Plants and THC

When do plants start to get thc?
and
What is the reason the plant has the chemical THC? Is like for defence or does it make it stronger or something?

A complex, thorough insight into THC can be found here:

http://www.hempfood.com/IHA/iha01201.html

and an even better one here:

http://cannabisculture.com/articles/2159.html

damn HARDDON, your just a wealth of knowage.. nice links