The nominative ending shows that the word is the subject of a sentence. The genus name is a noun. These adjectives must match the genus name in gender, number, and case. These genitives often commemorate the first collector of a species. Many of these names in apposition are names for the plant in other languages taken over as specific epithets.
Generic names are nouns that can come from any source whatsoever.
In practice, they tend to come from a few main sources. This is probably the largest category of plant generic names. Probably the second largest category of generic names. Stearn's dictionary of plant names for gardeners. The best book in English for origins of plant names.