Germination is an amazing event. It is when a potential new plant emerges from the seed - that seemingly dead organ which contains the embryonic plant. However, seeds are not dead but dormant. They wait for the right conditions of light, temperature and water to begin the process of germination.
Germination is triggered by different factors in different plants. For temperate plants shedding seeds in autumn, it would not be good for seeds to germinate straight away because plants would be young and vulnerable going into winter, the coldest part of the year. So for temperate plants, many seeds have inhibitors within their seed coats which wash away over time to allow water in. These inhibitors may be waxy products which block the pores in the seed coat or they mat be chemicals like ascorbic acid which actively prevent germination processes taking place. Many seeds have physical dormancy which means they have a thick seed coat to protect them from damage when passing through the gut of an animal or for them to get through winter and germinate in spring.
Germination itself takes place when the seed imbibes water. A seed may swell to over twice its original size. Water is the substance which starts enzymic reactions within the endosperm of the seed and which leads to the germination process. The hypocotyl, which is the embryonic shoot, pushes its way up and the radicle , the embryonic root, pushes its way down.
Seeds have either one or two embryonic leaves and this is used ot classify plants as monototyledons (one seed leaf) or dicotyledons (two seed leaves).
Either the seed leaf emerges in the soil and has to push its way towards the surface in which case this is hypogealgermination or the seed leaves emerge above the ground on a shoot . This is epigeal germination.
Most plants germinate either by epigeal or hypogeal germination but there is a third type of germination which occurs in some tropical plants. Here, 1 seed leaf grows above the ground . This leaf, however is not a seed leaf or cotyledon. The cotyledon itself remains below the ground. This first leaf above the ground has as its purpose to protect the new emerging leaf which follows. It often has an umbrella like appearance and is attached to a stalk It is green and can photosynthesise and enables the plant to gather energy for emergence and,more importantly, replacement in case of damage, of the actual first true leaves. This type of germination is called cleistogeal germination.
All seeds will germinate only when conditions are right. Some are scattered near the parent plant and discouraged from germinating by chemicals exuded from the parent plant's roots. However, if the parent plant dies, dormant seeds will germinate as the inhibitors are no longer present. Light plays an important part in germination as well and some seeds such as tiny seeds of begonia require high light for germination as well as higher temperatures whilst some larger seeds (which would normally be deposited in dung and have to push up to the surface) are inhibited from germination by light.
Woodland seeds as fascinating because for many of them to germinate, they require far red light. In summer with tree canopies in place, the trees interrupt the far red light , preventing it from reaching the ground but in autumn and spring when the far reds light reaches the ground and the trees do not have leaves, or if a tree falls, leaving a gap, the seeds get far red light wavelength and are stimulated to germinate, thus filling in gaps in the flora of the wood.
Seeds are fascinating organs, not dead but dormant just waiting for the right time to germinate. It is wonderful to think that in a tiny seed is contained all the genetic information, embryonic parts and potential for it to become a tiny plant or even a giant tree.