Cannabis is an annual, dioecious, flowering herb. The leaves are palmately compound or digitate, with serrate leaflets. The first pair of leaves usually have a single leaflet, the number gradually increasing up to a maximum of about thirteen leaflets per leaf (usually seven or nine), depending on variety and growing conditions. At the top of a flowering plant, this number again diminishes to a single leaflet per leaf. The lower leaf pairs usually occur in an opposite leaf arrangement and the upper leaf pairs in an alternate arrangement on the main stem of a mature plant.

 

Underside of Cannabis sativa leaf, showing diagnostic venation

The leaves have a peculiar and diagnostic venation pattern that enables persons poorly familiar with the plant to distinguish a Cannabis leaf from unrelated species that have confusingly similar leaves (see illustration). As is common in serrated leaves, each serration has a central vein extending to its tip. However,the serration vein originates from lower down the central vein of the leaflet, typically opposite to the position of, not the first notch down, but the next notch. This means that on its way from the midrib of the leaflet to the point of the serration, the vein serving the tip of the serration passes close by the intervening notch. Sometimes the vein will actually pass tangent to the notch, but often it will pass by at a small distance, and when that happens a spur vein (occasionally a pair of such spur veins) branches off and joins the leaf margin at the deepest point of the notch. This venation pattern varies slightly among varieties, but in general it enables one to tell Cannabis leaves from superficially similar leaves without difficulty and without special equipment. Tiny samples of Cannabis plants also can be identified with precision by microscopic examination of leaf cells and similar features, but that requires special expertise and equipment.

Cannabis normally has imperfect flowers, with staminate “male” and pistillate “female” flowers occurring on separate plants.It is not unusual, however, for individual plants to bear both male and female flowers. Although monoecious plants are often referred to as “hermaphrodites,” true hermaphrodites (which are less common) bear staminate and pistillate structures on individual flowers, whereas monoecious plants bear male and female flowers at different locations on the same plant. Male flowers are normally borne on loose panicles, and female flowers are borne on racemes. “At a very early period the Chinese recognized the Cannabis plant as dioecious,” and the (ca. 3rd century BCE) Erya dictionary defined xi “male Cannabis” and fu (or ju ) “female Cannabis“.

 

Cannabis growing as weeds at the foot of Dhaulagiri.

All known strains of Cannabis are wind-pollinated and the fruit is an achene. Most strains of Cannabis are short day plants, with the possible exception of C. sativa subsp. sativa var. spontanea (= C. ruderalis), which is commonly described as “auto-flowering” and may be day-neutral.

Cannabis, like many organisms, is diploid, having a chromosome complement of 2n=20, although polyploid individuals have been artificially produced. The first genome sequence of Cannabis, which is estimated to be 820 Mb in size, was published in 2011 by a team of Canadian scientists.The plant is believed to have originated in the mountainous regions northwest of the Himalayas. It is also known as hemp, although this term is often used to refer only to varieties of Cannabis cultivated for non-drug use. Cannabis plants produce a group of chemicals called cannabinoids, which produce mental and physical effects when consumed.

Cannabinoids, terpenoids, and other compounds are secreted by glandular trichomes that occur most abundantly on the floral calyxes and bracts of female plants.As a drug it usually comes in the form of dried flower buds (marijuana), resin (hashish), or various extracts collectively known as hashish oil. In the early 20th century, it became illegal in most of the world to cultivate or possess Cannabis for sale or personal use.

  • Root system side view

  • Root system top view

  • Micrograph sativa (left), indica (right)

Taxonomy

 

Cannabis sativa leaf, dorsal aspect

The genus Cannabis was formerly placed in the Nettle (Urticaceae) or Mulberry(Moraceae) family, and later, along with the Humulus genus (hops), in a separate family, the Hemp family (Cannabaceae sensu stricto).Recent phylogenetic studies based on cpDNA restriction site analysis and gene sequencing strongly suggest that the Cannabaceae sensu stricto arose from within the former Celtidaceae family, and that the two families should be merged to form a single monophyletic family, the Cannabaceae sensu lato.

Various types of Cannabis have been described, and variously classified as species, subspecies, or varieties:

  • plants cultivated for fiber and seed production, described as low-intoxicant, non-drug, or fiber types.
  • plants cultivated for drug production, described as high-intoxicant or drug types.
  • escaped, hybridised, or wild forms of either of the above types.

Cannabis plants produce a unique family of terpeno-phenolic compounds called cannabinoids, which produce the “high” one experiences from consuming marijuana. There are 483 identifiable chemical constituents known to exist in the cannabis plant,and at least 85 different cannabinoids have been isolated from the plant.The two cannabinoids usually produced in greatest abundance are cannabidiol (CBD) and/or Δ9-tetrahydrocannabinol (THC), but only THC is psychoactive.Since the early 1970s, Cannabis plants have been categorized by their chemical phenotype or “chemotype,” based on the overall amount of THC produced, and on the ratio of THC to CBD. Although overall cannabinoid production is influenced by environmental factors, the THC/CBD ratio is genetically determined and remains fixed throughout the life of a plant. Non-drug plants produce relatively low levels of THC and high levels of CBD, while drug plants produce high levels of THC and low levels of CBD. When plants of these two chemotypes cross-pollinate, the plants in the first filial (F1) generation have an intermediate chemotype and produce similar amounts of CBD and THC. Female plants of this chemotype may produce enough THC to be utilized for drug production.

 

Top of Cannabis plant in vegetative growth stage

Whether the drug and non-drug, cultivated and wild types of Cannabis constitute a single, highly variable species, or the genus is polytypic with more than one species, has been a subject of debate for well over two centuries. This is a contentious issue because there is no universally accepted definition of a species. One widely applied criterion for species recognition is that species are “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”Populations that are physiologically capable of interbreeding, but morphologically or genetically divergent and isolated by geography or ecology, are sometimes considered to be separate species.Physiological barriers to reproduction are not known to occur within Cannabis, and plants from widely divergent sources are interfertile.However, physical barriers to gene exchange (such as the Himalayan mountain range) might have enabled Cannabis gene pools to diverge before the onset of human intervention, resulting in speciation.It remains controversial whether sufficient morphological and genetic divergence occurs within the genus as a result of geographical or ecological isolation to justify recognition of more than one species.