The seed germination is affected by both the external as well as the internal factors. The important external or physical factors are:
a. Water, b. Oxygen, c. Temperature, d. Light.
Water: The proper amount of water is essential for the seed germination. As the water is absorbed by seeds, the enzymes get activated which will direct the germination process. That is moisture will trigger the mechanism of germination. The hydrolytic enzymes will then break down the stored food reserve which will help the cells of the embryo to divide. The embryo will be swells up and which will breaks the softened seed coat so that the seedlings will emerge out and is known as the seed germination.
Oxygen: Light is a factor that mediates the process of germination. It also acts an important factor that triggers the germination process. Some of the seeds will not germinate until the obtain sufficient amount of light. Whereas some others have hindering effects on the seed germination.
Temperature: Each seeds have minimum and maximum temperature at which the seed germinates. The light requirement in the form of alternating temperature helps in germination of seeds. When this alternating light is not received when the seeds are deeply buried in the soil it thereby prevents the germination of the seeds.
Light: Most of the plants require light for seed germination especially for the small seeded annuals. The light mediated germinating seeds have the advantages. In them the seed germination can be prevented when they are placed in the shaded region and also when they are deeply buried in the soil.