Product Code Database
In plant anatomy 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. 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.
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 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" closely related to early land plants (). 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.
See also
|
|
