Bad taxonomic keys -some su
warren_lamboy at QMRELAY.MAIL.CORNELL.EDU
Fri Sep 29 10:15:23 CDT 1995
OFFICE MEMO Bad taxonomic keys -some suggestions Date:28/09/1995
I have just begun using a taxonomic key to a group of flowering plants and am
dismayed by the persistent use of the couplet:
X. Plants with character states x, y, and z.
XX. Plants without the above combination of character states.
It has been at least 5 years since I used a taxonomic key with which I was not
already familiar, and I had forgotten how utterly dreadful couplets like the
above are to a newcomer to a taxonomic group.
One may rightly ask "What's the problem with a couplet such as the above? If
one spends the appropriate amount of time learning the definitions and
meanings of the terms used in the keys, then one should have no problems using
In theory, of course, this is quite true. A user of a taxonomic key must
invest some time in learning the key features of the taxa of interest. Even
if one does this, however, there are still problems with keys of the above
sort. First, one may not have all of the requisite characters available in a
particular specimen (this is hardly the fault of the maker of the key,
however!), or the state of the characters may be ambiguous on the specimen.
The latter is particularly true with one character that has been a bugaboo for
me every since I began working in plant taxonomy: extent and type of plant
hairs (to pick a character out of my hat). Such ambiguity is inevitable with
continuous characters that must arbitrarily be divided into categories for use
in taxonomic keys, without there being any clear dividing lines between them,
and with characters that change state with the seasons (such as plant hairs
that fall off or are rubbed or blown off during the growing season.).
The suggestion I am proposing, which is hardly original but which I think
needs reemphasis [the key that is giving me grief is only two years old], is
that the possible character states in the second part of the couplet be stated
explicitly (and not simply by stating "not x, not y, or not z"!). Although
the second part of the couplet may then be considerably longer than the first,
a user of the key would have some hope of keying out the specimen if some
character states are lacking. If one objects to having a long second half to
the couplet, then I recommend constructing a polytomous key, with more than
two choices for each entry (although some editors and reviewers would object
strongly to such a practice). A nice example of this that I am familar with
is the General Key on P. 3 of Norman C. Fassett's "A Manual of Aquatic
Plants", where there are 17 entries! I am sure there are other nice examples
of this type of key.
I am a key-maker myself, and I am aware of the difficulties involved in
putting into practice my suggestions. I think that we taxonomists need to be
as GENEROUS as possible to newcomers to taxonomic groups and construct keys
keeping in mind that many workers will not have all possible character states
in front of them for comparison at any one time and that some character states
are not as well-defined as those of us who have worked for years on a group
might imagine they are. And please, let's all agree to abandon the use of
vestiture or indument as a plant character! :-) Good.
USDA-ARS Plant Genetic Resource Unit
and Dept. of Horticultural Sciences
Geneva, New York 14456-0462
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