Biology
Vitis is distinguished from other genera of Vitaceae by having petals which remain joined at the tip and detach from the base to fall together as a calyptra or 'cap'. The flowers are unisexual or modified to act functionally as unisexual, they are pentamerous with a hypogynous disk. The calyx is greatly reduced or nonexistent in most species and the petals are joined at the summit into one unit but separated at the base. Flower buds are formed later in the growing season and overwinter for blooming in spring of the next year. There are two types of flowers produced, sterile flowers with five long filaments and erect stamens with undeveloped pistils and fertile flowers with well-developed pistils and that have five undeveloped reflexed stamens. The fruit is a berry, normally produced with four or less per flower by way of aborted embryos, ovoid in shape and juicy.
In the wild, all species of Vitis are normally dioecious, but under domestication, variants with perfect flowers appear to have been selected.
Most Vitis species have 38 chromosomes (n=19), but 40 (n=20) in subgenus Muscadinia. In that respect the Muscadinia are the same as other Vitaceae such as Ampelocissus, Parthenocissus, and Ampelopsis.
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