A group of flowering plant species is known to germinate in less than 24 h from imbibition, but this phenomenon is often overlooked in the current literature. Here, I review this topic by searching the literature published since 1967 and listing the 28 most detailed cases found. Of these, 20 are species of Amaranthaceae (all formerly treated as Chenopodiaceae); 15 of these are from the subfamily Salsoloideae, which is characterized by the possession of spiral embryos. The non-chenopods listed are small numbers of species from the families Acanthaceae, Cruciferae, Gramineae (one species) and Salicaceae (Populus and Salix). Seeds of the Salsoloideae contain fully differentiated embryos. On imbibition, the embryo cells elongate and the spiral embryo uncoils and ruptures the thin seed coat. This can occur in as little as 10 min. Nearly all of the families showing very fast germination have small to very small seeds and little or no endosperm. Most species have soft, thin seed coats that imbibe water readily. All are from high-stress habitats, either arid or saline or from active floodplains, where they can rapidly exploit temporarily favourable conditions for germination. They exhibit one of two contrasting germination behaviours, either having seeds which all germinate within a very short time of wetting or having seed persistence whereby small amounts of rain cause germination of small fractions of seed from a long-living soil seed bank. Serious confusion in the literature in the use of the term ‘opportunistic’ is pointed out and clarified.