One of the greatest things about growing marijuana is getting to watch the entire life cycle of the plant all within the space of a single year or just three months. It's a long and sometimes arduous process to get the plant from an inactive seed to a massive plant, but the end result is something that you cannot quantify. Marijuana undergoes distinct changes at each of its distinct stages in life, which is a process that is mimicked in all sorts of animals and plants on earth.
Of course, all marijuana plants start out as seeds. These seeds are the product of the male plant fertilizing the female plant. As a general rule, these seeds are relatively impervious to anything you throw at them and will survive for quite some time. They are, however, susceptible to being "frozen to death." They become unusable if they've been exposed to temperatures below 20 degrees Fahrenheit for any length of time.
If you managed to keep your marijuana seeds from being frozen, the next stage will be called the sprouting stage (or germination). In this stage, the seed will have been exposed to moisture and light to coax it out of its shell. It doesn't necessarily need soil to sprout, but it will need some kind of nutrient value once it starts to grow. The germination phase is usually indicated by two cotyledon leaves appearing above the soil.
Once germination ends, the seedling stage begins. Here, you will start to see real leaf growth with the standard ridges you associate with marijuana. The cotyledon leaves that appear during the germination stage are round and quite unlike the final product. The seedling stage generally lasts between 3 to 6 weeks. During this period, the plant will start to develop a sturdier root system that really delves deep into the soil. It also sets the stage for the production of chlorophyll that will be a necessity later on.
Indeed, during the vegetative state, the plant starts to really take shape. The plant can grow as much as 2 inches per day during this stage, which is largely a result of its increased ability to absorb nutrients. You'll find that, during this stage, the plants will be responsive to changes in light. In fact, indoor growers can keep plants in a vegetative state for longer than usual just by manipulating the light pattern. At around the fourth week of the vegetative stage, the plants will start to show clear signs of their sex.
The final stage is referred to as the flowering state. At this point, the plants are mature enough to start reproducing. You can see this usually in the almost measurable quantities of resin that drip off the buds and leaves. This resin is actually full of the active ingredient in marijuana called THC. This is the point at which the marijuana is ripe for harvesting because the THC has achieved its highest levels. At that point, the life cycle ends and you will likely begin preparations for the offspring of that batch of marijuana.