Plants are more varied in the mode of reproduction than us mammals.
- Some flowering plants have 'perfect' flowers, which contain both male and female parts. (Tomato is an example)
- Some flowering plants are 'monoecious' - meaning they have male and female flowers on the same plant, but in different places (Corn is an example - the tassel at the top is male, the ear is female)
- Some flowering plants are 'dioecious' - meaning they have male flowers on one plant, and female flowers on another (Spinach is an example)
- Some plants can also reproduce asexually, from cuttings, tubers, or rhizomes.
- Some plants can express multiple modes of reproduction - cannabis is one of these!
Depending on the genetic background, cannabis can be monoecious or dioecious, and can also be propagated by cuttings (clones).
For those unfamiliar with the reproductive organs of cannabis, here are a couple of photos of young male and female flowers.
The most interesting cannabis product in terms of THC/CBD production is the unpollinated female flower bud - sinsemilla (spanish for 'without seeds') - so people avoid growing male plants and monoecious plants, both of which would reduce the quality of the buds by fertilizing the female flowers and producing seeds.
Sex is (mostly) genetically controlled in cannabis, so if you pollinate a female with a male, just as with people, about half of the offspring will be male, and half female. This is fine for seed production, but a pain for cannabis growers using seeds, as half of the seeds will be both unproductive (no female flowers) and in fact will threaten their sinsemilla crop with pollen.
Fortunately for cannabis producers, cannabis is also quite flexible in its sexual expression. If you alter the hormonal signalling in the plant, you can over-ride the 'normal' (genetically determined) sexual phenotype. In this way, a genetically female plant can be tricked into producing pollen. And when you pollinate a female plant with female-source pollen you get ..... 100% female offspring rather than 50% male. This is how feminized seed is produced. This trick has been used for some time in hybrid seed production of cucumber and other cucurbits.
Why do the plants do this? One possible story: It is possible that this evolved as a mechanism for plants to reproduce in otherwise bad conditions. Imagine you are a genetically female cannabis plant, growing in a stressful place (not enough water or food, too much competition). It takes a lot of energy to produce seeds, and clearly the place the plant is growing is not an ideal place to have kids. So instead, the plant will produce pollen, which will be carried on the wind for miles, potentially contributing to the next generation, and maybe finding a more fertile place to grow.
What this also means is that, whether they are growing regular or feminized seeds, cannabis producers need to be careful to monitor their crop and check for stressed-out plants that might be considering pollen-production as a back-up plan.