Cannabis

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This article is about the plant genus Cannabis. For use as a psychoactive drug in the genus, see Cannabis (drug).
Cannabis
common hemp
common hemp
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Rosales
Family: Cannabaceae
Genus: Cannabis
L.
Species

Cannabis sativa L.[1]
Cannabis indica Lam. (putative)[1]
Cannabis ruderalis Janisch. (putative)
Cannabis Museum in Amsterdam

Cannabis (Cán-na-bis) is a genus of flowering plants that includes three putative species, Cannabis sativa L.,[1] Cannabis indica Lam.,[1] and Cannabis ruderalis Janisch. These three taxa are indigenous to central Asia and surrounding regions. Cannabis has long been used for fibre (hemp), for medicinal purposes, and as a psychoactive. Industrial hemp products are made from Cannabis plants selected to produce an abundance of fiber and minimal levels of THC (Δ9- tetrahydrocannabinol), a psychoactive molecule that produces the "high" associated with marijuana. The psychoactive consists of dried flowers and leaves of plants selected to produce high levels of THC. Various extracts including hashish and hash oil are also produced from the plant.[2] The cultivation and possession of Cannabis for recreational use is outlawed in most countries.

Contents

Etymology

Main article: Cannabis (etymology)

The plant name cannabis is from Greek κάνναβις (kánnabis), via Latin cannabis, a Scythian or Thracian word, also loaned into Persian as kanab. English hemp (Old English hænep) may be an early loan (predating Grimm's Law) from the same source. In Hebrew, the word is קַנַּבּוֹס [qanːa'boːs]. Old Akkadian qunnabtu, Neo-Assyrian and Neo-Babylonian qunnabu were used to refer to the plant.[3][4]

The further origin of the Scythian term is uncertain.

Description

Cannabis is an annual, dioecious, flowering herb. The leaves are palmately compound, 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.

Cannabis normally has imperfect flowers, with staminate "male" and pistillate "female" flowers occurring on separate plants.[5] It is not unusual, however, for individual plants to bear both male and female flowers.[6] 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 bare 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.[7]

All known strains of Cannabis are wind-pollinated[8] and produce "seeds" that are technically called achenes.[9] Most strains of Cannabis are short day plants,[8] 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 is naturally diploid, having a chromosome complement of 2n=20, although polyploid individuals have been artificially produced.[10] 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.[11] As a drug it usually comes in the form of dried flower buds (marijuana), resin (hashish), or various extracts collectively known as hashish oil.[2] In the early 20th century, it became illegal in most of the world to cultivate or possess Cannabis for drug purposes.

Taxonomy

Leaf of a Cannabis plant.

The genus Cannabis was formerly placed in the Nettle (Urticaceae) or Mulberry (Moraceae) family, but is now considered along with hops (Humulus sp.) to belong to the Hemp family (Cannabaceae).[12] Recent phylogenetic studies based on cpDNA restriction site analysis and gene sequencing strongly suggest that the Cannabaceae arose from within the Celtidaceae clade, and that the two families should be merged to form a single monophyletic group.[13][14]

Various types of Cannabis have been described, and classified as species, subspecies, or varieties:[15]

  • 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 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 smoking marijuana. 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.[16] 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.[17] 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.[16][18]

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.[19] 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."[20] 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.[20] Physiological barriers to reproduction are not known to occur within Cannabis, and plants from widely divergent sources are interfertile.[10] 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.[21] 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.[22][23][24]

Early classifications

Relative size of types of cannabis

The Cannabis genus was first classified using the "modern" system of taxonomic nomenclature by Carolus Linnaeus in 1753, who devised the system still in use for the naming of species.[25] He considered the genus to be monotypic, having just a single species that he named Cannabis sativa L. (L. stands for Linnaeus, and indicates the authority who first named the species). Linnaeus was familiar with European hemp, which was widely cultivated at the time. In 1785, noted evolutionary biologist Jean-Baptiste de Lamarck published a description of a second species of Cannabis, which he named Cannabis indica Lam.[26] Lamarck based his description of the newly named species on plant specimens collected in India. He described C. indica as having poorer fiber quality than C. sativa, but greater utility as an inebriant. Additional Cannabis species were proposed in the 19th century, including strains from China and Vietnam (Indo-China) assigned the names Cannabis chinensis Delile, and Cannabis gigantea Delile ex Vilmorin.[27] However, many taxonomists found these putative species difficult to distinguish. In the early 20th century, the single-species concept was still widely accepted, except in the Soviet Union where Cannabis continued to be the subject of active taxonomic study. The name Cannabis indica was listed in various Pharmacopoeias, and was widely used to designate Cannabis suitable for the manufacture of medicinal preparations.[28]

20th Century

In 1924, Russian botanist D.E. Janichevsky concluded that ruderal Cannabis in central Russia is either a variety of C. sativa or a separate species, and proposed C. sativa L. var. ruderalis Janisch. and Cannabis ruderalis Janisch. as alternative names.[15] In 1929, renowned plant explorer Nikolai Vavilov assigned wild or feral populations of Cannabis in Afghanistan to C. indica Lam. var. kafiristanica Vav., and ruderal populations in Europe to C. sativa L. var. spontanea Vav.[18][27] In 1940, Russian botanists Serebriakova and Sizov proposed a complex classification in which they also recognized C. sativa and C. indica as separate species. Within C. sativa they recognized two subspecies: C. sativa L. subsp. culta Serebr. (consisting of cultivated plants), and C. sativa L. subsp. spontanea (Vav.) Serebr. (consisting of wild or feral plants). Serebriakova and Sizov split the two C. sativa subspecies into 13 varieties, including four distinct groups within subspecies culta. However, they did not divide C. indica into subspecies or varieties.[15][29] This excessive splitting of C. sativa proved too unwieldy, and never gained many adherents.

Ruderal Cannabis.

In the 1970s, the taxonomic classification of Cannabis took on added significance in North America. Laws prohibiting Cannabis in the United States and Canada specifically named products of C. sativa as prohibited materials. Enterprising attorneys for the defense in a few drug busts argued that the seized Cannabis material may not have been C. sativa, and was therefore not prohibited by law. Attorneys on both sides recruited botanists to provide expert testimony. Among those testifying for the prosecution was Dr. Ernest Small, while Dr. Richard E. Schultes and others testified for the defense. The botanists engaged in heated debate (outside of court), and both camps impugned the other's integrity.[22][23] The defense attorneys were not often successful in winning their case, because the intent of the law was clear.[30]

In 1976, Canadian botanist Ernest Small[31] and American taxonomist Arthur Cronquist published a taxonomic revision that recognizes a single species of Cannabis with two subspecies: C. sativa L. subsp. sativa, and C. sativa L. subsp. indica (Lam.) Small & Cronq.[27] The authors hypothesized that the two subspecies diverged primarily as a result of human selection; C. sativa subsp. sativa was presumably selected for traits that enhance fiber or seed production, whereas C. sativa subsp. indica was primarily selected for drug production. Within these two subspecies, Small and Cronquist described C. sativa L. subsp. sativa var. spontanea Vav. as a wild or escaped variety of low-intoxicant Cannabis, and C. sativa subsp. indica var. kafiristanica (Vav.) Small & Cronq. as a wild or escaped variety of the high-intoxicant type. This classification was based on several factors including interfertility, chromosome uniformity, chemotype, and numerical analysis of phenotypic characters.[16][27][32]

Professors William Emboden, Loran Anderson, and Harvard botanist Richard E. Schultes and coworkers also conducted taxonomic studies of Cannabis in the 1970s, and concluded that stable morphological differences exist that support recognition of at least three species, C. sativa, C. indica, and C. ruderalis.[33][34][35][36] For Schultes, this was a reversal of his previous interpretation that Cannabis is monotypic, with only a single species.[37] According to Schultes' and Anderson's descriptions, C. sativa is tall and laxly branched with relatively narrow leaflets, C. indica is shorter, conical in shape, and has relatively wide leaflets, and C. ruderalis is short, branchless, and grows wild in central Asia. This taxonomic interpretation was embraced by Cannabis aficionados who commonly distinguish narrow-leafed "sativa" drug strains from wide-leafed "indica" drug strains.[38]

Ongoing research

Molecular analytical techniques developed in the late twentieth century are being applied to questions of taxonomic classification. This has resulted in many reclassifications based on evolutionary systematics. Several studies of Random Amplified Polymorphic DNA (RAPD) and other types of genetic markers have been conducted on drug and fiber strains of Cannabis, primarily for plant breeding and forensic purposes.[39][40][41][42][43] Dutch Cannabis researcher E.P.M. de Meijer and coworkers described some of their RAPD studies as showing an "extremely high" degree of genetic polymorphism between and within populations, suggesting a high degree of potential variation for selection, even in heavily selected hemp cultivars.[17] They also commented that these analyses confirm the continuity of the Cannabis gene pool throughout the studied accessions, and provide further confirmation that the genus comprises a single species, although theirs was not a systematic study per se.

Karl W. Hillig, a graduate student in the laboratory of long-time Cannabis researcher Paul G. Mahlberg[44] at Indiana University, conducted a systematic investigation of genetic, morphological, and chemotaxonomic variation among 157 Cannabis accessions of known geographic origin, including fiber, drug, and feral populations. In 2004, Hillig and Mahlberg published a chemotaxomic analysis of cannabinoid variation in their Cannabis germplasm collection. They used gas chromatography to determine cannabinoid content and to infer allele frequencies of the gene that controls CBD and THC production within the studied populations, and concluded that the patterns of cannabinoid variation support recognition of C. sativa and C. indica as separate species, but not C. ruderalis.[18] The authors assigned fiber/seed landraces and feral populations from Europe, central Asia, and Asia Minor to C. sativa. Narrow-leaflet and wide-leaflet drug accessions, southern and eastern Asian hemp accessions, and feral Himalayan populations were assigned to C. indica. In 2005, Hillig published a genetic analysis of the same set of accessions (this paper was the first in the series, but was delayed in publication), and proposed a three-species classification, recognizing C. sativa, C. indica, and (tentatively) C. ruderalis.[21] In his doctoral dissertation published the same year, Hillig stated that principal components analysis of phenotypic (morphological) traits failed to differentiate the putative species, but that canonical variates analysis resulted in a high degree of discrimination of the putative species and infraspecific taxa.[45] Another paper in the series on chemotaxonomic variation in the terpenoid content of the essential oil of Cannabis revealed that several wide-leaflet drug strains in the collection had relatively high levels of certain sesquiterpene alcohols, including guaiol and isomers of eudesmol, that set them apart from the other putative taxa.[46] Hillig concluded that the patterns of genetic, morphological, and chemotaxonomic variation support recognition of C. sativa and C. indica as separate species. He also concluded there is little support to treat C. ruderalis as a separate species from C. sativa at this time, but more research on wild and weedy populations is needed because they were underrepresented in their collection.

In September 2005, New Scientist reported that researchers at the Canberra Institute of Technology had identified a new type of Cannabis based on analysis of mitochondrial and chloroplast DNA.[47] The New Scientist story, which was picked up by many news agencies and web sites, indicated that the research was to be published in the journal Forensic Science International. When the article was finally published, there was no mention of "Rasta."[48]

As of 2007, most taxonomy web sites continue to list Cannabis as a single species.[49][50][51][52]

Popular usage

The scientific debate regarding taxonomy has had little effect on the terminology in widespread use among cultivators and users of drug-type Cannabis. Cannabis aficionados recognize three distinct types based on such factors as morphology, native range, aroma, and subjective psychoactive characteristics. "Sativa" is the term used to describe the most widespread variety, which is usually tall, laxly branched, and found in warm lowland regions. "Indica" is used to designate shorter, bushier plants adapted to cooler climates and highland environments. "Ruderalis" is the term used to describe the short plants that grow wild in Europe and central Asia.

Breeders, seed companies, and cultivators of drug type Cannabis often describe the ancestry or gross phenotypic characteristics of cultivars by categorizing them as "pure indica," "mostly indica," "indica/sativa," "mostly sativa", or "pure sativa."

Wild Cannabis

In the U.S., wild (feral) "ditch weed" grows in great abundance in mid-western states such as Kansas and Nebraska. These plants have escaped from cultivation, which was encouraged during World War II. They produce very little THC, and are not valued for recreational use. In many areas, wild or naturalized populations of Cannabis are considered invasive species, and are often targeted by government-sponsored eradication programs. Feral C. sativa subsp. sativa (= C. sativa var. spontanea) shows great local variation; for example, in temperate climates it can reach heights up to 20 feet (6 m), but in colder regions it may only reach 1 foot (30 cm) in height at maturity. Feral Cannabis plants have long, thin, "airy" inflorescences, and may be branched or unbranched. Wild C. sativa subsp. indica (= C. indica var. kafiristanica), which primarily grows in Asia, has tiny seeds that fall from the plant at maturity, and is sometimes used by the locals for hashish production.

Reproduction

Breeding systems

Some Cannabis sativa seeds
Female Cannabis flowers.
Male Cannabis pollen sacs.

Cannabis is predominantly dioecious,[8][53] although many monoecious varieties have been described.[54] Subdioecy (the occurrence of monoecious individuals and dioecious individuals within the same population) is widespread.[55][56][57] Many populations have been described as sexually labile.[41][58][59]

As a result of intensive selection in cultivation, Cannabis exhibits many sexual phenotypes that can be described in terms of the ratio of female to male flowers occurring in the individual, or typical in the cultivar.[60] Dioecious varieties are preferred for drug production, where only the female plants are used. Dioecious varieties are also preferred for textile fiber production, whereas monoecious varieties are preferred for pulp and paper production. It has been suggested that the presence of monoecy can be used to differentiate licit crops of monoecious hemp from illicit drug crops.[55] However, the so-called "sativa" drug strains often produce monoecious individuals, probably as a result of inbreeding.

Mechanisms of sex determination

Cannabis has been described as having one of the most complicated mechanisms of sex determination among the dioecious plants.[60] Many models have been proposed to explain sex determination in Cannabis.

Based on studies of sex reversal in hemp, it was first reported by K. Hirata in 1924 that an XY sex-determination system is present.[58] At the time, the XY system was the only known system of sex determination. The X:A system was first described in Drosophila spp in 1925.[61] Soon thereafter, Schaffner disputed Hirata's interpretation,[62] and published results from his own studies of sex reversal in hemp, concluding that an X:A system was in use and that furthermore sex was strongly influenced by environmental conditions.[59]

Since then, many different types of sex determination systems have been discovered, particularly in plants.[53] Dioecy is relatively uncommon in the plant kingdom, and a very low percentage of dioecious plant species have been determined to use the XY system. In most cases where the XY system is found it is believed to have evolved recently and independently.[63]

Since the 1920s, a number of sex determination models have been proposed for Cannabis. Ainsworth describes sex determination in the genus as using "an X/autosome dosage type".[53]

Dense raceme of carpellate flowers typical of drug-type varieties of Cannabis

The question of whether heteromorphic sex chromosomes are indeed present is most conveniently answered if such chromosomes were clearly visible in a karyotype. Cannabis was one of the first plant species to be karyotyped; however, this was in a period when karyotype preparation was primitive by modern standards (see History of Cytogenetics). Heteromorphic sex chromosomes were reported to occur in staminate individuals of dioecious "Kentucky" hemp, but were not found in pistillate individuals of the same variety. Dioecious "Kentucky" hemp was assumed to use an XY mechanism. Heterosomes were not observed in analyzed individuals of monoecious "Kentucky" hemp, nor in an unidentified German cultivar. These varieties were assumed to have sex chromosome composition XX.[64] According to other researchers, no modern karyotype of Cannabis had been published as of 1996.[65] Proponents of the XY system state that Y chromosome is slightly larger than the X, but difficult to differentiate cytologically.[66]

More recently, Sakamoto and various co-authors[67][68] have used RAPD to isolate several genetic marker sequences that they name Male-Associated DNA in Cannabis (MADC), and which they interpret as indirect evidence of a male chromosome. Several other research groups have reported identification of male-associated markers using RAPD and AFLP.[17][41][69] Ainsworth commented on these findings, stating,

It is not surprising that male-associated markers are relatively abundant. In dioecious plants where sex chromosomes have not been identified, markers for maleness indicate either the presence of sex chromosomes which have not been distinguished by cytological methods or that the marker is tightly linked to a gene involved in sex determination.[53]

Environmental sex determination is known to occur in a variety of species.[70] Many researchers have suggested that sex in Cannabis is determined or strongly influenced by environmental factors.[59] Ainsworth reviews that treatment with auxin and ethylene have feminizing effects, and that treatment with cytokinins and gibberellins have masculinizing effects.[53] It has been reported that sex can be reversed in Cannabis using chemical treatment.[71] A PCR-based method for the detection of female-associated DNA polymorphisms by genotyping has been developed.[72]

Human uses

Cannabis plants, more commonly known as hemp for this end, is used for a wide variety of purposes, including the manufacture of cordage of varying tensile strength, clothing, and nutritional products. Industrial hemp products are made from Cannabis plants selected to produce an abundance of fiber and minimal levels of THC (Δ9- tetrahydrocannabinol). The bast fibers can be used in 100% hemp products, but are commonly blended with fibers such as flax, cotton or silk, for apparel and furnishings, most commonly at a 55%/45% hemp/cotton blend. The inner two fibers of hemp are more woody, and are more often used in non-woven items and other industrial applications, such as mulch, animal bedding and litter. The oil from the fruits ("seeds") dries on exposure to air (similar to linseed oil) and is sometimes used in the manufacture of oil-based paints, in creams as a moisturising agent, for cooking, and in plastics. Hemp seeds have been used in bird seed mix.[73] Due to the psychoactive molecule that produces the "high" associated with marijuana, the cultivation and possession of Cannabis for recreational use is outlawed in most countries. Various extracts including hashish and hash oil are also produced from the plant.[2]

Aspects of Cannabis production and use

Cannabis field seized by authorities

Gallery of images

Cannabis plants

Cannabis plants

Cannabis plants

Cannabis plants

See also

Cannabis portal

References

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Further reading

External links

Sister project Wikispecies has information related to: Cannabis
Sister project Wikimedia Commons has media related to: Cannabis sativa
Sister project Wikiquote has a collection of quotations related to: Cannabis
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