DICOTYLEDONS: CHARACTERISTICS, CLASSIFICATION AND EXAMPLES OF SPECIES - BIOLOGY - 2024

The dicots are a group of plants of angiosperms characterized by the presence of two or cotyledons primary leaves in the "body" of the embryo that is within their seeds.

Angiosperms belong to the group of spermatophytes, that is, plants with seeds, and correspond to the group of flowering plants. Plants belonging to this taxonomic level have traditionally been classified as monocots and dicots, mainly on the basis of the characteristics of the embryo in its seeds, although both groups differ in many other aspects.

A dicotyledonous seedling (Source: Gnan Sri Varsh via Wikimedia Commons)

However, the term “dicotyledonous” is not used in the formal taxonomic nomenclature, since some molecular and morphological analyzes have shown that certain members of this group are more related to monocots than to other dicots, so there are some discrepancies between the plant taxonomists.

Evolution and other data

Although it is not yet fully elucidated, there are two hypotheses for the phylogenetic "position" of dicots in the evolutionary history of angiosperms: the first states that seed plants are a monophyletic group and that dicots are part of the most common species. ancestral of the group of flowering plants.

The second, on the other hand, supported by some bioinformatic analyzes, proposes that the plants with seeds are not of monophyletic origin (the same common ancestor) and that possibly the most “distal” ancestor of the group of angiosperms is a monocotyledonous plant or similar (a pteridophyte).

Ignoring the logical inconvenience of determining the origin of the group, it is important to establish that it is an extremely important group of plants, both from the biodiversity point of view and from the anthropocentric point of view (based on the human being).

That said, it is good to know that the plants belonging to this group are the most abundant in the plant kingdom, accounting for more than 75% of flowering plants.

There are approximately 200 thousand species of dicotyledons, among which are almost all plants domesticated by man for food and industrial exploitation (except cereals and other grasses, since these are monocotyledons).

Dicotyledonous characteristics

Photograph of a Phaseolus vulgaris plant, a dicotyledonous plant (Source: Rainer Zenz, via Wikimedia Commons)

Depending on the text that is consulted, dicotyledonous plants are described as belonging to a monophyletic or paraphyletic group. According to some molecular and morphological analyzes, all dicots come from a common ancestor or arose in the same evolutionary event, that is, they are monophyletic.

However, the fact that not all dicots have exactly the same characteristics and that, in fact, some seem to be more closely related to some species of monocots (and vice versa) raises doubts regarding the monophyly of the group. Rather, it may be a set of plants that evolved at different points in history, from different ancestors (paraphyletic).

True cotyledons

To solve this small phylogenetic "problem" of dicots, many authors have proposed the "creation" or "grouping" of plants into a stricter group, which is known as the eudicotyledons or true dicots.

No matter what the phylogenetic conception of the group is, these plants, in general, share many fundamental physiological and anatomical aspects. Namely:

The seeds

Differences between a monocot and a dicot (Source: Flowerpower207 via Wikimedia Commons)

The "classic" taxonomic character that is used to distinguish a dicot plant from another monocot is the structure of the embryo that contains its seed.

The seeds of dicotyledonous plants have an embryo with two embryonic leaves, primordial or cotyledonous, generally fleshy and rich in reserve substances that nourish the embryo during the early stages of its development and during the initial process of germination.

The embryo of a dicot is anatomically organized in such a way that the following can be distinguished:

- An embryonic stem or plumule, which will later become the stem of the adult plant

- An embryonic root or radicle, from which the main root will develop

- Two cotyledons or embryonic leaves, which represent the first leaves of the seedling once the seed has germinated, and

- A hypocotyl, which is the portion between the plumule and the radicle.

Pollen grains

The monophyly of the eudicotyledons is based on an apomorphy ("novel" trait) of their pollen grains: they all have tricolpal pollen grains or derived from tricolpal grains.

That a pollen grain is tricolp means that it has three openings, equally spaced and more or less parallel to the polar axis of the pollen grain. These openings correspond to differentiated regions of the pollen grain through which the pollen tube can "exit" during pollination.

Those dicotyledons that have more than three openings in their pollen grains are considered "more recent" or "derived" from those that have three-lobed grains. There are also dicotyledons with unopened pollen grains, polyporates and polycorporated, all derived from the tricolpates.

Flowers

All the plants belonging to the clade of the eudicotyledons (and a large part of all the dicotyledons) have "cyclic" flowers, which means that they are organized in "whorls" whose pieces, the calyx and the corolla, are alternated. In addition, they have very thin staminal filaments that share well differentiated anthers.

The floral whorls of these plants are generally found in multiples of 4 or 5, which is used as a taxonomic character.

Leaves

Young castor oil plant showing its two prominent embryonic leaves (cotyledons), which differ from adult leaves. No machine-readable author provided. Rickjpelleg assumed (based on copyright claims).

Dicots have large leaves, with a reticulated vein pattern, which could also be described as broad and branched.

This particular character is very useful to distinguish these plants from monocots, which have narrow leaves with nerves or veins parallel to the length of the leaf (one next to the other).

The stems and vascular system

Dicotyledonous plant. Raj.palgun13

Dicots have relatively "hard" stems, which are distinguished from the stems of herbaceous plants (monocots) in that they are not structures composed of leaves, but rather by secondary thickening or deposition of resistant substances on the stem.

In these plants, the vascular system that is inside the stem is arranged in a circular shape, surrounded by a special tissue called endodermis. Vascular bundles are arranged in such a way that the xylem corresponds to the furthest portion of the endodermis, the cambium is between the xylem and the phloem, and the phloem is at a fraction of vascular sclerenchyma.

Between the endodermis and the epidermis, which is the tissue that covers the stem, a “cortex” or parenchyma can be distinguished.

Classification of dicots

Most flowering plants (angiosperms) are dicotyledonous; speaking in approximate percentage terms, eudicots (which comprise a large part of dicots) represent more than 75% of all known angiosperms in the biosphere.

The following classification is based on the tricolor pollen character and the rbcL, atpB, and 18S ribosomal DNA sequences.

This group is divided into the following groups:

Basal or early divergent eudicotyledons:

- Buxales

- Trocodendrales

- Ranunculales

- Proteals

Central eudicotyledons:

- Berberidopsidales

- Dilenials

- Gunnerales

- Caryophilales

- Santalales

- Saxifragales

- Rósidas

- Asteridas

Among the rosids and asterids are, perhaps, the most representative and abundant groups of dicotyledons. The orders Geraniales, Myrtales, Celastrales, Malpighiales, Oxalidales, Fabales, Rosales, Cucurbitales, Brassicales, Malvales and Sapindales are classified as rosids.

The orders Ericales, Gentianales, Lamiales, Solanales, Garryales, Aquifoliales, Apiales, Asterales and Dipsacales are classified as asterids.

Examples of Dicotyledonous Plant Species

There are almost 200 thousand species of dicotyledonous plants in nature. Many of the plants that support both humans and other animals are dicotyledonous, as well as others of industrial, medicinal and therapeutic interest, etc.

Virtually all trees are dicotyledonous, except for those belonging to gymnosperm species, which can have more than two cotyledons.

Among some of the most representative species of these plants, the following could be highlighted:

Calendula officinalis

Also known as "buttercup" or simply as "calendula", this plant of South European origin has great anthropocentric value from a medicinal point of view, as it is used directly or in different preparations to alleviate ailments of different types; it is also popular for the beauty and radiance of its golden or orange flowers.

It is a dicotyledonous plant, belonging to the Asteraceae family. It has herbaceous characteristics and can be annual or perennial.

Helianthus annuus

Helianthus annuus inflorescence photograph (Source: H. Zell via Wikimedia Commons)

Commonly known as "common sunflower", H. annuus is also an asteraceae, whose seeds are widely exploited as food or for the extraction of edible oils. It is a plant of North American and Central American origin, but it is cultivated in many regions of the world.

Myristica fragrans

The fruit produced by the M. fragrans trees is known worldwide as "nutmeg", a very important spice, produced mainly in Indonesia, where it originated. It belongs to the group of magnoliales (dicotyledons) and it is a tree with evergreen or permanent leaves.

It is highly exploited in the food industry, especially in Asian countries, although it has great value in the European market and in North America.

Persea americana

Photograph of a Persea americana (avocado) fruit (Source: Petruss via Wikimedia Commons)

Known worldwide as "avocado", "palto" or "Creole avocado", this dicotyledonous plant belongs to the Lauraceae family of the Laurales order. It is native to Mexico and Guatemala and is one of the trees whose fruits are highly demanded worldwide.

The individuals of this species are trees whose size can be up to 18 meters high. They produce a berry-like fruit of different sizes (depending on the cultivar) that has great global economic importance.

The country that leads the production of this item is Mexico, followed by Guatemala, Peru, Indonesia and Colombia. It is consumed for its delicious taste and for its benefits and nutritional properties. In addition, many industries are dedicated to the extraction of avocado oil, which also has important nutritional and antioxidant properties.

Lens culinaris

Also called "lentil", it is a dicotyledonous plant belonging to the Fabaceae family and to the Fabales order of Angiosperms. It is a plant native to the Mediterranean, West Asia and Africa, and is one of the oldest plants cultivated for human consumption.

It is a legume with high fiber and protein content, popular in the food of the Middle East and many other countries in the world. These plants can grow up to 18 inches tall and produce tendril-like stem modifications to hold onto adjacent surfaces.

As well as these there are many more examples of dicotyledonous plants, since fruits such as apples, pears, plums, peaches, oranges and mandarins belong to this group. All cucurbits (squash, cucumber, cantaloupe, and watermelon, for example) are also dicotyledonous plants.

References

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