This is a collection of very similar species and forms the first part of a plant’s scientific name. For example Alchemilla in Alchemilla Mollis and as such it is very important in the naming of plants. Ideally it would be best to have a clear definition as to what constitutes a genus and the International Code of Nomenclature for algae, fungi, and plants goes into great detail as to how a genus should be named. It does not though make any attempt to nail down what actually constitutes a genus and for a very good reason – you can’t. In practice it would be impossible, plants evolve into genera in what ever way evolution takes them and only much later to people come along and try to group them into genera, species, etc. In the end a genus is a collection one or more species which a consensus has been arrived at that they should be placed together because of there botanical similarities.