PLANT NAMING (Linnaeum system)
Naming a plant by observation is incredibly difficult when in many cases there are hundreds of cultivars. Nurseries and professionals are constantly trying to improve upon old varieties and produce new hybrids, partly because people like new colours and forms, but of course mainly driven by competition and the need to increase profitability. Rose propagation is one of the oldest plants to be hybridised, and in this case augmented by the grafting process (to optimise root growth and produce healthy plants more quickly).
It goes like this: GENUS.....SPECIES.....FORM (f)......VARIETY(v)....(CULTIVAR)
Form usually just means a small difference like colour shade.
You can usually get to the Genus and species ok, but getting the right variety is almost impossible with a leaf or flower alone. You often need extras like growth habit/bark colour/twig colour and bud shape etc. So I am very hesitant to attempt variety naming.
Here is an example:
(It's pretty clear) that it is an Acer palmatum, but it could be
Acer palmatum f. atropurpureum OR
Acer palmatum v. 'Bloodgood'
So I think the best you will ever be safe with as an amateur gatdener Is to use just the Genus and Species names. At least you won't be wrong, even if you don't have the complete name. This applies to lots of plants, but Acers are a particularly good example of the problem because there are so many cultivated variations.
The naming conventions and classification world is in the process of massive change. For the last 20 years or so researchers have been analysing the DNA 'fingerprint' of plant material in order to provide a new and genetically based method for linking plants together. It is a very complicated process, and if you are interested you can start at Kew Gardens or The Linnean Society where much of the groundwork has been carried out. There are a lot more institutions researching this, and a web search produces lots of results. Organisations Like the RHS are slowly moving towards adopting the APG III (Angiosperm Phylogeny Group) system. This will eventually compliment and possibly replace the Linnaeum system, which is observation based. Some surprising links are being revealed, where plants are much more closely related than would be expected from observation.