Other characters that have been found of great value are those connected with the seed. The ability to use the seed characters, however, cannot readily be acquired except by the use of an illustrated manual and some experience in selecting the seeds, as they are quite variable on the same plant. The weight of the seeds differs in different species, but varies so much inside the species that it is not of much value from a systematic standpoint. In general, it may be said that the Labruscas have the largest and heaviest seed of our American grapes; Riparia has the smallest seed, with Aestivalis occupying an intermediate position. The size of the seeds in Aestivalis, however, is more noticeable on account of the small size of the fruit. The color-plate illustrates the different characters to be found in grape seeds and a study of this plate with the technical descriptions of the several species will show how important seeds become in classifying grapes.
Attention is called to the characters given by Bartram as distinguishing the European from the American species. The first difference that he mentions is in the shape of the fruit, that of the Vinifera being more or less oval and that of American species roundish or oblate. Recent technical descriptions of our American species give the fruit as spherical where the shape is mentioned at all. On the other hand it is known that most of the cultivated varieties of European grapes are oval. Does this mean that all of our cultivated American varieties which show oval berries, such as Isabella, Catawba, and others, contain Vinifera blood? It could not be said without careful study that this is true but it is certainly worthy of consideration. This point seems to have escaped the attention of our later-day botanists.
The sexual status of the grape has always been a source of misunderstanding. The earlier botanists spoke of American vines as dioecious, that is, bearing staminate and pistillate flowers on separate individuals. In this, as was noted on page 98, they were corrected by Bartram, in so far as American species were concerned, he stating that the vines of America were polygamous (showing staminate and hermaphrodite plants). Bartram did not presume to speak as to the sex of the flowers of the Old World grape. Later it was determined that the cultivated varieties of Europe were always hermaphrodite and that staminate forms were unknown. Engelmann[121] explains this so well and with such apparent satisfaction that we cannot do better than quote him here. “All the true Grape-vines bear fertile flowers on one stock and sterile flowers on another separate stock, and are therefore called polygamous, or, not quite correctly, dioecious. The sterile plants do bear male flowers with abortive pistils, so that while they never produce fruit themselves, they may assist in fertilizing the others; the fertile flowers, however, are hermaphrodites containing both organs—stamens and pistils—and are capable of ripening fruit without the assistance of the male plants. Real female flowers without any stamens do not seem ever to have been observed. Both forms, the male and the hermaphrodite, or if preferred those with sterile and those with complete flowers, are found mixed in their native localities of the wild plants, but of course only the fertile plants have been selected for cultivation, and thus it happens that to the cultivator only these are known; and as the Grape-vine of the Old World has been in cultivation for thousands of years, it has resulted that this hermaphrodite character of its flowers has been mistaken for a botanical peculiarity, by which it was to be distinguished, not only from our American Grape-vines, but also from the wild grapes of the old world. But plants raised from the seeds of this as well as any other true Grape-vine, generally furnish as many sterile as fertile specimens, while those propagated by layering or by cuttings, of course, only continue the individual character of the mother-plant or stock.” The accompanying plate shows various forms of grape flowers.
He further says in a foot-note: “These fertile plants, however, are of two kinds; some are perfect hermaphrodites, with long and straight stamens around the pistil, the others bear smaller stamens, shorter than the pistil which soon bend downward and curve under it; these may be called imperfect hermaphrodites, approaching females, and they do not seem to be as fruitful as the perfect hermaphrodites, unless otherwise fertilized.”
Beach[122] tested many of our cultivated varieties by sacking the clusters at blooming time and thus determined their capacity to fertilize themselves. From the data thus secured he divides them into four classes: 1st. Those that are able to fertilize themselves so that the clusters are perfect or varying from perfect to somewhat loose. 2d. Those in which the clusters are marketable, varying from moderately compact to loose. 3d. Clusters so loose as to be unmarketable. 4th. Those which are self-sterile or showing no fruit on covered clusters. Of 169 varieties tested, he found 38 belonging to the first class, 66 to the second class, 28 to the third, and 37 to the fourth.
Later it was found that the reason why certain varieties were self-sterile was on account of impotent and abortive pollen, the percentage of abortive pollen grains varying with different varieties and this percentage determining the degree of self-sterility. The upright or depressed stamen is not an invariable criterion of the condition of the pollen although it is usually. There are a few instances in which upright stamens bear impotent pollen but these are very exceptional. Munson made similar tests of vines of twenty-two[123] American species of vines secured from their habitats. In every case he found that they showed only two forms, the staminate vines and the self-sterile hermaphrodite, no perfect hermaphrodites being found. While of some of the species the number of vines tested was a half dozen or less, in most instances many vines were tested from different places. This is particularly interesting in that it becomes a puzzle as to where our perfectly hermaphrodite cultivated forms could have come from if such forms are not present in the wild vines of our woods and prairies.