Calyx
Also known as bract, this tear-shaped organ is responsible for protecting the flowers. Easier to perceive when the buds are well-formed, they are considered specialised leaves that can attract pollinating insects with their colours, which can be as vivid as that of the flowers' petals. Pollination represents a risk to the calyx, as once it occurs, the structure becomes an ovary with the sole task of carrying a seed.
The problem here is that the flowers are not able to realise their full potential, resulting either in compact, overly hard clusters comprised of tiny calyxes - especially in the case of indicas - or in larger calyxes grouped in clusters that are not compact enough - especially in the case of sativas. Rich in trichomes (i.e. resin), THC and other cannabinoids, the calyx is highly appreciated by cannabis users.