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plant anatomy Plant anatomy or phytotomy is the general term for the study of the internal structure of plants. Originally it included plant morphology, the description of the physical form and external structure of plants, but since the mid-20th century plan ...
and
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
a microphyll (or lycophyll) is a type of
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
leaf A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...
with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the
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 ...
record, and few such plants exist today. In the classical concept of a microphyll, the leaf vein emerges from the protostele without leaving a
leaf gap A leaf gap is a space in the stem of a plant through which the leaf grows. The leaf is connected to the stem by the leaf trace, which grows through the leaf gap. The leaf gap is a break in the vascular tissue of a stem Stem or STEM may refer ...
. Leaf gaps are small areas above the node of some leaves where there is no vascular tissue, as it has all been diverted to the leaf. Megaphylls, in contrast, have multiple veins within the leaf and leaf gaps above them in the stem.


Leaf vasculature

The
clubmosses 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 ...
and horsetails have microphylls, as in all extant species there is only a single vascular trace in each leaf. These leaves are narrow because the width of the blade is limited by the distance water can efficiently diffuse cell-to-cell from the central vascular strand to the margin of the leaf.How the Earth Turned Green: A Brief 3.8-Billion-Year History of Plants
/ref> Despite their name, microphylls are not always small: those of '' Isoëtes'' can reach 25 centimetres in length, and the extinct ''
Lepidodendron ''Lepidodendron'' is an extinct genus of primitive vascular plants belonging to the family Lepidodendraceae, part of a group of Lycopodiopsida known as scale trees or arborescent lycophytes, related to quillworts and lycopsids (club mosses). Th ...
'' bore microphylls up to 78 cm long.


Evolution

The enation theory of microphyll evolution posits that small outgrowths, or enations, developed from the side of early stems (such as those found in the
Zosterophyll The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zoste ...
s).WN Stewart & GW Rothwell (1993) Palaeobotany and the evolution of plants. 2nd edition. Cambridge University Press. Outgrowths of the protostele (the central vasculature) later emerged towards the enations (as in ''
Asteroxylon ''Asteroxylon'' ("star-shaped xylem") is an extinct genus of vascular plants of the Division Lycopodiophyta known from anatomically preserved specimens described from the famous Early Devonian Rhynie chert and Windyfield chert in Aberdeenshire, ...
''), and eventually continued to grow fully into the leaf to form the mid-vein (such as in ''
Baragwanathia ''Baragwanathia'' is a genus of extinct lycopsid plants of Late Silurian to Early Devonian age (), fossils of which have been found in Australia, Canada, China and Czechia. The name derives from William Baragwanath who discovered the first specim ...
''). The fossil record appears to display these traits in this order, but this may be a coincidence, as the record is incomplete. The telome theory proposes instead that both microphylls and megaphylls originated by the reduction; microphylls by reduction of a single ''telome'' branch, and megaphylls by evolution from branched portions of a telome. The simplistic evolutionary models, however, do not correspond well to evolutionary relationships. Some genera of ferns display complex leaves that are attached to the pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap. Horsetails (''
Equisetum ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass ...
'') bear only a single vein, and appear to be microphyllous; however, the fossil record suggests that their forebears had leaves with complex venation, and their current state is a result of secondary simplification. Some
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 bear needles with only one vein, but these evolved later from plants with complex leaves. An interesting case is that of '' Psilotum'', which has a (simple) protostele, and enations devoid of vascular tissue. Some species of ''Psilotum'' have a single vascular trace that terminates at the base of the enations. Consequently, ''Psilotum'' was long thought to be a "
living fossil A living fossil is an extant taxon that cosmetically resembles related species known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living foss ...
" closely related to early land plants ( rhyniophytes). However, genetic analysis has shown ''Psilotum'' to be a reduced fern. It is not clear whether leaf gaps are a homologous trait of megaphyllous organisms or have evolved more than once. While the simple definitions (microphylls: one vein, macrophylls: more than one) can still be used in modern botany, the evolutionary history is harder to decipher. File:Vein sceleton hydrangea ies.jpg, Megaphylls have a complex network of veins. File:Psilotum.jpg, '' Psilotum'' has secondarily lost leaves, and bears enations resembling the microphylls of early land plants.


See also

*
Vegetation classification Vegetation classification is the process of classifying and mapping the vegetation over an area of the earth's surface. Vegetation classification is often performed by state based agencies as part of land use, resource and environmental management. ...


References

{{reflist Plant physiology Plant anatomy Leaves