A tumbleweed is a structural part of the above-ground anatomy of a number of species of plants, a diaspore that, once it is mature and dry, detaches from its root or stem, and tumbles away in the wind. In most such species, the tumbleweed is in effect the entire plant apart from the root system, but in other plants, a hollow fruit or an inflorescence might serve the function. Tumbleweed species occur most commonly in steppe and arid ecologies, where frequent wind and the open environment permit rolling without prohibitive obstruction. Apart from its propagules (that is, its seeds or spores), the tissues of the tumbleweed structure are dead; their death is functional because it is necessary for the structure to degrade gradually and fall apart so that the propagules can escape during the tumbling, or germinate after the tumbleweed has come to rest in a wet location. In the latter case, many species of tumbleweed open mechanically, releasing their seeds as they swell when they absorb water. The tumbleweed diaspore disperses propagules, but the tumbleweed strategy is not limited to the seed plants; some species of spore-bearing Cryptogams such as Selaginella form tumbleweeds, and some fungi that resemble puffballs dry out, break free of their attachments and are similarly tumbled by the wind, dispersing spores as they go.