''Lepidodendron'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of primitive vascular plants belonging to the family Lepidodendraceae, part of a group of

Lycopodiopsida Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...

known as scale trees or arborescent lycophytes, related to quillworts and

lycopsid Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...

s (club mosses). They were part of the

coal forest Coal forests were the vast swathes of wetlands that covered much of the Earth's tropical land areas during the late Carboniferous ( Pennsylvanian) and Permian times.Cleal, C. J. & Thomas, B. A. (2005). "Palaeozoic tropical rainforests and their e ...

flora. They sometimes reached heights of , and the trunks were often over in diameter. They thrived during the Carboniferous Period (358.9 to 298.9

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago) ...

). Sometimes erroneously called "giant club mosses", the genus was actually more closely related to modern quillworts than to modern club mosses. Within the

form classification Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of s ...

system used within paleobotany, ''Lepidodendron'' is both used for the whole plant as well as specifically the stems and leaves.

Etymology

The name ''Lepidodendron'' comes from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

λεπίς ', scale, and δένδρον ''dendron'', tree.

Growth

During the early stages of growth, ''Lepidodendron'' grew as single, unbranched trunk, with leaves growing out of the scale leaf bases (cushions). Towards the end of the trees growth, the leaves on the lower part of the trunk were shed, and in ''Lepidodendron'', the upper part of the trunk dichotomously branched into a

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...

Description and biology

''Lepidodendron'' species were comparable in size to modern trees. The plants had tapering trunks as wide as at their base that rose to about and even , arising from an underground system of horizontally spreading branches that were covered with many rootlets. Though the height of the trees make the plants similar to modern trees, the constant dichotomy of branches created a habit that contrasts with that of modern trees. At the ends of branches were oval-shaped

cones A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines conn ...

that had a similar shape to modern cones of a spruce or fir. The stem of the trees had a unifacial

vascular The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...

cambium, contrasting with the bifacial vascular cambium of modern trees. Though the bifacial cambium of modern trees produces both secondary

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...

and

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

, the unifacial cambium of ''Lepidodendron'' trees produced only secondary xylem. As the trees aged, the wood produced by the unifacial cambium decreased towards the top of the plant such that terminal twigs resembled young ''Lepidodendron'' stems. Compared to modern trees, the stems and branches of the trees contained little wood with the majority of mature stems consisting of a massive cortical

meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...

. The nearly-uniform growth of this cortical tissue indicates no difference in growth during changing seasons, and the absence of dormant

bud In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be spec ...

s further indicates the lack of

seasonality In time series data, seasonality is the presence of variations that occur at specific regular intervals less than a year, such as weekly, monthly, or quarterly. Seasonality may be caused by various factors, such as weather, vacation, and holidays a ...

in ''Lepidodendron'' species. The outermost cortex of oldest stems developed into the bark-like

lycopodiopsid Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...

periderm Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood and consist ...

. The bark of the trees was somewhat similar to that of '' Picea'' species, as

leaf scar A leaf scar is the mark left by a leaf after it falls off the twig. It marks the site where the petiole attached to the stem. A leaf scar is typically found below a branch as branches come from axillary buds located above leaf scars. Formation Le ...

s formed peg-like projections that stretched and tore as the bark stretched. To resist the bending force of wind, ''Lepidodendron'' trees depended on their outer bark rather than their vascular tissues, as compared to modern trees that rely mostly on their central mass of wood. Lepidodendron PAMuseum

The leaves of the trees were needle-like and were densely spiraled about young shoots, each possessing only a single

vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenat ...

. The leaves were similar to those of a fir in some species and similar to those of ''

Pinus roxburghii ''Pinus roxburghii'', commonly known as chir pine or longleaf Indian pine, is a species of pine tree native to the Himalayas. It was named after William Roxburgh. Description ''Pinus roxburghii'' is a large tree reaching with a trunk diameter ...

'' in others, though in general the leaves of ''Lepidodendron'' species are indistinguishable from those of ''

Sigillaria ''Sigillaria'' is a genus of extinct, spore-bearing, arborescent (tree-like) plants. It was a lycopodiophyte, and is related to the lycopsids, or club-mosses, but even more closely to quillworts, as was its associate ''Lepidodendron''. Fossil ...

'' species. The

decurrent ''Decurrent'' (sometimes decurring) is a term used in botany and mycology to describe plant or fungal parts that extend downward. In botany, the term is most often applied to leaf blades that partly wrap or have wings around the stem or petio ...

leaves formed a cylindrical shell around branches. The leaves were only present on thin and young branches, indicating that, though the trees were evergreen, they did not retain their needles for as long as modern conifers. The leaf-cushions were fusiform and elongated, growing at most to a length of and a width of . The middle of leaf-cushions were smooth, where

s were created when an

abscission Abscission () is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed. In zoology, abscission is the intentional shedding of a body part, such as the shedding of a claw, husk, or the autotomy of a ...

layer cut a leaf from its base. Each leaf scar was composed of a central circular or triangular scar and two lateral scars that were smaller and oval-shaped. This central scar marks where the main vascular bundle of the leaf connected to the vascular system of the stem. This xylem bundle was composed only of primary

trachea The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...

. The two outer scars mark the forked branches of a strand of vascular tissue that passed from the

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

of the stem into the leaf. This forked strand is sometimes referred to as the "parichnos". Surrounding this strand were parenchyma cells and occasionally thick-walled elements. Surrounding both conducting tissues was a broad sheath of transfusion

tracheid A tracheid is a long and tapered lignified cell in the xylem of vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms use another type of tracheary element, called vessel elements, to transport water through th ...

s. Below the leaf scar the leaf-cushion tapered to a basal position. In this tapering area, circular impressions with fine pits were present. These impressions were continuous with the parichnos scars near the top of the tapering portion. This is because the impressions are formed by

aerenchyma Aerenchyma or aeriferous parenchyma or lacunae, is a modification of the parenchyma to form a spongy tissue that creates spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and th ...

tissue that developed in closely with the parichnos. Above the leaf scar was a deep triangular impression known as the "ligular pit" for its similarities to the ligule of ''

Isoetes ''Isoetes'', commonly known as the quillworts, is the only extant genus of plants in the family Isoetaceae, which is in the class of lycopods. There are currently 192 recognized species, with a cosmopolitan distribution but with the individual ...

''. In some leaf-cushions a second depression was present above the ligular pit. Though its purpose is unclear, it has been suggested that the depression may mark the position of a sporangium. As the branch of a ''Lepidodendron'' tree grew the leaf-cushion only grew to a certain extent, past which the leaf-cushion stretched. This stretching widened the groove that separated the leaf-cushions, creating a broad, flat channel. Hyphae are occasionally present in the tissues of ''Lepidodendron'' trees, indicating the presence of mycorrhizal associations. Different

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

genera have been described to name the various levels of decay in ''Lepidodendron'' bark fossils. The name ''Bergeria'' describes stems that have lost their epidermises, ''Aspidiariu'' is used when cushions have been removed by deep decay, and ''Knorria'' is used when the leaf cushions and the majority of cortical tissues has decayed, with a shallow "fluted" surface remaining. However, it has been suggested that these are more likely growth forms than preserved bark types, as entire fossilized trunks have been discovered with dissimilar forms; if decay is assumed to be constant throughout the trunk, then different forms indicate growth rather than levels of decay. It is likely that the trunk of ''Lepidodendron'' trees were subject to the growth forms ''Knorria'', ''Aspidiaria'', and ''Bergeria'' progressing up the trunk, respectively.

Reproduction

''Lepidodendron'' species had a life cycle of 10 to 15 years composed of a growth cycle, in which the trees grew to a predetermined height, and a subsequent reproductive cycle, in which the trees produced reproductive organs, after which the trees died, similar to the life cycle of a Mauna Kea silversword. Rather than reproduce with seeds, ''Lepidodendron'' trees reproduced with spores. The spores were stored in

sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...

situated on fertile stems that grew on or near the main trunk. The fertile stems grew together in cone-like structures that clustered at the tips of branches.

Distribution

The lack of growth rings and of dormant buds indicate no seasonal growth patterns and modern plants with similar characteristics tend to grow in

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

conditions, but ''Lepidodendron'' species were distributed throughout subtropical conditions. The trees inhabited an extensive area compared to tropical flora of the same time period, with trees growing as far north as Spitsbergen and as far south as

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

, in a latitudinal range of 120°.

Decline and extinction

By the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

era, the giant lycopsids had died out and were replaced by conifers as well as smaller quillworts. This may have been the result of competition from the emerging woody

gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...

s. ''Lepidodendron'' is one of the more common plant

s found in Pennsylvanian-age (

Late Carboniferous Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

) rocks. They are closely related to other extinct lycopsid genera, ''

'' and '' Lepidendropsis''.

Etymology

Growth

Description and biology

Reproduction

Distribution

Decline and extinction

See also

References

Further reading