Roger Whitehead wrote:
And how hard it can sometimes be to identify plants from photos. So many
of the normal clues and cues are missing.

Indeed. Also, pictures often show just one or a few flowers (leaves,
stems, ...), and therefore one can never be sure if the thing shown is
typical for the specimen or species at hand (unless the photographer has
taken the trouble to add a note about that or take many pictures). This
is related to Pauls' observation yesterday, with respect to one of
Peter's pictures, that is was a pity that that pictures just showed one
flower in which floral parts could properly be counted.

As anyone who has ever used identification keys surely is aware, it
happens rather frequently to find, e.g., a few tetramerous flowers in
inflorescences that have otherwise tens of pentamerous flowers and stuff
like that. Some documented examples that illustrate such things can be
found below.

Kerria japonica: most flowers pentamerous, but tetramerous flowers can
be found occasionaly as well -
http://www.plantsystematics.org/cgi-...ank=bi nomial

Ptelea trifoliata: in the tree that I had a look at, most flowers were
tetramerous, but quite some pentamerous flowers could be found as well
-
http://www.plantsystematics.org/cgi-...ank= binomial

Paris quadrifolia: plants typically with four leaves, but specimens
with, e.g., five leaves can be found as well -
http://www.plantsystematics.org/cgi-...ank= binomial

Best regards

-- Jan