Seed dormancy is defined as a state in which seeds are prevented from germination even under environmental conditions normally favorable for germination. In nature dormancy is a mechanism that allows various species to survive in particular environments and dormancy mechanisms assure that seed will germinate at the proper time. In some dormant seeds morphological changes must take place before germination can start. For others, parts of the seed must undergo physiological changes before germination can occur. Under natural conditions necessary changes take place gradually under varying combinations climatic. In general, there are two types of seed dormancy: seed coat dormancy and internal dormancy. Seeds with seed coat dormancy usually have a seed coat that is impermeable to oxygen and/or water. Under natural conditions these seeds remain on or in the ground without germinating until they have weathered sufficiently, to allow penetration of water, exchange of gases, or neutralization of inhibiting chemicals. Methods of breaking seed coat dormancy include scarification, light, hot water and acid. Internal dormancy is a general term encompassing a number of physiological conditions that delay germination. Common methods for breaking internal dormancy are after ripening, cold stratification. In some cases, the use of hormones can be substituted for part or the entire stratification requirement. Multiple dormancy factors also occur. In one general type there is seed coat dormancy plus internal dormancy. Seeds with this dormancy combination must be treated for the impermeable seed coat first, then for internal dormancy.