Horticultural Varieties of Citrus
In general appearance and other respects, the citrus fruits of principal commercial importance fall into four, reasonably-well-defined horticultural groups: the oranges, the mandarins, the pummelos and grapefruits, and the common acid members. The common acid group includes three subgroups the citrons, lemons, and limes. While the writer's competence does not extend to all the citrus fruits that have horticultural importance, the considerable number with which he is acquainted all exhibit horticultural resemblances with one or more of these groups and subgroups that suggest some degree of relationship. In most instances, it is not difficult to determine the group of closest resemblance and probable or possible relationship. Therefore, in this treatment, for each of the natural groups presented there is a subsection covering fruits of horticultural importance that most closely resemble the group in question. In some instances, however, lack of first-hand acquaintance with a fruit has necessitated provisional placement.
In addition to the fruit groups mentioned above, all of which belong to the genus Citrus, there are the kumquats, which belong to the closely related genus Fortunella, and the so-called but much more distantly related trifoliate orange, Poncirus trifoliata (L.) Raf. The kumquats comprise a group of considerable importance for their fruits. The trifoliate orange, together with its hybrids, is of significance as a rootstock.