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54-581: Lepidodendrales (from the Greek for "scale tree") or arborescent lycophytes are an extinct order of primitive, vascular, heterosporous , arborescent ( tree -like) plants belonging to Lycopodiopsida . Members of Lepidodendrales are the best understood of the fossil lycopsids due to the vast diversity of Lepidodendrales specimens and the diversity in which they were preserved; the extensive distribution of Lepidodendrales specimens as well as their well-preservedness lends paleobotanists exceptionally detailed knowledge of

108-517: A protostele in which the outer xylem matured first (exarch), but the later and higher portion of the trunk developed as an ectophloic siphonostele in which the xylem was flanked by phloem tissue on both its inner and outer side. The most common fossil specimens of Lepidodendrales, as well as the most recognizable, are the compressions of stem surfaces marked with constant, though partially asymmetric, rhomboidal leaf cushions. These fossils look much like tire tracks or alligator skin, lending

162-402: A central axis with sporophylls arranged helically; sporangia are on the adaxial surface of the sporophylls and are upturned distally to overlap sporophylls above. Part of the sporophyll typically extends downward to create a heel or other distal extension. A ligule can be found in a small pit distal to the sporangium. Though Lepidostrobus is the most common name for Lepidodendrales cones,

216-596: A combination of these theories, that tectonic activity caused changes in floral composition which triggered climate change, in turn resulting in this decline. Amongst Lycopodiopsida , Lepidodendrales are considered to be more closely related to Isoetales (which includes modern quillworts ) than to club mosses or spikemosses . Some authors do not use Lepidodendrales, and instead include arborescent lycophytes within Isoetales. Various specimens of Lepidodendrales have been historically categorized as members of Lepidodendron ,

270-410: A gametophyte, but does not stop two gametophytes that originated from the same sporophyte from mating. This specific type of self-fertilization is termed as sporophytic selfing, and in extant plants it occurs most commonly among angiosperms . While heterospory stops extreme inbreeding from occurring, it does not prevent inbreeding altogether as sporophytic selfing can still occur. A complete model for

324-453: A genus defined by morphology of leaf cushions. DiMichele established Diaphorodendron to dissuade ambiguity over these widely ranging specimens, which includes some structurally preserved specimens which were previously members of Lepidodendron . Diaphorodendron was later divided into the two genera Diaphorodendron and Synchysidendron , and the genera were placed in the new family Diaphorodendraceae. Synapomorphies of this new family are

378-741: A large effect on tree uprooting, and since arborescent lycopsids had little secondary xylem and bushy crowns they may have been better suited to standing upright. The reproductive organs of the Lepidodendrids consisted of strobili or cones on distal branches in the crown. In Synchysidendron the cones occur on late-formed crown branches, while in Diaphorodendron the cones occur on deciduous lateral branches. The cones could grow to be considerably large, as in Lepidostrobus goldernbergii specimens are over 50 cm (20 in) long. The cones consist of

432-441: A medullated protostele and a dorsiventrally flattened megasporangium. Synapomorphies of the family Lepidodendraceae are a bilaterally flattened megasporangium and infrafoliar parichnos which extend below the leaf scar. The generic names Lepidodendron and Diaphorodendron today describe both cellularly preserved stem segments and entire plants, including their foliar organs, underground organs, and reproductive organs. Specifically,

486-531: A name for stems with nearly all tissues outside the xylem absent. The pattern of stem growth in Lepidodendrales can be reconstructed by analyzing their cortical growth patterns. When plants are immature, the cortex is extensive and the outer stem surface is covered with many rows of leaf bases. As the tree continues to grow, the secondary xylem and periderm originate from the vascular cambium and phellogen . This increase in stem tissue and stem diameter results in

540-736: A preference for disturbed habitats. The large quantities of biomass that were responsible for the formation of globally widespread Carboniferous coal seams were predominantly produced by arborescent lycophytes. Lepidodendrales are suggested to be responsible for almost 70% of the plant material in the Westphalian coal-swamp forests of America, though at the end of the Westphalian period Lepidodendrales members were in decline and had become responsible for only 5% of coal biomass. Arborescent lycopsids were largely becoming extinct in North America and Europe by

594-411: A separate flowering plant, the name Lepidophylloides is used today instead. Along the entire lamina of Lepidophylloides , a single vascular bundle is bordered by shallow grooves on the abaxial surface. Stomata are sunken in pits aligned in rows parallel to these grooves. A hypodermal zone of fibers surrounds the vascular bundle of the leaf. The underground organs of the Lepidodendrales are assigned

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648-401: A shared structure for all lycopsids. Bordering outside the secondary xylem is a section of cells with thin walls, representing the vascular cambium . Though modern seed plants have a bifacial cambium, the Lepidodendrids have a unifacial cambium, producing a secondary xylem only on their inner face. The phloem zone is separated from this secondary xylem by a section of thin-walled cells known as

702-404: A similar carbon fixation mechanism to modern quillworts , where carbon was uptaken from the surrounding sediment, and enriched carbon dioxide concentrations within internal gas spaces allowed increased carbon absorption. Most parts of the plant, including the leaves, stems and parts of the rooting rhizophore structures, were likely photosynthetic. Arborescent lycophytes are suggested to have had

756-475: Is larger and in turn consists of larger parenchyma cells. This section is characterized by radially extending lacunae in young stems, while in older stems the middle cortex is usually not preserved save for a few parenchyma cells. The outer cortex has no definite arrangement, but its cells have slightly thicker walls. The periderm is produced in the outer cortex. The periderm is bizonate in Diaphorodendron, where

810-410: Is only a small portion of the diameter of the stem, as the extensively developed periderm is responsible for the large trunks. The primary and secondary xylem tracheids are scalariform and have Williamson striations, or fimbrils, between these scalariform lines. The fimbrils characterize wood in arborescent lycopsids, though similar structures occur in modern club and spike mosses, and these fimbrils are

864-464: Is present in each rootlet, surrounded by a compact inner cortex. Outside this inner cortex is a hollow middle cortex and a thin outer cortex; sometimes a connection extends from the inner cortex to the outer cortex. The primary xylem of Stigmaria is endarch and arranged in a series of bands surrounded by vascular cambium. The secondary xylem tracheids are arranged in radial lines and contain scalariform wall thickenings with fimbrils identical to those in

918-544: The sporangium . Many of these different plant organs have been assigned both generic and specific names as relatively few have been found organically attached to each other. Some specimens have been discovered which indicate heights of 40 and even 50 meters and diameters of over 2 meters at the base. The massive trunks of some species branched profusely, producing large crowns of leafy twigs; though some leaves were up to 1 meter long, most were much shorter, and when leaves dropped from branches their conspicuous leaf bases remained on

972-627: The Diaphorodendraceae. Above the leaf scar is a mark from a former ligule . A waxy cuticle covered the stem surface, including leaf cushions but not including the stem scars. The simple epidermis lacks specialized cells like trichomes or epidermal glands. Stomata are frequent and sunken in shallow depressions. Stems of Lepidodendrales could be protostelic , as in Diaphorodendron , have a mixed pith , or be siphonostelic , as in Diaphorodendron and Lepidodendron . In mixed pith stems, parenchyma cells are scattered while tracheids are placed in

1026-409: The Greek name "Lepidodendrales," meaning "scale trees." These leaf cushions are actually the expanded leaf base which remained after the leaves fell, as the abscission of the leaf was not flush to the stem surface. The rhomboidal shape arises from the acute angle of the top and bottom of the cushions, known as leaf bolsters, and the rounded angle of the side. The actual leaf scar is present slightly above

1080-497: The Late Carboniferous also had secondary xylem. Lepidodendrales had tall, thick trunks that rarely branched and were topped with a crown of bifurcating branches bearing clusters of leaves . These leaves were long and narrow, similar to large blades of grass, and were spirally-arranged. The vascular system of the erect trunk was unusual in that it switched its morphological development as the plant grew. The young trunk began as

1134-457: The aerial branches. No secondary phloem has been found in Stigmaria fossil specimens, and the vascular cambium was unifacial with translocation enabled by the primary phloem. The radially aligned tracheids in most Stigmaria axes were produced by a thickening meristem rather than a vascular cambium. The development of underground organs of Lepidodendrales was likely similar to the development of

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1188-407: The aerial leaves of Lepidodendrales but modified to serve anchoring and absorbing functions. This implies that the underground organs of the plants arose as evolutionary modification of the aerial organs. Despite the towering 40 m (130 ft) height of some Lepidodendrales plants, their stigmarian system was typically shallow, and therefore it is dubious how the underground organs could support

1242-408: The aerial stems. However, some features of these organs have yet to be identified in function and some modern features of roots are absent in Stigmaria . The helical arrangement of the rootlet appendages is unlike the irregular arrangement of modern roots. No root hairs have been identified, though fungi in some cortical parenchyma cells may have functioned as mycorrhizae. The monarch vascular bundle in

1296-853: The base of the trunk as four major axes extending horizontally, leading to a relatively shallow rooting system. Lateral appendages are attached to each axis in a helical pattern. These appendages would abscise as the plant grew, resulting in the characteristic circular external scars of Stigmaria fossil specimens. Although these appendages are often called “stigmarian rootlets,” their helical arrangement and growth abscission are actually more characteristic of leaves than modern lateral roots. The four primary axes of Stigmaria dichotomize often, forming an extensive underground system possibly ranging up to 15 m (49 ft) in radius. The rootlets range in size, being up to 40 cm (16 in) long and 0.5–1 cm (0.20–0.39 in) wide, and typically taper distally and do not dichotomize. A small monarch vascular strand

1350-412: The coal-swamp giants’ reproductive biology, vegetative development, and role in their paleoecosystem. The defining characteristics of the Lepidodendrales are their secondary xylem , extensive periderm development, three-zoned cortex , rootlike appendages known as stigmarian rootlets arranged in a spiralling pattern, and megasporangium each containing a single functional megaspore that germinates inside

1404-478: The cone Bothrodendrostrobus . The embryo begins as an unvascularized globular structure found within megagametophyte tissue, and in more mature specimens two vascularized appendages extend through the trilete suture, representing the first shoot and first root. Gametophyte generation of Lepidodendrales is poorly understood and based on few specimens, but the Flemingites schopfii cones exhibit well-preserved signs of

1458-469: The developing seedling. During the Devonian period there were many species that utilized vertical growth to capture more sunlight. Heterospory and separate sporangia probably evolved in response to competition for light. Disruptive selection within species resulted in there being two separate sexes of gamete or even the whole plant. This may first have led to an increase in spore size and ultimately resulted in

1512-608: The end of the Carboniferous as tree ferns began to rise to prominence, though arborescent lycopsids persisted in China until the Middle Permian . Some scientists have suggested that the decline of lepidodendrids during this period was a result of Variscan tectonic activity creating unstable conditions by reducing the size of the coal-swamp ecosystems, while others suggest that their decline was due to climate change; some scientists suggest

1566-491: The gametophytes of both sexes are very highly reduced and contained within the spore wall. The microspores of both exosporic and endosporic species are free-sporing, distributed by wind, water or animal vectors, but in endosporic species the megaspores and the megagametophyte contained within are retained and nurtured by the sporophyte phase. Endosporic species are thus usually dioecious , a condition that promotes outcrossing . Some exosporic species produce micro- and megaspores in

1620-593: The generic name Lepidodendron is typically used to describe compression specimens which feature a particular type of leaf cushion morphology. In addition, many "organ taxa" have been identified to the Lepidodendrales: roots ( Stigmaria ), leaves, and cones ( Lepidostrobus ) were originally given a different genus and species name before it could be shown that they belonged to the same organism. Heterospory Heterospory evolved due to natural selection that favoured an increase in propagule size compared with

1674-460: The generic name Stigmaria . These structures are one of the most common lycopsid fossils and are the main organ found in the clay layer beneath most Carboniferous coal deposits; this clay layer represents the soil layer which the plants were rooted in. Despite the existence of multiple species of Stigmaria , our understanding of the underground organs is based primarily on the widespread species Stigmaria ficoides . The stigmarian organs originate from

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1728-425: The huge trees, especially since many plants grew in supersaturated, watery soil that was largely unstable. Different suggestions have arisen to explain their stature and root system: it may be that the extensive horizontal growth of the root axes provided enough support, or that the crowns of adjacent trees could entangle and provide mutual support. The nature of the wood and density of the crown of modern trees can have

1782-641: The inner zone consists of alternatingly thick and thin walled cells, and the outer zone contains dark, “resinous” cells. The homogenous or bizonate periderm is massive in Lepidodendron. The loose construction of the cortex and the large amounts of thin-walled periderm contributed to the sloughing of tissue layers during the fossilization process. This led to a variety of decorticated fossils often presumed to be external stem and trunk features but lacking leaf cushions and other features. Various generic names have been given to decorticated specimens, including Knorria ,

1836-406: The inside of spore. Both heterospory and endospory seem to be one of the many precursors to seed plants and the ovary. Heterosporic plants that produce seeds are their most successful and widespread descendants. Seed plants constitute the largest subsection of heterosporic plants. Microspores are haploid spores that in endosporic species contain the male gametophyte, which is carried to

1890-424: The largest diameters have the longest leaves, a pattern correlated with the determinate growth of the plants. Many organ taxa established for detached Lepidodendrales leaves were likely produced by the same kind of plant and differ in morphology only because of their position on the plant. The generic name Lepidophyllum is the original name for preserved Lepidodendrid leaves, but as this name had already been used for

1944-409: The leaves growing directly out of the leaf cushions/bases. Later in the growth cycle, depending on the species, the trunk produced either a series of laterally (perpendicular to the trunk) growing branches with a dichotomizing growth pattern, or a crown of dichotomising branches. Some Sigillaria species are suggested to not have branched at all. During the later stages of growth, the leaf laminae on

1998-580: The lower portions of the trunk were shed, though the rate of shedding was not rapid, as large stems have been found with the leaves remaining attached. The leaf bases remained on the trunk until in the largest stems they were sloughed off to expose the periderm. The rate of growth of arborescent lycophytes is disputed, some authors contend that they had a rapid life cycle, growing to their maximum size, reproducing and then dying in only 10 to 15 years, while other authors argue that these growth rates are overestimated. It has been proposed that arborescent lycophytes had

2052-463: The male gametophyte originating from the microspore. This results in the formation of a fertilized diploid zygote , that develops into the sporophyte embryo. While heterosporous plants produce fewer megaspores, they are significantly larger than their male counterparts. In exosporic species, the smaller spores germinate into free-living male gametophytes and the larger spores germinate into free-living female gametophytes. In endosporic species,

2106-431: The megaspores by wind, water currents or animal vectors. Microspores are not flagellated, and are therefore not capable of active movement. The morphology of the microspore consists of an outer double walled structures surrounding the dense cytoplasm and central nucleus. Megaspores contain the female gametophytes in heterosporic plant species. They develop archegonia that produce egg cells that are fertilized by sperm of

2160-464: The micro and megagametophyte phases. Compared to gametophytes of modern lycopsids, F. schopfii has microgametophytes most similar to extant Selaginella , while the megagametophytes are more similar to Isoetes . Other well-preserved Lepidodendrid gametophytes have been found in spores of Lepidodendron rhodumnense fossilized in chert from the late Viséan . During the early stages of growth, arborescent lycophytes grew as unbranched trunks, with

2214-415: The middle, though the tracheids exist in a short, squat, parenchymatous shape; this is cited as evidence that the pith in Lepidodendrales originated as immature parenchymatous cells which failed to properly differentiate into tracheids. Around the primary xylem of Lepidodendrales may be a secondary xylem, which can be several centimeters thick. Unlike modern woody trees, the secondary xylem of Lepidodendrales

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2268-402: The midpoint of the cushion and is roughly elliptical in shape. On the leaf scar, three small pitted impressions can sometimes be found. The central and always present pit results from a vascular bundle that extended into the leaf from the stem, known as “parichnos,” a system of aerating tissues. Two other parichnos channels can be found on Lepidodendron stem surfaces, though these do not occur in

2322-512: The most distal branches, where only a tiny protostele, no secondary tissues, and few rows of leaves exist; this distal stage of development is known as “apoxogenesis.” These small, distal twigs cannot develop into larger branches over time, a growth pattern known as determinate growth; this contrasts with the modern indeterminate growth pattern of most modern woody plants. Leaves of Lepidodendrales plants are linear, with some 1–2 m (3 ft 3 in – 6 ft 7 in) long. Stems with

2376-506: The name has been used for specimens of any form of preservation and for both monosporangiate and bisporangiate forms, so taxonomic problems often ensue. Attempts to dissuade these taxonomic confusions have been made. Some have suggested the name Lepidostrobus should only describe monosporangiate cones and the name Flemingites describe bisporangiate cones, while others have used cone morphology to attempt to better differentiate species within Lepidostrobus . Embryo specimens have been found in

2430-566: The origin of heterospory, known as the Haig-Westoby model, establishes a connection between minimum spore size and successful reproduction of bisexual gametophytes. For the female function, as minimum spore size increases so does the chance for successful reproduction. Glossary of botanical terms#stele This glossary of botanical terms is a list of definitions of terms and concepts relevant to botany and plants in general. Terms of plant morphology are included here as well as at

2484-431: The rootlets is bilaterally symmetrical, but modern roots have radially symmetrical vascular tissue, though vascular bundles in leaves are bilaterally symmetrical. In addition, the rootlets underwent abscission from the axis regularly as the plant grew in a similar fashion to the process of foliar abscission. However, root abscission is unknown in modern plants. These features of the rootlets suggest that they are homologous to

2538-652: The same sporangium , a condition known as homoangy, while in others the micro- and megaspores are produced in separate sporangia (heterangy). These may both be borne on the same monoecious sporophyte or on different sporophytes in dioicous species. Heterospory was a key event in the evolution of both fossil and surviving plants. The retention of megaspores and the dispersal of microspores allow for both dispersal and establishment reproductive strategies. This adaptive ability of heterospory increases reproductive success as any type of environment favors having these two strategies. Heterospory stops self-fertilization from occurring in

2592-508: The sloughing off of outer tissues including leaf bases; hence, in older areas of the plant the outer surface of the trunk is protected by the periderm. Many older drawings of Lepidodendron incorrectly illustrate leaf bases extending to the ground in older trees. At higher, younger levels of the tree, the branches have fewer rows of smaller leaves. In these sections less secondary xylem and periderm are produced. This reduction in stele size and secondary tissue production continues to taper towards

2646-496: The smaller spores of homosporous plants. Heterosporous plants, similar to anisosporic plants , produce two different sized spores in separate sporangia that develop into separate male and female gametophytes. It is proposed that the emergence of heterosporous plants started with the separation of sporangia, which allowed for the development of two different spore types; numerous small spores that are easily dispersed, and fewer, larger spores that contain adequate resources to support

2700-405: The species Lepidophloios , also known as the scale tree, has been shown in fossils to have been heterosporous; The scale tree had separate cones containing either male or female spores on the same plant. Modern heterosporous plants such as many ferns exhibit endospory , in which a megagametophyte is fertilized by a microgametophyte all while still inside the spore wall, gaining nutrients from

2754-525: The species in which it first appeared are now extinct. Heterospory is thought to have emerged in the Devonian era, mostly in wet/damp places based on fossil record evidence. In addition to being an outcome of competition for light, it is thought that heterospory was more successful in wetter areas because the megaspore could move more easily around in an aquatic environment while microspores were more easily dispersed by wind. Differing sized spores have been observed in many fossilized plant species. For example,

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2808-423: The species producing larger megaspores as well as smaller microspores. Heterospory is advantageous in that having two different types of spores increases the likeliness that plants would successfully produce offspring. Heterosporous spores can respond independently to selection by ecological conditions in order to strengthen male and female reproductive function. Heterospory evolved from homospory many times, but

2862-478: The surface of branches. Strobili could be found at the tips of distal branches or in an area at the top of the main trunk. The underground organs of Lepidodendrales typically consisted of dichotomizing axes bearing helically arranged, lateral appendages serving an equivalent function to roots. Sometimes called "giant club mosses", they are believed to be more closely related to extant quillworts based on xylem, although fossil specimens of extinct Selaginellales from

2916-434: The “parenchyma sheath.” Current evidence suggests that there was no secondary phloem present within arborescent lycopsids. The cortex of Lepidodendrids typically consisted of the inner, middle, and outer cortex, distinguished by their cell types. The inner cortex is the narrowest and consists of small parenchyma cells; secretory cells, lacunae , and various sclerotic cells also can be found in this section. The middle cortex

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