Anatomy of the Stamen and Microsporogenesis
The staminal filament is covered externally by an epidermis with a thin cuticle, while its interior consists of parenchymal tissue. A vascular bundle extends along the central axis of the staminal filament, reaching the anther. A young anther (Fig. 35) consists of a connective, flanked on the right and left by two pollen locules. However, upon the opening of a mature anther via longitudinal slits, the transverse septa between each pair of locules rupture, so that the opened anther contains only 2 pollen sacs instead of 4. The connective shares the same anatomical structure as the staminal filament. A cross-section through the wall of a very young anther reveals that it is covered by an epidermis with a thickened outer wall, adjacent to which on the inner side are 2–3 layers of small parietal cells and 1 layer of flat cells known as the "tapetum" (Fig. 36).
With age, the layers of the anther wall undergo various modifications, so that by the time of anthesis, its wall consists of only a single layer of enlarged cells, bounded on the outer side by a faintly visible epidermis. The layer of tapetal cells is completely destroyed by this time, as it serves to nourish the pollen mother cells, from which the mature pollen is formed through a series of transformations.
The pollen mother cells (Fig. 36) are large, polygonal cells closely adjacent to the tapetum in three layers along the inner surface of the anther. Gradually, the pollen mother cells round off and become more or less isolated from one another. Concurrently, reduction division of the nuclei takes place within them, and immediately following the formation of the daughter nuclei, the second homeotypic division begins. As a result of the second division, four microspore cells are formed within each mother cell, establishing the so-called "tetrads." In the tetrad stage, the nuclei of the microspore cells are always arranged in a specific order: 3 nuclei in a triangle, with the 4th either above or below (Fig. 36). After the breakdown of the mother cell wall, the tetrads dissociate into individual microspores.
The microspores increase in volume, assume a rounded shape, and become enclosed in an outer wall ("exine"). A fully formed microspore possesses a large nucleus and highly vacuolated protoplasm with one large and several small vacuoles. Subsequently, nuclear division occurs within the microspore, resulting in the formation of 2 nuclei: a larger "vegetative" nucleus located in the center of the cell, and a smaller "generative" nucleus lying near the microspore wall. Following this, the generative nucleus is separated from the rest of the microspore cavity by a thin wall, forming a small, elongated generative cell. The formation of the generative cell completes the process of microspore transformation into a pollen grain ready for fertilization.
Pollen development in "female" grapevine cultivars proceeds completely normally up to the formation of tetrads. In some cultivars, degenerative processes affect the nuclei of mature microspores even before their division, while in others, the generative and vegetative nuclei degenerate shortly after their formation (Fig. 33). In most cases, the generative nucleus is the first to degenerate, though the reverse is occasionally observed. Very rarely, pollen grains are found that retain a normal vegetative nucleus, but the generative nuclei are invariably destroyed.
Anatomy of the Pistil, Ovule, and Megasporogenesis
The stigma of the grapevine flower is constructed of rod-shaped and swollen cells loosely connected to one another, which makes the upper surface of the stigma appear papillose. The style is covered by a thin-walled epidermis and mainly consists of small-celled parenchymal tissue, in which giant raphide-containing cells are very frequently encountered. In the central part of the style, the cells are loosely arranged, leaving large intercellular spaces that form a channel-like structure, facilitating the penetration of pollen tubes to the ovules. The ovary walls and the septum between the locules are also constructed of parenchymal tissue and are lined externally and internally by an epidermis. The ovary contains vascular bundles, numbering 15–18. Approximately at the level of the nectaries, each bundle splits into two branches: one directed straight into the style along the inter-locular septum, and the other deflecting sideways into the ovary wall, where it breaks up into a fine network of nerves. At the level of the style base, these two branches merge again into a single bundle. Smaller vascular bundles also branch off to each of the ovules, each stamen, the sepals, and the petals (Fig. 37).
The ovule possesses two integuments—outer and inner (Fig. 38). The vascular bundle entering the ovule through its funiculus bypasses the nucellus from above and terminates in the chalaza. Along the long axis of the ovule lies the embryonic sac, surrounded by the nucellus tissue.
The development of the embryonic sac in grapevines occurs identically across all cultivars (Fig. 39). The archesporial cell is initiated quite deeply within the tissues of the young ovule. Initially, a reduction division takes place in the archesporial cell, resulting in 2 daughter cells. These cells divide in turn, forming 4 megaspore cells arranged in a vertical row one above the other. Immediately after formation, the three upper megaspores begin to degenerate, flattening under the pressure of the expanding lower megaspore, and soon become completely unnoticeable. Consequently, only the lower megaspore becomes the mother cell of the embryonic sac.
Within this cell, three nuclear divisions take place in succession, without cell wall formation. After the first division, the two nuclei migrate to opposite ends of the mother cell (the two-nucleate stage of the embryonic sac) and there divide twice in succession (the four-nucleate and eight-nucleate stages of the embryonic sac). From the three nuclei at the chalazal end of the embryonic sac, three small antipodal cells are formed, while the fourth, the so-called "lower polar" nucleus, remains free and moves toward the center of the embryonic sac. The three nuclei at the micropylar end of the embryonic sac form the egg apparatus, consisting of three cells: the egg cell and two synergids. The fourth "upper polar" nucleus moves toward the center, where both polar nuclei fuse into a single "secondary" nucleus of the embryonic sac. In this state, the embryonic sac is ready for fertilization. The antipodals in grapevines exist for a very short duration and disintegrate shortly after the fusion of the polar nuclei.
