The time of bloom is an easy mark of distinction between several species of grapes and helps to distinguish varieties in a species as well. Most species of grapes bear fertile flowers on one vine and sterile flowers on another and are, therefore, polygamous-dioecious. Sterile vines bear male flowers with abortive pistils so that, while they never produce fruits themselves, they usually assist in fertilizing others. Fertile flowers are capable of ripening fruits without cross-pollination. Vines with female flowers only are seldom found. In most species of the grape, plants with sterile flowers and those with complete flowers are found mixed in the wild state, but usually only the fertile plants have been selected for cultivation. Plants raised from seeds of any of the species, however, furnish many sterile vines. [Illustration: FIG. 53. The grape flower. I. Opening bud showing the way in which the cap becomes loosened at the base. II. Diagrammatic illustration of grape stamens.] The degree of fertility of blossoms is also a fine mark of distinction in species and varieties of the grape. Fertile vines are of two kinds in most species. The flowers on one kind are perfect hermaphrodites, while in the other kind the stamens are smaller and shorter than the pistil and eventually bent down and curved under. The two kinds of stamens are shown in Figs. 53 and 54. These may be called imperfect hermaphrodites since they are seldom as fruitful as the perfect hermaphrodites unless fertilized from another plant. Examined with a microscope, it is found that self-sterile plants usually bear abortive pollen and that the percentage of abortive pollen grains varies greatly in different varieties. The upright or depressed stamen does not always indicate the condition of the pollen, since there are many instances in which upright stamens bear impotent pollen and occasionally the depressed stamens bear perfect pollen. [Illustration: FIG. 54. Grape flowers. _Left_, upright stamens of Delaware; _right_, depressed stamens of Brighton.] _The leaf._ _Blade_: The expanded portion of the leaf. _Lobe_: The more or less rounded division of the leaf. _Sinus_: The recess or bay between two lobes. _Petiole_: The leaf-stalk. _Petiolar sinus_: The sinus about the petiole. _Basal sinuses_: The two sinuses toward the base of the blade. _Lateral sinuses_: The two sinuses toward the apex of the blade.