Not as above

[Mike Crisp]

Let's clarify this discussion with some real examples.  These are taken
from the last key I wrote, to 121 species and 24 subspecies comprising the
genus Daviesia (Fabaceae). As you would appreciate, writing a key to a
large genus with many closely related species is difficult, and it is
necessary to use combinations of characters.  It is also necessary to use
the dreaded 'not as above' in some places, even though this is undesirable.
Even so, I managed to get away with only 7 instances in 150 couplets.
Here thy are, with the reasons I used 'not as above':

In the first two couplets, I only used 'not as above' for only 1 character
out of 2.  Thus it is possible to key the specimen without considering 'not
as above'.  In both couplets the characters contrasted with 'not as above'
are very distinctive, and not confusable with the various alternatives.  It
is not necessary to spell the latter out, which would require a paragraph.

12(7)   Unit-inflorescence umbelliform; flowers subtended by an involucre of
        bracts 3-20 mm long; bracts enlarging considerably to enclose pods,
        becoming scarious or coriaceous (D. cordata group)
18

        Unit-inflorescence not umbelliform, or if so, then bracts not as
above  13

141(137)  Phyllodes rhombic with conspicuously reticulate venation; flowers
          in axillary clusters                                       D.
rhombifolia

          Phyllodes not as above; flowers solitary (rarely 2) in each axil
142

The species in couplet 27 was very difficult to key out.  It has not unique
character, but combines those in a group of closely related species.  The
simplest way to deal with it was to pull it out first using its unique
_combination_ of characters.  If I had not done so, I would have had to key
it out in several different places.

27(26)  Plants prostrate or procumbent; phyllode apex acuminate or cuspidate,
        semi-pungent; inflorescence a 2-4(-5)-flowered umbel; pedicel dilated
        upwards, with an annular thickening at the apex               D.
lancifolia

        Not the above combination
28

The following are species with distinctive (in 2 cases bizarre)
autapomorphic features.  Nothing following in the key looks anything like
them.  It is pretty obvious if you have 'not as above'.  Also, some of the
distinctive features in all species are vegetative, which means that you
are guaranteed to be able to observe them.  In the case of the sequence 132
to 134, I was picking off the odd-looking species before tackling a large
group.  If I had described all the alternatives to 'not as above', I would
have written an essay.

79(76)  Branchlets erect, 6-10 mm diam., appearing succulent but filled with
        pith, glaucous to pruinose; phyllodes reduced to recurved spines 3-5 mm
        long                                                        D.
euphorbioides

        Not as above
80

132(128)  Untidy plant with zigzag branchlets; phyllodes spreading widely
          or retrorse at base but curving round so that the apex points
          forward, narrowly elliptic or obovate, with margins longitudinally
          folded down to more or less conceal the abaxial surface, with midrib
          obscure or absent                                             D.
tortuosa

          Not as above
133

133(132)  Phyllodes more or less opposite, ovate to orbicular, cordate, with
          prominent reticulate venation; bracts conspicuous, more or less
          orbicular, 4-5 mm diam. in flower, enlarging to c. 2 cm when enclosing
          fruit                                                        D.
crenulata

          Not as above
134

134(133)  Phyllodes forming dense fascicles terminating each seasonal shoot,
          conspicuously pilose, linear, broadest at apex and tapering to base,
          2-4 cm long                                                 D.
croniniana

          Not as above
135

To sum up, I use 'not as above' sparingly, but sometimes it simplifies
things a lot.  In particular, I try to avoid enumerating a long list of
alternatives which would make leads too long.  One of the great
difficulties I find in using some keys is excessive verbosity.  A key is
not meant to be a set of descriptions.  Using only 1 character in a couplet
(see Laferriere's example below) is risky - the specimen may not show that
feature, or a simple mistake will more easily lead the user astray.

On Fri, 1 Sep 1995, Mike Crisp wrote:

>> Richard Jensen says:
>>
>> >I disagree with Murray Fletcher's view that any couplet having "not as
>> >above" is a poor couplet.  Some (many, most?) taxa are polythetic entities
>> >or are especially variable in individual characters - a simple way to
>> >isolate them in a key (rather than following them through all possible
>> >leads) is to specify the combination of characters that allows
>> >identification.  As long as no other taxon in the key has that precise
>> >combination of features, then the couplet works, and that's one key to a
>> >good key.
>>
>> I agree.
>>
>> I use 'not as above' frequently for taxa that have obvious diagnostic
>> characters but otherwise have nasty combinations of characters that don't
>> allow them to fall into either group of a major split I want to make
>> further down in the key.  If a taxon is very distinctive, either by virtue
>> of its autapomorphic characters or, as R.J. says, a unique combination,
>> then the meaning of 'not as above' is clear.

Joseph Laferriere replied:

>I am afraid I disagree with both of you. This "not as above" method of
>writing keys may make them easier to write, but it makes them extremely
>difficult to use. It is sometimes necessary to do this, but it should be
>done only as a last resort. I have seen long keys to complex groups
>composed solely along the "not as above" method, which require inhuman
>patience to use.

On Tue, 5 Sep 1995, Richard Jensen wrote:

>> Well, let's consider this.  I have a group of species some of which have
>> leaves densely pubescent on the abaxial surface and some of which are
>> distinctly obtrullate in outline.  Only one species has both characters,
>> so I argue that the following is a good couplet:
>>
>>       A. Leaves obtrullate and densely pubescent abaxially.....species X
>>       A. Leaves not as above
>>
>> Yes, I could construct a key with species X keyed under both
>> "characters," but the above couplet simplifies the key.
>>

Joseph Laferriere replied:

>I would very much prefer the following:
>
>A. Leaves densely pubescent abaxially
>     Leaves obtrullate    ......   X
>     Leaves not obrtullate .....   Y
>A. Leaves not densely pubescent abaxially
>    Leaves obtrullate ..........    Z
>    Leaves not obrtullate ......    A
>
>Doing it your way is infinitely more confusing. I have seen keys with 6
>or 8 characters strung together in your kind of a key, with the user
>being required to match not one but all before being able to proceed to
>the next leg of the key. Besides, I do not like the "obtrullate" vs. "not
>obtrullate" choice. Give the reader more info on what the second
>alternative is. Say "obtrullate" vs "linear" or something. This will save
>the reader from having to decide on something which might be sort of
>obtrullate but not quite matching the textbook definition of the term.