Dicotyledoneae

Magnoliopsida is a plant taxon that was used in very popular classification systems such as Cronquist's (Cronquist 1981, 1988), whose circumscription coincides with the circumscription of dicots. (so called because they typically have two cotyledons during the early stages of development, called Dicotyledonea in classification systems such as Engler's), a name that is therefore a synonym of this taxon. This taxon has been entrenched in classification systems for a long time, but it is now known to be paraphyletic with respect to monocots, and therefore its characters (for example, the embryo with two cotyledons) are characters that already existed at some point. common ancestor to dicots and monocots (they are "plesiomorphic" characters, common to all angiosperms or even to all seed plants) and monocots do not possess them because they lost them after having acquired them. According to taxonomy as it is mostly understood today, paraphyletic taxa should be abandoned, and only monophyletic taxa should be used in classifications, so in modern classification systems such as APG III (2009) and APWeb (2001 onwards), the taxon was abandoned.

Dicotyledons are made up of 6 monophyletic groups present in modern classifications, the largest of which was called Eudicotyledoneae (eu- means "true"). Eudicots comprise 75% of angiosperm species, and possess many of the characters previously attributed to dicots, such as tricolpate pollen (and derivatives thereof). The other groups are Amborellaceae, Nymphaeaceae, Austrobaileyales, Magnoliidae (which now includes Chloranthaceae), and Ceratophyllales.

Description

Theoretical Introduction in Descriptive Terminology of Plants

The characters belonging to the ancestral angiosperm and lost in the branch from which the monocots originated are:

• The radicle of the embryo does not abort as in monocothylenes but gives rise to a persistent primary root (in monocothylenes the absorption function is fulfilled by the adventic roots).
• The nerves of the leaves form pine or patted patterns ("reticulated venation") unlike the monocotyledons that acquired a parallel venation.
• The verticils of the flowers are made up of 4 or 5 pieces generally (instead of 3 as in the monocotyledons).

A character that was traditionally attributed to dicotyledons, which is tricolpate pollen and its derivatives (in monocots the pollen is monosulcate), is actually a character originating in a subgroup of dicotyledons called Eudicotyledoneae, being the pollen from the rest of the dicots monosulcate, which is how it is found in monocots (and in the ancestral angiosperm).

The presence of some characters traditionally considered characteristic of dicots, are found in both dicots and gymnosperms, therefore they are plesiomorphies of all seed plants, which were lost in monocots, namely:

• the presence of two or more cotiledons in the embryos, unlike the monocotyledons that possess only one,
• in the cross-section of the stem ("stella") it is observed that the xylema and the floem are arranged forming a ring, according to a pattern called eustela (unlike the atactostela of the monocothylenes),
• the stem grows in thickness with the formation of wood or wood ("secondary growth"), unlike the monocotyledons that have lost the ability to grow in thickness of that form (some monocotyleddons, such as the palm trees, grow in thickness by increase of the parenchymatic tissue, without wood formation).

Phylogeny

Cladistic analyzes of morphology and chloroplastid, mitochondrial, and nuclear DNA sequences do not support the division of angiosperms into monocots and dicots (Olmstead et al. 1992a, Chase et al. 1993, Doyle 1996, 1998, Doyle et al. 1994, Donoghue and Doyle 1989, Graham and Olmstead 2000, Mathews and Donoghue 1999, Savolainen et al. 2000, Soltis et al. 2000, Zimmer et al. 2000, Hilu et al. 2003, Zanis et al. 2003). In all the analyses, the dicotyledons are a paraphyletic group, and the morphological characters already mentioned are plesiomorphic within the angiosperms (except tricolpate pollen and its derivatives).

Dicotyledons are made up of the monophyletic groups:

A discussion of the phylogeny of the group can be found in Angiosperm Systematics.

Most dicots belong to the clade Eudicotyledoneae (also called "tricolpad dicots"), which have tricolpad pollen (and subsequent modifications) as synapomorphies, and the DNA sequences rbcL, atpB, matK, and 18S rDNA.

Taxonomy

Theoretical Introduction in Taxonomy

An updated classification to the year 2009 can be found in the APG III classification system, and in the APWeb. They abandon the dicot taxon and use the aforementioned monophyletic groups instead.

The previous classification of dicots, that of Cronquist (1981 and 1988), into subclasses, orders and families can be seen by displaying the following box.