Discuss the external factors that affect seed germination.

Explanation

External Factors:
a. Water: It activates the enzymes which digest the complex reserve foods of the seed. If the water content of the seed goes below a critical level, seeds fail to germinate.
b. Temperature: Seeds fails to germinate at very low and high temperature. The optimum temperature is `25^@C` to `35^@C` for most tropic species.
c. Oxygen: It is necessary for germination. Since aerobic respiration is a physiological requirement for germination most will germinate well in air containing 20% oxygen.
d. Light: There are many seeds which respond to light for germination and these seeds said to be photoblastic.
e. Soil Conditions: Germination of seed in its natural habit is influenced by soil conditions such as holding capacity, mineral composition and aeration of the soil.