There are fifteen interrelated families of alismatid monocots, a group of flowering plants named after their largest order, Alismatales. Like other monocots, they have a single embryonic leaf (cotyledon) in most of their seeds, and are generally characterized by leaves with parallel veins, scattered vascular systems, flowers with parts in groups of three or multiples of three, and roots that can develop in more than one place along the stems. The alismatids were the first species to diverge from the other monocots, occurring during the Cretaceous period. Like the earliest monocots, many of them are aquatic, and some grow completely submerged. Apart from the sweet-flag family of wetland plants, which form the order Acorales (species pictured), all alismatid families are in Alismatales. (Full list...)