Cistaceae Cochlospermaceae Convolvulaceae Cucurbitaceae Dipterocarpaceae Geraniaceae Fabaceae Malvaceae Nelumbonaceae Rhamnaceae Sarcolaenaceae Sapindaceae Physical dormancy has been recorded in a few species of Cycadales with thick sclerotesta and slow embryo development, for example Cycas revoluta and Zamia floridana, but not in any of the other groups of extant gymnosperms. Generally, physical dormancy is the result of one or more palisade layers in the fruit or seed coat. These layers are lignified with malpighian cells tightly packed together and impregnated with water-repellent. In the families Anacardiaceae and Nelumbonaceae the seed coat is not well developed. Therefore, palisade layers in the fruit perform the functional role of preventing water uptake . While physical dormancy is a common feature, several species in these families do not have physical dormancy or produce non-dormant seeds. Specialised structures, which function as a "water-gap", are associated with the impermeable layers of the seed to prevent the uptake of water. The water-gap is closed at seed maturity and is opened in response to the appropriate environmental signal. Breaking physical dormancy involves the disruption of these specialised structures within the seed, and acts as an environmental signal detector for germination. For example, legume (Fabaceae) seeds become permeable after the thin-walled cells of lens (water-gap structure) are disrupted and pull apart to allow water to contact the seed. Other water-gap structures include carpellary micropyle, bixoid chalazal plug, imbibition lid and the suberised "stopper". In nature, the seed coats of physically dormant seeds are thought to become water permeable over time through repeated heating and cooling over many months or years in the soil seedbank. For example, the high and fluctuating temperatures during the dry season in northern Australia promote dormancy break in impermeable seeds of Stylosanthes humilis and S.hamata (Fabaceae). Generally, the weight of physically dormant seeds (e.g., Abrus precatorious) remains relatively constant over long periods of time, even under different environmental conditions (moisture, temperature) due to the impermeability of seed coat towards water and air. Indigenous people recognizing that the weight of Abrus precatorious seeds remains stable have used them as a weighing unit (Ratti). Mechanical dormancy Mechanical dormancy when seed coats or other coverings are too hard to allow the embryo to expand during germination. In the past this mechanism of dormancy was ascribed to a number of species that have been found to have endogenous factors for their dormancy instead. These endogenous factors include low embryo growth potential.