plant anatomy Plant anatomy or phytotomy is the general term for the study of the internal Anatomy, structure of plants. Originally, it included plant morphology, the description of the physical form and external structure of plants, but since the mid-20th centu ...

and

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

a microphyll (or lycophyll) is a type of

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

leaf A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leav ...

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 preserve ...

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. 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 also known as lycopsids, lycopods, or lycophytes. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching stems bearing simple leaves ...

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 lycopodian vascular plants belonging the order Lepidodendrales. It is well preserved and common in the fossil record. Like other Lepidodendrales, species of ''Lepidodendron'' grew as large-tree ...

'' 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 Zostero ...

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''), and eventually continued to grow fully into the leaf to form the mid-vein (such as in '' Baragwanathia''). 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) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass Equisetidae, which ...

'') 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 ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...

s 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 a Deprecation, deprecated term for an extant taxon that phenotypically 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 or ...

" closely related to early land plants (

rhyniophyte The rhyniophytes are a group of extinct early vascular plants that are considered to be similar to the genus ''Rhynia'', found in the Early Devonian (around ). Sources vary in the name and Taxonomic rank#Ranks in botany, rank used for this group, ...

s). 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 enations resembling the microphylls of early land plants.

References

{{reflist Leaf morphology Plant physiology Plant anatomy

Leaf vasculature

Evolution

See also

References