Dicots/What is a dicot?

 

Description

Taking a very simplified overview, there are plants that may have been regarded historically as Dicots, such as Waterlillies (Nymphaceae) which may have diverged from the common ancestor many millions of years ago. Also, the Monocots which are a monophyletic group and can therefore legitimately claim to be a Clade seem to nest within the Dicots and as Stevens[2] points out:

The basic problem with paraphyletic groups is that users do not generally expect a family to be nested inside another family, a genus within another genus, etc. - this flouts the basic principles of classifications of organisms as evident in folk taxonomies

Among the points he is trying to make here I believe is the logical one that for things to make sense there needs to be a set of principles which should be adhered to. Otherwise, systems of classification have little or no validity. However, much more could be said and there are proponents for paraphyletic clades. Therefore I shall not delve further, but will conclude by saying that the word ‘Dicot’ (or Dicotyledon) is a convenient term for referring to a large group of plants that are related to each other. The key phrase being convenient term. Clearly there is much more to this subject, but I will draw a line in the sand here. Readers seeking further enlightenment should consider linking through to the resources listed below this page.

Please note that whilst I do not take responsibility for the content of external pages linked to from here, I fully acknowledge the authors ownership of such material and respect their copyright therein. Readers seeking to use other authors work in their own projects should seek permission from the them before doing so. Go to Library/Dicots

 

A Dicot has always been thought of as a plant that has two seed leaves and until fairly recently the umbrella term Dicotyledon referring to the plant group named Dicotyledonae was universally accepted.  Strictly speaking; or to use the latin term ‘Sensu Stricto’ the taxonomic group no longer exists, but as a convenient catch-all for plants that have two seed leaves as opposed to one, it still has uses. However, for the beginner to Botany and the associated science of Taxonomy, this can all seem no less peculiar than particle physics. So a brief explanation is now called for.

Essentially what has happened is that with the ability to sequence genes, scientists can now directly see how closely species are related. And the answers they have been getting to such questions is quite often; ‘not very.’ The two terms that are relevant here are: Monophyletic and Paraphyletic. The Angiosperm Phylogeny Website[1] defines them in the following ways:

  1. 1.monophyletic: of any taxon that includes its common ancestor and all its descendents, a monophyletic group is a clade, q.v., cf. paraphyletic, polyphyletic.(a)

  2. 2.paraphyletic: a taxon made up of members which, given a particular phylogenetic tree and classification based on it, include only but not all the descendents (of)(b) a common ancestor, likely to be a grade, cf. monophyletic, polyphyletic.

The Dicots are by the above definition paraphyletic. In other words the plant group that we thought was a unified whole all springing from one common ancestor with all descendants related to it, isn’t. Instead it is a group of organisms that have a lot of similarity, but which cannot lay full claim to the title Clade.

(a&b) Minor alterations made here for clarity.

References:

  1. 1.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 12, July 2012 [and more or less continuously updated since] (Accessed August 11th 2012)

  2. 2.http://www.mobot.org/MOBOT/research/APweb/welcome.html (Accessed August 11th 2012)

Resources:

  1. 1.Angiosperm Phylogeny Poster (Downloadable pdf, Accessed August 11th 2012)

  2. 2.ANGIOSPERMS: The flowering plants A chart of flowering plant families (Downloadable pdf, Accessed August 11th 2012)

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