Typha are aquatic or semi-aquatic, rhizomatous, herbaceous perennial plants.:925 The leaves are glabrous (hairless), linear, alternate and mostly basal on a simple, jointless stem that bears the flowering spikes. The plants are monoecious, with unisexual flowers that develop in dense racemes. The numerous male flowers form a narrow spike at the top of the vertical stem. Each male (staminate) flower is reduced to a pair of stamens and hairs, and withers once the pollen is shed. Large numbers of tiny female flowers form a dense, sausage-shaped spike on the stem below the male spike. In larger species this can be up to 30 centimetres (12 in) long and 1 to 4 centimetres (0.4 to 2 in) thick. The seeds are minute, 0.2 millimetres (0.008 in) long, and attached to fine hairs. When ripe, the heads disintegrate into a cottony fluff from which the seeds disperse by wind.