There is a third species of Marijuana aside from Indica and Sativa known as "Rudaralis".
This species contains no THC, and is useless medically as far as I know, except in one way.
have bred Rudiralis plants with Sativa and Indica plants to make
strains that can flower, without ever going into a flowering cycle.
plants "veg" on a 24hr cycle (meaning never turn the lights off) or
18/6 (which means 18hrs with lights on, 6 off), and then must be put in
flower for 50-80 days for a "flowering" cycle, which is 12/12 (12hrs on,
12 off), strains bred with Rudiralis and one of the others only need
about a month to flower, and don't have to be switched into flowering,
they will flower while in their "Veg" cycle.
These are called "Autoflower" strains, and are good for new growers. Some even have pretty high THC contents :D