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 plant ...

and evolution a microphyll (or lycophyll) is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil 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

enation Enations are scaly leaflike structures, differing from leaves in their lack of vascular tissue. They are created by some leaf diseases and occur normally on ''Psilotum''. Enations are also found on some early plants such as '' Rhynia'', where th ...

s, developed from the side of early stems (such as those found in the Zosterophylls).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'') 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 gymnosperms bear needles with only one vein, but these evolved later from plants with complex leaves. An interesting case is that of ''

Psilotum ''Psilotum'' is a genus of fern-like vascular plants. It is one of two genera in the family Psilotaceae commonly known as whisk ferns, the other being ''Tmesipteris''. Plants in these two genera were once thought to be descended from the earlies ...

'', 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 fossi ...

" 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, ''

'' has secondarily lost leaves, and bears

s resembling the microphylls of early land plants.

References

{{reflist Plant physiology Plant anatomy Leaves

Leaf vasculature

Evolution

See also

References