. The cones are large and very spiny. Seeds are present at the base of the bracts. This genus grew in subtropical mountain forests. Typical height 100 ft.Araliopsoides -- this genus is characterized by large leaves. Araliopsoides grew in warm, temperate to sub-tropical deciduous forests. Typical height 33 ft.  This Mesozoic flora was the vegetation eaten by dinosaurs, other reptiles, and mammal herbivores during this era.

l Three (and possibly four) early seed plant species have been found at Red Hill. They are uncommon at Red Hill; they account for less than 1% of the total identifiable plant fossils and are found in only a few bedding planes. On the other hand, their diversity exceeds that found in most other Late Devonian localities. They are also among the earliest reports of seed plants in the fossil record.At least two of the Red Hill species are cupulate seed plants. It is unclear whether a third cupulate form, which is represented by only a single specimen, is a separate species. (Cupulate seed plants have a cupule, a cup-like structure that partially encloses the ovule. It may have served to enhance wind pollination or help to protect the ovule.) These cupulate species are contemporaneous with and morphologically similar to cupulate taxa found in West Virginia (Elkinsia polymorpha) and Belgium (Moresnetia zalesskyi and Dorinnotheca streelii). lHowever, the state of preservation in the Red Hill specimens makes identification elusive.The other Red Hill seed plant species is aculpate (i.e., lacks cupules).It compares favorably (c.f.) with Aglosperma quadripartita, a slightly younger seed plant from the Late Devonian of Wales.Microgametophytes in heterosporous plants produce sperm that must swim to the megagametophyte before fertilization can occur. In contrast, the entire microgametophye of seed plants is reduced to only a few cells, encased in a protective shell and then transported to the megagametophyte. This modified microgametophyte is best known by its more familiar name: pollen.The megagametophyte of seed plants is retained and nourished by the parent plant. It’s also enclosed in a protective integument. Together they form the ovule. Once the pollen grain settles into an intimate proximity to the ovule (i.e., pollination), it must deliver its sperm. The mechanism for delivering the sperm differs between the earliest seed plants and their later relatives. In later seed plants, the ovule is completely encased in a protective integument. Following pollination, the pollen grain grows a pollen tube that transports the sperm through a specialized opening in the integument called the micropyle. Once the pollen tube reaches the megagametophyte, the tube bursts and fertilization occurs.

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References

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