The following points highlight the seven hypotheses of origin of monocots.
1. According to Engler and his associates, Isoetales, Ophioglossales and a few other pteridophytes are ancestors to monocots. The herbaceous monocots have been considered to be more primitive than woody dicotyledons by them.
However, this view is contradicted by the fact that, the woody habit is now generally accepted to be more primitive, and monocots most probably have diverged from some primitive dicotyledons in the early history of evolution, which also implies the monophyletic origin of angiosperms as a group.
2. Several workers have proposed the dicot families such as Lardizabalaceae , Menispermaceae, Ranunculaceae and Berberidaceae as possible ancestors of monocots. Hutchinson’s view is generally not accepted as majority of monocots are characterized by monocolpate pollen, while the pollen in Ranunculaceae is tricolpate, pantocolpate or pantoporate and is never monocolpate.
Takhtajan and Eames are of the view that ancestors of monocots must therefore be the dicots with primitive monocolpate pollen. In dicots, monocolpate pollen is found in Magnoliales, some Laurales, Nymphaeales (excluding Nelumbo), majority of the Piperales and in the genus Saruma of Aristolochiaceae.
3. Takhtajan , Cronquist, Arber, Hallier and Parkin are of the opinion that some extinct vessel less herbaceous plants, with apocarpous gynoecia and monocolpate pollen, resembling the modern Nymphaeales, were most likely the immediate ancestors of monocots.
This view is based on the fact that vessels originated independently in dicots and monocots, which has been proved by comparative anatomical studies.
Thus, the ancestors of monocots must have been the vessel less dicots such as Magnoliales and Nymphaeales.
There are several common features between the primitive monocots and Nymphaeales, which include features of leaf structure and development, stem with scattered, closed vascular bundles, more or less reduced primary root and their root structure and apocarpous gynoecia with “diffuse” placentation.
The primitive monocots and Nymphaeales only differ is the presence of two cotyledons in the latter.
4. According to various workers, monocots have an aquatic ancestry as well. Takhtajan has considered the order Nymphaeales as a hygrophilous derivative of some ancient Magnoliales, the vegetative organs of which have more or less degenerated in an aquatic environment.
The monocot line was most probably initiated from such ancient herbaceous dicots of Nymphaealean type, but more primitive and less reduced than their present-day representatives.
5. Sargant proposed that monocots have arisen as a result of adaptation to a geophilous (marsh loving) habit and called them “aquatic geophytes”. The common ancestors of both Nymphaeales and monocots were apparently the amphibious geophytes, in which geophily at first arose under terrestrial conditions, most probably under the forest canopy or in the forest margin.
6. On the basis of comparative study of various dicot and monocot embryos, Hegelmaier suggested that, the monocot embryo (consisting of a single cotyledon) arose as a result of the failure of one of the two cotyledons of a typical dicot embryo (consisting of two cotyledons) to develop.
This abortion-hypothesis has also been agreed upon by Eames,Henslow ,Metcalfe, Winkler, and many others. However, Stebbins believes that the single cotyledon of monocots is derived through intercalary concrescence of the petioles of two original cotyledons by way of syncotyly and subsequent reduction of lobes.
7. Burger proposed the dicots, very similar to Chloranthaceae of the order Piperales, which are more closely related to monocots than other living dicots, as probable ancestors of monocots. He developed a hypothesis suggesting that the simple flowers, similar to those of Chloranthaceae, came together by the loss of internodes to form three-parted flowers of Piperales and many monocots.
He also postulated that the ancestral simple flowers had a single small bract, two stamens and a single adaxial pistil. Evidence from Lactoridaceae. Saururaceae, Potamogetonaceae, Alismatales and others support his hypothesis.