Gramma schreef
Well yes, I did think there would be some kind of standard features of a
plant that *must* be included in a botanical drawing.
As there isn't , I have learned something.

***
I suppose that roughly speaking there are such features.
But only roughly speaking
* * *

What matters for the illustration is the important features of the
particular plant-species portrayed. Features which likely are unique to
that group of plants.

I assumed for the purpose I want, which initially is one illustration of
a chosen plant, that I would be able [that it would be correct to do so ]
to take a living plant and draw it from life.

***
Oh yes, do draw one from life.
But the angle from which you draw, the parts depicted and indeed the very
choice of specimen all depend on what you need to show. These must be
educated choices.
* * *

I may be phrasing that wrongly due to my lack of botanical vocabulary, but
would this be considered an incorrect botanical illustration because it
was a stand alone drawing without any comparison to others of the plant
species?

***
Stand-alone drawings are the norm.
Best advice is to look at a range of monographs, dealing with quite
different plants, such as trees with catkins, trees with "real" flowers,
several herbaceous plants and grasslike plants. That will convey the idea
better than anything anybody can say.
PvR