There are four distinct types of plant hairs, better known as trichomes. There is a half-bubble, larger, egg-shaped resin gland that clings to the surface of the leaves and bud and contains only smell and taste terpenes (no THC): this is called a bulbous trichome. It is basically a plant hair without a stalk.
There is also a crystolith trichome, which basically looks like a hair with a long, pointed tip. These trichomes appear primarily on the underside of leaves and their main purpose is to protect the plant from insects and spider mites. They are protective trichomes that discourage insects and mites from eating foliage. They also contain no THC.
The third trichome is called a sessile stalked capitate trichome. It is several times larger than the bulbous glandual trichome, but is still very small. It begins to develop in the plant’s vegetative stage and stays very close to the plant’s surface. It contains very low levels of terpenes and THC and is considered a contaminant to hash makers.
Stalked capitate glandular trichomes, the fourth — which under the microscope appear more or less like translucent mushrooms — are made up of a stalk or clear column with a resin head. They form on buds/bracts and smaller surrounding leaves. The resin head on top is psychoactive: this is the one that produces Delta 9 THC, so that’s what we’re looking for.
Excerpted from “Marijuana Horticulture Fundamentals: A Comprehensive Guide to Cannabis Cultivation and Hashish Production” by K from Trichome Technologies.