As with most useful plants, there are also different varieties of the grapevine, which in viticulture are called grapevine varieties (cultivars). The morphological description is contained under the keyword vine. The science of describing grapevine varieties is called ampelography (grapevine varietal science). Botanical taxonomy (hierarchical systematics based on relationships) is described under vine systematics. Over the course of thousands of years, thousands of new grapevine varieties have been created through spontaneous (natural) crossing and sowing, random mutations and, above all, from the 19th century onwards, through targeted new breeding. Primarily, they always originate from fertilised nuclei that have reached the soil or from targeted nucleus sowing, i.e. they are products of generative (sexual) reproduction.