The blooming of the vine had little significance to the grape-grower, the blooming period being so late that grapes are seldom caught by frost, until the discovery was made that many varieties of grapes are unable to fertilize themselves, and that failure of crops of these varieties was often due to the self-sterility of the variety. Until this discovery, the uncertainty attending the setting of the grape in these varieties was one of the discouragements of grape-growing. Following investigations of the self-sterility of the tree-fruits, an investigation of the grape showed that the vines of this fruit are often self-sterile. This knowledge has in some degree modified the planting of all home collections and has more or less affected the plantings of commercial sorts.

Varieties of American grapes show most remarkable differences in the degree of self-fertility. Many sorts fruit perfectly without cross-pollination. Others set no fruit whatsoever if cross-pollination is not provided for. Most varieties, however, are found in groups between the two extremes, neither self-fertile nor self-sterile. [Figure 51] shows staminate and perfect clusters on one vine. Some varieties show no variation in the degree of self-sterility or self-fertility; others behave differently in regard to these characters under different environment. Now and then the widest variations are to be found in a variety in respect to self-fertility.

Fig. 51. Staminate and perfect clusters on one vine; right, staminate; left, perfect.

Following the lead of Beach at the New York Agricultural Experiment Station, several workers have made careful studies of the self-fertility of the grape, and now the cultivated varieties of native grapes are divided into four groups in accordance with the degree of self-fertility. Class I includes self-fertile varieties having perfect or nearly perfect clusters; Class II includes self-fertile varieties having clusters loose but marketable; Class III includes varieties which are so imperfectly self-fertile that the clusters are generally too loose to be marketable; Class IV includes self-sterile varieties. The following is a list of commonly cultivated grapes classified according to the divisions just given:

CLASSIFICATION OF GRAPES ACCORDING TO SELF-FERTILITY

Class I. Clusters perfect or varying from perfect to somewhat loose.