# Cannabis plant biology: the parts and what they do

> Cannabis sativa is a fast-growing annual herb in the hemp family. Most plants are either male or female, it flowers in response to long nights (unless it carries autoflower genetics), and the cannabinoids and terpenes growers want are made in glandular trichomes concentrated on the female flower heads.

- Canonical: https://genetixbay.com/knowledge/basics/cannabis-plant-biology.html
- Category: Basics · Level: Beginner
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What are the parts of a cannabis plant?** A cannabis plant is made up of a taproot and fibrous roots, a stem with nodes where leaves and branches arise, [fan leaves](https://genetixbay.com/knowledge/glossary/fan-leaf.md) with usually 5-9 serrated leaflets, small resin-coated [sugar leaves](https://genetixbay.com/knowledge/glossary/sugar-leaf.md), and either [female flower](https://genetixbay.com/knowledge/glossary/pistillate-flower.md) clusters (bracts with stigmas) or [male flowers](https://genetixbay.com/knowledge/glossary/staminate-flower.md) with anthers. Glandular trichomes on the flowers make the resin, and a fertilised female flower forms a single seed in a hard shell.

**Where are THC and terpenes made in the plant?** THC and terpenes are made in glandular trichomes, mainly the [capitate-stalked](https://genetixbay.com/knowledge/glossary/capitate-stalked-trichome.md) 'mushroom' glands densest on the bracts of unpollinated female flowers. The plant actually makes THCA, the acid form, in the trichome's secretory cells and stores it in the gland head. It becomes neutral THC mainly through heat ([decarboxylation](https://genetixbay.com/knowledge/glossary/decarboxylation.md)), not on the living plant. This trichome localisation is established science.

**What is a trichome?** A trichome is a tiny hair-like outgrowth on the plant surface. On cannabis, the glandular types, large capitate-stalked 'mushroom' glands plus smaller sessile and bulbous ones, are resin factories: secretory cells make cannabinoid acids and terpenes and store them in the gland head. They are densest on female bracts and sugar leaves, which is why sugar leaves are so resin-coated.

**Is cannabis a male and female plant?** Yes, cannabis is dioecious: individual plants are normally either male or female, with sex set by [X and Y chromosomes](https://genetixbay.com/knowledge/glossary/xy-sex-determination.md) (XX female, XY male). That separation has been stable for millennia. Intersex or 'hermie' plants are unwanted outliers, which GenetixBay attributes mainly to poor breeding. Growers usually keep only females, because unpollinated female flowers produce the seedless, resin-rich buds called sinsemilla.

**What is the difference between a calyx, a bract and a pistil?** On a female cannabis flower, the bract is the small leafy sheath wrapping the single ovary; growers usually call it the 'calyx', though botanically it is a bract. The 'pistils' or 'hairs' growers talk about are really the two stigmas that stick out to catch pollen. Bracts carry most of the glandular trichomes, so they matter far more for resin than the stigmas do.

**How many chromosomes does cannabis have?** Cannabis is diploid with 2n = 20 chromosomes: nine pairs of autosomes plus one pair of sex chromosomes, XX in females and XY in males. Cytogenetic research established this XY system. The genes that decide THC versus CBD sit in large, repeat-rich, low-recombination regions of the genome, which is why chemotype tends to be inherited as a single block.

**Short answer.** Cannabis (*[Cannabis sativa](https://genetixbay.com/knowledge/glossary/cannabis-sativa.md)* L., family Cannabaceae, which it shares with hops) is an annual, wind-pollinated herb. Plants are either male or female (dioecious), with sex set by X and Y chromosomes; that separation has been stable for millennia, and intersex ("hermie") plants are unwanted outliers, mostly from poor breeding (see [male, female and hermaphrodite plants](https://genetixbay.com/knowledge/basics/sex-male-female-hermaphrodite.md)). The resin that growers value (cannabinoids such as THCA and CBDA, plus terpenes) is made in glandular trichomes, which are densest on the bracts of unpollinated female flower heads.

## What's happening

Cannabis grows fast in long days, then switches to flowering when nights get long (photoperiod types) or after a fixed age ([autoflowering](https://genetixbay.com/knowledge/glossary/autoflower.md) types). The flowering switch is less "turning on flowers" than a change in shoot architecture: the [growing tips](https://genetixbay.com/knowledge/glossary/apical-meristem.md) stop making long internodes and start building the compact, branched flower clusters we call colas [2].

The genome is diploid with 2n = 20 chromosomes: nine pairs of autosomes plus the sex chromosomes (XX female, XY male) [3]. The genes that decide THC versus CBD sit in large, repeat-rich, low-recombination regions, which is why chemotype tends to be inherited as a block (see [chemotype inheritance](https://genetixbay.com/knowledge/breeding/chemotype-inheritance.md)) [4].

Taxonomy is argued over. Most botanists today treat cannabis as one species, *Cannabis sativa*, with subspecies and varieties; the familiar "sativa / indica / ruderalis" labels are a separate, popular vocabulary that does not map cleanly onto genetics (*Contested*; see [indica, sativa and ruderalis](https://genetixbay.com/knowledge/genetics-history/indica-sativa-ruderalis.md)) [1][5].

## The parts

| Part | What it is | Why a grower cares |
|---|---|---|
| Taproot and fibrous roots | Primary root plus a dense lateral mass | Root-zone health limits everything above it |
| Stem and nodes | Hollow-ish stem; leaves and branches arise at nodes | Node spacing (internode length) shows light and genetics |
| Fan leaves | Palmately compound leaves, usually 5-9 serrated leaflets (1-3 on seedlings and late flower) | Main [photosynthetic](https://genetixbay.com/knowledge/glossary/photosynthesis.md) surface; first place deficiencies show |
| Sugar leaves | Small, resin-coated leaves inside flower clusters | Trimmed at harvest; trichome-rich |
| Stipules and pre-flowers | Small structures at nodes; the first solitary flowers form here | First visual sign of sex [2] |
| Female flower | A single ovary wrapped in a **bract** (often called "calyx" by growers) with two **stigmas** ("pistils", "hairs") | Bracts carry most glandular trichomes; stigmas catch pollen |
| Male flower | Five tepals and five anthers hanging on short stalks | Sheds pollen about 1-3 weeks after opening; one plant can seed a room |
| Glandular trichomes | Capitate-stalked "mushroom" glands, plus smaller sessile and bulbous types | Where cannabinoids and terpenes are made and stored |
| Seed (achene) | Single seed in a hard shell, mottled or plain | Carries half its genes from each parent |

## Where the resin is made

Cannabinoids are made as acids (THCA, CBDA, CBGA) inside the secretory cells of glandular trichomes and pushed into the head's storage cavity. They become neutral THC or CBD mainly by heat (decarboxylation), not on the living plant (*Established*) [1]. Unpollinated female flowers keep building bracts and resin for longer, which is why drug-type crops are grown seedless (sinsemilla).

## Numbers that matter

| Parameter | Typical value | Notes |
|---|---|---|
| Chromosome number | 2n = 20 (18 autosomes + XX or XY) | [3] |
| Haploid genome size | ~818 Mb (female) / ~843 Mb (male) | Flow cytometry estimates |
| Leaflets per fan leaf | 5-9 mature | 1-3 on [first true leaves](https://genetixbay.com/knowledge/glossary/true-leaves.md) and at the top in flower |
| Height outdoors | 1-5 m (3-16 ft) | Strongly genotype and season dependent |
| Life cycle | 3-9 months seed to harvest | See [life cycle](https://genetixbay.com/knowledge/basics/life-cycle.md) |

## Common mistakes

- Calling the stigma the "pistil" and assuming its colour tells you ripeness. Stigma browning is a rough cue; trichome maturity is the better one.
- Assuming "sativa" and "indica" describe genetic ancestry. In modern hybrids they mostly describe shape and expected effect (*Contested*).
- Thinking male plants contain no cannabinoids. Males make resin too, just far less flower mass.

## Evidence

- Dioecy with XY sex determination and 2n = 20: *Established* [3].
- Trichome localisation of cannabinoid synthesis: *Established* [1].
- Photoperiod response mainly reshapes the shoot apex rather than starting flowers from zero: *Supported* (one detailed study on three medical cultivars) [2].
- Single-species treatment: *Supported* but debated [1][5].

## Sources

1. Small E. 2015. Evolution and classification of Cannabis sativa (marijuana, hemp) in relation to human utilization. Botanical Review 81:189-294. https://doi.org/10.1007/s12229-015-9157-3
2. Spitzer-Rimon B, Duchin S, Bernstein N, Kamenetsky R. 2019. Architecture and florogenesis in female Cannabis sativa plants. Frontiers in Plant Science 10:350. https://doi.org/10.3389/fpls.2019.00350
3. Divashuk MG, Alexandrov OS, Razumova OV, Kirov IV, Karlov GI. 2014. Molecular cytogenetic characterization of the dioecious Cannabis sativa with an XY chromosome sex determination system. PLoS ONE 9(1):e85118. https://doi.org/10.1371/journal.pone.0085118
4. Laverty KU, Stout JM, Sullivan MJ, et al. 2019. A physical and genetic map of Cannabis sativa identifies extensive rearrangements at the THC/CBD acid synthase loci. Genome Research 29(1):146-156. https://doi.org/10.1101/gr.242594.118
5. McPartland JM. 2018. Cannabis systematics at the levels of family, genus, and species. Cannabis and Cannabinoid Research 3(1):203-212. https://doi.org/10.1089/can.2018.0039