Early history Pre-Linnean by
John Ray 1682, after
Malpighi Monocots were first recognized as a group in
Matthias de l'Obel's
Stirpium adversaria nova. Searching for non-pharmacological characteristics to classify plants by, he chose on
leaf form and
venation, and observed that the majority of plants had broad leaves with net-like venation, but some had long and straight leaves with parallel veins. Formal description dates from
John Ray's studies of
seed structure in the 17th century. Ray, who is often considered the first botanical
systematist, observed the
dichotomy of
cotyledon structure in his examination of seeds. He reported his findings in a paper read to the
Royal Society on 17 December 1674, entitled "A Discourse on the Seeds of Plants". Since this paper appeared a year before the publication of
Malpighi's
Anatome Plantarum (1675–1679), Ray has the priority. At the time, Ray did not fully realise the importance of his discovery but progressively developed this over successive publications. And since these were in Latin, "seed leaves" became
folia seminalia and then
cotyledon, following
Malpighi. Malpighi and Ray were familiar with each other's work, and Malpighi in describing the same structures had introduced the term cotyledon, which Ray adopted in his subsequent writing. In this experiment, Malpighi also showed that the cotyledons were critical to the development of the plant, proof that Ray required for his theory. In his
Methodus plantarum nova Ray also developed and justified the "natural" or pre-evolutionary approach to classification, based on characteristics selected
a posteriori in order to group together taxa that have the greatest number of shared characteristics. This approach, also referred to as polythetic would last till
evolutionary theory enabled
Eichler to develop the
phyletic system that superseded it in the late nineteenth century, based on an understanding of the acquisition of characteristics. He also made the crucial observation
Ex hac seminum divisione sumum potest generalis plantarum distinctio, eaque meo judicio omnium prima et longe optima, in eas sci. quae plantula seminali sunt bifolia aut διλόβω, et quae plantula sem. adulta analoga. (From this division of the seeds derives a general distinction amongst plants, that in my judgement is first and by far the best, into those seed plants which are bifoliate, or bilobed, and those that are analogous to the adult), that is between monocots and dicots. He illustrated this by quoting from Malpighi and including reproductions of Malpighi's drawings of cotyledons (see figure). Initially Ray did not develop a classification of flowering plants (florifera) based on a division by the number of cotyledons, but developed his ideas over successive publications, coining the terms
Monocotyledones and
Dicotyledones in 1703, in the revised version of his
Methodus (
Methodus plantarum emendata), as a primary method for dividing them,
Herbae floriferae, dividi possunt, ut diximus, in Monocotyledones & Dicotyledones (Flowering plants, can be divided, as we have said, into Monocotyledons & Dicotyledons).
Post Linnean Although
Linnaeus (1707–1778) did not utilise Ray's discovery, basing his own classification solely on
floral reproductive morphology, the term was used shortly after his classification appeared (1753) by
Scopoli and who is credited for its introduction. Every
taxonomist since then, starting with
De Jussieu and
De Candolle, has used Ray's distinction as a major classification characteristic. In
De Jussieu's system (1789), he followed Ray, arranging his Monocotyledones into three classes based on stamen position and placing them between
Acotyledones and Dicotyledones.
De Candolle's system (1813) which was to predominate thinking through much of the 19th century used a similar general arrangement, with two subgroups of his
Monocotylédonés (Monocotyledoneae).
Lindley (1830) followed De Candolle in using the terms Monocotyledon and Endogenae interchangeably. They considered the monocotyledons to be a group of
vascular plants (
Vasculares) whose vascular bundles were thought to arise from within (
Endogènes or
endogenous). Monocotyledons remained in a similar position as a major division of the flowering plants throughout the nineteenth century, with minor variations.
George Bentham and
Hooker (1862–1883) used Monocotyledones, as would
Wettstein, while
August Eichler used Mononocotyleae and
Engler, following de Candolle, Monocotyledoneae. In the twentieth century, some authors used alternative names such as
Bessey's (1915) Alternifoliae and
Cronquist's (1966) Liliatae. Later (1981) Cronquist changed Liliatae to Liliopsida, usages also adopted by
Takhtajan simultaneously.
Thorne (1992) and
Dahlgren (1985) also used Liliidae as a synonym. Taxonomists had considerable latitude in naming this group, as the Monocotyledons were a group above the rank of family. Article 16 of the
ICBN allows either a
descriptive botanical name or a name formed from the name of an included family. In summary they have been variously named, as follows: • class Monocotyledoneae in the
de Candolle system and the
Engler system • class Monocotyledones in the
Bentham & Hooker system and the
Wettstein system • class Monocotyleae in the
Eichler system • class Liliatae then
Liliopsida in the
Takhtajan system and the
Cronquist system • subclass
Liliidae in the
Dahlgren system and the
Thorne system Modern era Over the 1980s, a more general review of the classification of
angiosperms was undertaken. The 1990s saw considerable progress in plant
phylogenetics and
cladistic theory, initially based on
rbcL gene sequencing and cladistic analysis, enabling a
phylogenetic tree to be constructed for the flowering plants. The establishment of major new
clades necessitated a departure from the older but widely used classifications such as Cronquist and Thorne, based largely on morphology rather than genetic data. These developments complicated discussions on
plant evolution and necessitated a major taxonomic restructuring. This
DNA based
molecular phylogenetic research confirmed on the one hand that the monocots remained as a well defined
monophyletic group or
clade, in contrast to the other historical divisions of the flowering plants, which had to be substantially reorganized. No longer could the angiosperms be simply divided into monocotyledons and dicotyledons; it was apparent that the monocotyledons were but one of a relatively large number of defined groups within the angiosperms. Correlation with morphological criteria showed that the defining feature was not cotyledon number but the separation of angiosperms into two major
pollen types,
uniaperturate (
monosulcate and monosulcate-derived) and triaperturate (tricolpate and tricolpate-derived), with the monocots situated within the uniaperturate groups. The formal taxonomic ranking of Monoctyledons thus became replaced with monocots as an informal clade. This is the name that has been most commonly used since the publication of the
Angiosperm Phylogeny Group (APG) system in 1998 and regularly updated since. Within the angiosperms, there are two major
grades, a small early branching
basal grade, the
basal angiosperms (ANA grade) with three
lineages and a larger late branching grade, the
core angiosperms (mesangiosperms) with five lineages, as shown in the
cladogram.
Subdivision While the monocotyledons have remained extremely stable in their outer borders as a well-defined and coherent monophylectic group, the deeper internal relationships have undergone considerable flux, with many competing classification systems over time. Historically,
Bentham (1877), considered the monocots to consist of four
alliances, Epigynae, Coronariae, Nudiflorae and Glumales, based on floral characteristics. He describes the attempts to subdivide the group since the days of
Lindley as largely unsuccessful. Like most subsequent classification systems it failed to distinguish between two major orders,
Liliales and
Asparagales, now recognised as quite separate. A major advance in this respect was the work of
Rolf Dahlgren (1980), which would form the basis of the
Angiosperm Phylogeny Group's (APG) subsequent modern classification of monocot families. Dahlgren who used the alternate name
Lilliidae considered the monocots as a
subclass of
angiosperms characterised by a single cotyledon and the presence of triangular protein bodies in the
sieve tube plastids. He divided the monocots into seven
superorders, Alismatiflorae, Ariflorae, Triuridiflorae,
Liliiflorae, Zingiberiflorae, Commeliniflorae and Areciflorae. With respect to the specific issue regarding Liliales and Asparagales, Dahlgren followed
Huber (1969) in adopting a
splitter approach, in contrast to the longstanding tendency to view
Liliaceae as a very broad
sensu lato family. Following Dahlgren's untimely death in 1987, his work was continued by his widow,
Gertrud Dahlgren, who published a revised version of the classification in 1989. In this scheme the
suffix -florae was replaced with
-anae (
e.g. Alismatanae) and the number of superorders expanded to ten with the addition of Bromelianae, Cyclanthanae and Pandananae. Molecular studies have both confirmed the
monophyly of the monocots and helped elucidate relationships within this group. The
APG system does not assign the monocots to a taxonomic rank, instead recognizing a monocots clade. However, there has remained some uncertainty regarding the exact relationships between the major lineages, with a number of competing models (including APG). The APG system establishes eleven orders of monocots. These form three grades, the
alismatid monocots,
lilioid monocots and the
commelinid monocots by order of branching, from early to late. In the following
cladogram numbers indicate
crown group (most recent common ancestor of the sampled species of the clade of interest) divergence times in
mya (million years ago). Of some 70,000
species, by far the largest number (65%) are found in two
families, the orchids and grasses. The orchids (
Orchidaceae,
Asparagales) contain about 25,000 species and the grasses (
Poaceae,
Poales) about 11,000. Other well known groups within the Poales
order include the
Cyperaceae (sedges) and
Juncaceae (rushes), and the monocots also include familiar families such as the palms (
Arecaceae, Arecales) and lilies (
Liliaceae,
Liliales).
Evolution In
prephyletic classification systems monocots were generally positioned between plants other than angiosperms and dicots, implying that monocots were more primitive. With the introduction of phyletic thinking in taxonomy (from the
system of Eichler 1875–1878 onwards) the predominant theory of monocot origins was the ranalean (ranalian) theory, particularly in the work of
Bessey (1915), which traced the origin of all flowering plants to a Ranalean type, and reversed the sequence making dicots the more primitive group. The monocots form a
monophyletic group arising early in the history of the
flowering plants, but the fossil record is meagre. The earliest fossils presumed to be monocot remains date from the
early Cretaceous period. For a very long time,
fossils of palm trees were believed to be the oldest monocots, first appearing 90 million years ago (
mya), but this estimate may not be entirely true. At least some putative monocot fossils have been found in strata as old as the eudicots. The oldest fossils that are unequivocally monocots are pollen from the Late
Barremian–
Aptian – Early
Cretaceous period, about 120-110 million years ago, and are assignable to
clade-
Pothoideae-Monstereae Araceae; being Araceae, sister to other
Alismatales. They have also found flower fossils of Triuridaceae (Pandanales) in Upper Cretaceous rocks in New Jersey, becoming the oldest known sighting of
saprophytic/
mycotrophic habits in
angiosperm plants and among the oldest known fossils of monocotyledons. Topology of the angiosperm
phylogenetic tree could imply that the monocots are among the oldest lineages of angiosperms, which would support the theory that they are just as old as the eudicots. The pollen of the eudicots dates back 125 million years, so the lineage of monocots should be that old too.
Molecular clock estimates Kåre Bremer, using
rbcL sequences and the
mean path length method for estimating
divergence times, estimated the age of the monocot crown group (i.e. the time at which the ancestor of today's
Acorus diverged from the rest of the group) as 134 million years. Similarly, Wikström
et al., using Sanderson's
non-parametric rate smoothing approach, obtained ages of 127–141 million years for the crown group of monocots. All these estimates have large error ranges (usually 15–20%), and Wikström
et al. used only a single calibration point, namely the split between
Fagales and
Cucurbitales, which was set to 84 Ma, in the late
Santonian period. Early molecular clock studies using strict clock models had estimated the monocot crown age to 200 ± 20 million years ago or 160 ± 16 million years, while studies using relaxed clocks have obtained 135-131 million years or 133.8 to 124 million years. Bremer's estimate of 134 million years has been used as a secondary calibration point in other analyses. Some estimates place the diversification of the monocots as far back as 150 mya in the
Jurassic period. The lineage that led to monocots (stem group) split from other plants about 136 million years ago or 165–170 million years ago.
Core group The age of the core group of so-called 'nuclear monocots' or 'core monocots', which correspond to all orders except
Acorales and Alismatales, is about 131 million years to present, and crown group age is about 126 million years to the present. The subsequent branching in this part of the tree (i.e.
Petrosaviaceae,
Dioscoreales + Pandanales and
Liliales clades appeared), including the crown
Petrosaviaceae group may be in the period around 125–120 million years BC (about 111 million years so far), and stem groups of all other orders, including
Commelinidae would have diverged about or shortly after 115 million years. These and many clades within these orders may have originated in southern
Gondwana, i.e. Antarctica, Australasia, and southern South America.
Aquatic monocots The aquatic monocots of Alismatales have commonly been regarded as "primitive". They have also been considered to have the most primitive foliage, which were cross-linked as Dioscoreales and
Melanthiales. Keep in mind that the "most primitive" monocot is not necessarily "the sister of everyone else". This is because the ancestral or primitive characters are inferred by means of the reconstruction of character states, with the help of the phylogenetic tree. So primitive characters of monocots may be present in some derived groups. On the other hand, the basal taxa may exhibit many
morphological autapomorphies. So although Acoraceae is the sister group to the remaining monocotyledons, the result does not imply that Acoraceae is "the most primitive monocot" in terms of its character states. In fact, Acoraceae is highly derived in many morphological characters, and that is precisely why Acoraceae and Alismatales occupied relatively derived positions in the trees produced by Chase
et al. and others. Some authors support the idea of an aquatic phase as the origin of monocots. The phylogenetic position of Alismatales (many water), which occupy a relationship with the rest except the Acoraceae, do not rule out the idea, because it could be 'the most primitive monocots' but not 'the most basal'. The Atactostele stem, the long and linear leaves, the absence of secondary growth (see the
biomechanics of living in the water), roots in groups instead of a single root branching (related to the nature of the
substrate), including
sympodial use, are consistent with a water source. However, while monocots were sisters of the aquatic
Ceratophyllales, or their origin is related to the adoption of some form of aquatic habit, it would not help much to the understanding of how it evolved to develop their distinctive anatomical features: the monocots seem so different from the rest of angiosperms and it is difficult to relate their morphology, anatomy and development and those of broad-leaved angiosperms.
Other taxa In the past, taxa which had
petiolate leaves with
reticulate venation were considered "primitive" within the monocots, because of the superficial resemblance to the leaves of
dicotyledons. Recent work suggests that while these taxa are sparse in the phylogenetic tree of monocots, such as fleshy fruited taxa (excluding taxa with aril seeds dispersed by ants), the two features would be adapted to conditions that evolved together regardless. Among the taxa involved were
Smilax,
Trillium (Liliales),
Dioscorea (Dioscoreales), etc. A number of these plants are
vines that tend to live in shaded habitats for at least part of their lives, and this fact may also relate to their shapeless
stomata. Reticulate venation seems to have appeared at least 26 times in monocots, and fleshy fruits have appeared 21 times (sometimes lost later); the two characteristics, though different, showed strong signs of a tendency to be good or bad in tandem, a phenomenon described as "concerted convergence" ("coordinated convergence").
Etymology The name monocotyledons is derived from the traditional botanical name "Monocotyledones" or
Monocotyledoneae in
Latin, which refers to the fact that most members of this group have one
cotyledon, or embryonic leaf, in their
seeds. == Ecology ==