Marijuana Cultivation/Fundamentals/Parts of the Plant
Parts of the Plant 
This section is added for the curious who don't know much about marijuana and how it is used. The entire plant contains the psychoactive ingredients which include Delta-9 THC and many cannabinoids that give each plant its own unique profile of properties and effects. However, not all parts contain them in the same concentrations.
The flowers or Buds of the plant contain the highest concentrations of plant resin which in turn contains the highest concentration of active ingredients. The unpollinated flowers of the female plants are the part of the plant that is used by recreational users because they produce copius amounts of potent plant resin in order to try to catch pollen. Marijuana is one of the few plants today that actually is split by gender and has distinct separate male and female plants. Female plants do not pollinate themselves in the absence of males but do contain the genetic capability to produce male flowers and may do so under stress. There are also hermaphrodite plants that produce both male and female flowers. Male plants and male flowers are not very potent and are not used recreationally
Leaves are generally not used recreationally and will generally give you a headache rather than get you high. There are however ways to extract the active ingredients from leaves and the different types of leaves have different potencies.
Large Shade Leaves 
These are the large classically shaped leaves on the plant and are the least potent. There are extraction methods that can be performed to get something useful out of these but the result won't be very pure and many growers just throw them away.
Grow Tips 
These clusters of small tender leaves are the point on the plant where new growth sprouts from during vegetation. They are more potent than Shade leaves but less potent than trim leaves or bud.
Trim Leaves 
These are the sugar coated leaves that are trimmed from around the buds during harvest. The sugar coating of trichomes can make these quite potent and they can be as potent as low quality flowers with seeds.
The stems are not really useful for psychoactive purposes but they are a great source of plant fiber for rope, paper, etc.
The seeds generally contain only trace amounts of psychoactive ingredients but they are one of the most nutritious foods currently known to man. They are one of only a handful of substances that man can sustain off indefinitely with no other food and provide all known amino acids.
Trichomes are the oil glands of the plant and contain the highest concentration of active ingredients of any part of the plant. The flowers are sought out because they contain the highest concentration of trichome and hash is made from collected and pressed trichomes. These are very useful for determining the maturity of a plant during the flowering stage.
The purpose of the roots is served when the plant is harvested. They have no useful psychoactive ingredients and aren't typically eaten.