CBSE Class 8 Answered
The conditions required for combustion are:
Combustible substance, Oxygen gas which supports combustion and ignition temperature.
Ignition temperature is the minimum temperature at which substance catches fire.
Wood from trees is a combustible substance, also presence of oxygen supports combustion
but it does not catches fire due to the absence of minimum temperature required for burning of combustible substance.
Hence, trees does not catches fire on their own until it reaches its ignition temperature.