MMJ Spots

The main active ingredient of Marijuana is THC (?9-tetrahydrocannabinol) there are hundreds of cannabinoids that play a part but do not concern us here.
THC is produced by the female plant, no one knows quite why but glands (or trichomes) mainly on the flowering buds (leaves, stems also but not nearly as much).
Basically when the plant starts to flower it starts to produce thc. With the absence of pollen from the male plant the female is forced to produce more thc rather than spending energy creating seeds. The less seeds the more thc it will produce but it all depends how well (unstressed) the plant is grown and of course the actual strain. You cannot make a plant produce more thc than the genes of the plant will alllow.
A plant high in thc content will have no seeds in the buds (obviously it depends on the strain as to how much thc it will produce). This is where the genetics come in to play and different strains high in thc content will be crossed together producing offspring with an even higher thc content. This is one of the things seed banks aim for but they also try to create strains that do not grow to high which is for the benefit for indoor growers.
A plant covered in white crystals (trichomes) is high in thc.
A favourite topic of mine I could go on and on !