10. The genus. In the same way that those individuals which possess some special set of peculiarities constitute a variety, and just as there may be several varieties in which the individuals are enough alike to form a species, so different species possessing in common certain features of a more general nature are grouped into a genus (plural genera).1

1 The beginner can hardly be expected to grasp more than vaguely the distinctions here presented between genus, species, and variety; and the same may be true as well of certain other distinctions to be considered presently. His conceptions are likely to grow more definite, however, as his acquaintance with plants increases, and his efforts to gain such wider acquaintance will be much facilitated by starting with some conception, vague though it be, of what botanists mean by the groups of different rank which they distinguish.

The name of the genus to which a given species belongs appears as the first component of the botanical name. In the examples already mentioned Gypsophila, Morus, Quercus, Salix, or Brassica is the generic part; fastigiata, alba, nigra, rubra, aquatica, or oleracea, the specific part of the name. Both parts are required to form the name of the species. Alba, nigra, etc., by themselves are not names.