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Taxonomic confusion

This message is hopefully only slightly off-topic, in that its not about
cultivating aquatic plants or about an algae problem, but I know that there
are a couple of botanists on the list and maybe they can shed some light on
a few questions I have.

For anyone _not_ interested in knowing anything about scientific
classification, I apologize....

I have been looking at a number of books and web sites which deal with the
taxonomy of plants and how scientists currently classify them and the
relationships between them. From the most recent version of the
International Code of Botanical Literature (St. Louis Code), I note that is
says, in part:

2.1. Every individual plant is treated as belonging to an indefinite number
of taxa of consecutively subordinate rank, among which the rank of species
(species ) is basic.

3.1. The principal ranks of taxa in descending sequence are: kingdom
(regnum), division or phylum (divisio, phylum), class (classis), order
(ordo), family (familia), genus (genus), and species (species). Thus, each
species is assignable to a genus, each genus to a family, etc.

4.1. The secondary ranks of taxa in descending sequence are tribe (tribus)
between family and genus, section (sectio) and series (series) between genus
and species, and variety (varietas) and form (forma) below species.

5.1. The relative order of the ranks specified in Art. 3 and 4 must not be

That all seems clear enough...... until I start looking at the actual plants
and different sources of information which are available.

Often, when discussing some closely related plants and trying to distinguish
between them, it appears that the concept of what constitutes a "species"
seems to break down. For example, in a lot of Cryptocorynes there appear to
be many intermediate "types" or variations that can make it very difficult
to say with any certainty exactly which "species" a particular plant belongs
to. The plant can't be wrong, so it must be our concept of "species" which n
eeds adjustment.

Turning from the very narrow focus of the "species" level, I have also been
looking at the way taxonomists have grouped related "families" of plants
together at the higher levels.

It seems that there is disagreement when you climb up the "family tree" and
arrive at the Class and Order levels of taxonomy - some books seem to use
quite different schemes to group subordinate families of plants together.
For example, in "The Families of Flowering Plants", found on the web at
http://biodiversity.uno.edu/delta/angio/index.htm, each separate flowering
plant family is described. The information listed includes alternative and
older names and gives a list of the Genera which belong to each Family. The
higher taxonomic classification of each Family is also given, often with
several alternative "family trees" cited, each one prefaced by the name of
the taxonomist who proposed it.

In several instances, when I try to match aquatic plants and the taxonomy
given for them in hobby related sources, it seems that some adjustments have
been made at the Family level - for example, the aquatic plants from the
Genus Lilaeopsis are said by some sources to be members of the Family
Apiaceae, yet in the above noted document that family is described as being
included in the Family Umbelliferae Juss.

The higher taxon placement of many plants seems in similar disarray. It
appears that there are several schools of thought on the matter, with some
texts following the arrangement set out by Dahlgren (1980, for Dicots) and
by Dahlgren, Clifford and Yeo (et al 1985, for Monocots) with others siding
with Cronquist (1981, for Dicots).

When I backtracked and looked up the source material (regarding Dahlgren and
Cronquist and their individual schemes) I found that not only is the number
of Families different, in many cases so is the name given to individual

Again, I understand that for actually growing the things in aquariums you
need not know this sort of thing, but for some of us it can add a further
level of enjoyment to the hobby to be able to understand how the plants we
grow relate to the wider world we live in.

Who is to be believed? I can understand that this sort of thing is fluid and
can only reflect current thinking and it is all possibly subject to change
as time goes on, but is there any resource which is considered "standard" at
the present time? Is any particular "scheme" (i.e. Dahlgren vs. Cronquist)
more authoritative that the others?


James Purchase