With this brief history of the formation of the genus Vitis as it now stands we pass to a consideration of the botanical characters of Vitis.
From the time when botanists first commenced to work on the grape there has been a constant search for taxonomic characters for separating the various species clearly and distinctly. Many of the earlier descriptions, while they are correct so far as they go, do not mention enough characters to enable one to distinguish between similar species. It has been found that dependence upon the shape of the leaf, size of berry, size of plant, date of ripening, and similar characters, is very uncertain and unsatisfactory and that, while these characters are always mentioned in descriptions as indicating the intrinsic value of a species, they are of little value from a systematic standpoint. There are, however, several characters of Vitis which have great taxonomic importance.
One of the fundamental characters which determine a species is continuous or intermittent tendrils, first noticed by Professor A. Braun[120] of Berlin. Vitis labrusca, the common Fox grape, is peculiar in that there are tendrils, or an inflorescence, opposite nearly every leaf; this arrangement is known as continuous tendrils. All other species have two leaves with a tendril opposite each and a third leaf without a tendril; such species are said to have intermittent tendrils. Continuity of tendrils is a variable character and to ascertain which of the two forms is present it is necessary to have vigorous, healthy, typical canes. The lowest leaves of canes usually have no opposite tendrils. This character is shown in the color-plates of the several species.
A closely related character is that of the number of inflorescences borne by a species. All species excepting Vitis labrusca average two inflorescences to the cane but the last named species, at least in some of its subdivisions, may bear from three to six inflorescences, each of course in the place of a tendril opposite a leaf.
Professor Millardet of Bordeaux first called attention to the value of that part of the cane known as the diaphragm as a means of distinguishing species. The cane of the grape vine contains a large pith, and in most species this pith is interrupted by woody tissue at the joints; this woody tissue is the diaphragm. The presence or absence of the diaphragm and its thickness are of taxonomic value. In Rotundifolia, the southern Fox grape, the diaphragm is absent; in Riparia, the Riverbank grape, it is very thin; in Rupestris it is slightly thicker; while Cordifolia, Aestivalis, and Labrusca have thick diaphragms. This character is studied best in the year-old canes of the grape. The color-plate of canes shows the range in thickness of diaphragms as they occur in several species.
The time of flowering is of considerable value in distinguishing species. Unfortunately it requires live plants and a certain time of year in order that this character be noted. The first American species to flower is Riparia. Rupestris flowers shortly after; next, Labrusca; Aestivalis a little later, although the Lincecumii variety of Aestivalis blooms slightly before Labrusca; Cordifolia is very late in coming into bloom, and Cinerea still later. Vinifera, the European grape, blooms shortly after Labrusca. The cultivated offspring of all wild grapes retain the blooming characters of the species from which they are derived.