The peduncle of the bunch and the pedicel of the berry exhibit the same anatomical structure as a young grapevine shoot. The growth of the grapevine berry is caused exclusively by the expansion of the cells from which the ovary is constructed; no neoformation of tissues occurs during this process. The superficial cell layers of the grapevine berry expand mainly in the tangential direction, whereas the cells of the deeper layers of the pulp expand in the radial direction (Fig. 46). During the ripening process of the berry, the dimensions of its cells no longer increase, but the cells become filled with liquid contents (cell sap) and their cellulose walls become extremely thin and delicate.
The skin of a mature grapevine berry consists of several layers of more or less flat, polygonal parenchymal cells with thicker walls than those of the pulp cells. Externally, it is covered by a single-layered epidermis and a very thin cuticle. In red and black grapevine cultivars, the cells of the epidermis and the skin cells immediately adjacent to it contain coloring matter; the number of colored cell layers of the skin in different cultivars varies from 1 to 6, rarely reaching 8.
At the boundary between the skin and the pulp of the berry lies a fine-meshed network of vascular bundles. The pulp of the berry is formed of large (300–400 μ in diameter), rounded parenchymal cells containing a very thin layer of parietal cytoplasm and huge vacuoles filled with cell sap. In the vast majority of grapevine cultivars, the sap filling the pulp cells is completely colorless; only in the so-called dyer grapes ("Teinturiers") and in certain American species is this sap colored red.
Simultaneously with the ripening of the berries, the fertilized ovules transform into seeds, whereby the embryo is formed from the egg cell through repeated cell division, the endosperm is formed from the secondary nucleus of the embryonic sac, and both integuments of the ovule, after a series of changes, transform into the outer coverings of the seed. The embryo is located in the lower, pointed part of the seed, which is called the "beak" (Fig. 47).
A normally formed embryo consists of two cotyledons folded together, between which the apical bud is concealed, a hypocotyl, and a short primary root; all these organs are constructed of meristematic tissue. The entire remaining cavity of the grapevine seed is filled with endosperm, the cells of which contain a very large number of aleurone grains and small spherical droplets of fatty oil. The aleurone globoids in grapevine are highly characteristic and reach exceptional sizes; they can fill up to two-thirds of the cell cavity.
The outer covering of the grapevine seed consists of three distinct coats (Fig. 48):
- The "inner" coat, immediately adjacent to the endosperm, represents the slightly modified inner integument of the ovule and consists of only three layers of cells.
- An extremely hard "middle" coat of the seed, formed from the outer integument of the ovule and consisting of two to three layers of radially elongated stone cells.
- An "outer" coat composed of several layers of parenchymal cells, which are very rich in tannin and starch grains.
At full maturity of the seed, its outer coat dries up completely and easily detaches from the underlying stone cells, which thus assume the function of direct protection of the seed against unfavorable external factors.
In longitudinal and transverse sections of a mature grapevine seed (Fig. 47), it can be observed that different parts of its hard coat have unequal thickness: this coat is thinnest at the chalaza and on the ventral side, where it forms two folds extending deeply into the seed and dividing the endosperm as if into three lobes. However, this hard coat creates significant difficulties for the germination of grapevine seeds. Under normal conditions of spring sowing in open ground, the primary root of the embryo emerges from the "beak" of the grapevine seed no earlier than 25–30 days. If, however, the hard coat is carefully removed from the tip of the grapevine seed, seed germination can be obtained as early as the 3rd to 10th day. In mass seed sowings, applying this painstaking method is rather difficult. To achieve the same goal, one can limit the treatment to a certain thinning of the hard coat on the beaks of the seeds, for example, by rubbing them with emery paper.
