Letter to the editor: Small, Ernest. 2015. Evolution and classification of Cannabis sativa (Marijuana, Hemp) in relation to human utilization. Botanical review 81(3): 189-294.
In his comprehensive literature review Ernest Small covers diverse topics such as hemp fibers and processing, hemp seed nutrition, cannabinoid biosynthesis, cannabinoid-receptor interactions, medical uses and modes of action, among many more. The author's concise narrative style, and excellent illustrations (including naive art), add to the educational tone of this long contribution. However, the authors of this rebuttal challenge Small's use of outdated terminology, question his renaming of the functional evolutionary and taxonomic groupings, support the use of cultivar names, are confused by his conclusions concerning resin gland function, and wish to elaborate on the particulars of drug Cannabis domestication.
Throughout his review Small uses the terms "narcotic" and "intoxicant" to describe drug Cannabis plants and their psychoactive products. The Oxford English Dictionary Online (http://www.oxforddictionaries.com. Accessed September 2, 2015) defines a "narcotic" as a "drug or other substance affecting mood or behavior and sold for nonmedical purposes, especially an illegal one" while the online Merriam-Webster Unabridged Dictionary (http://www.merriam-webster.com. Accessed September 2, 2015) defines a "narcotic" as "a drug (such as cocaine, heroin, or marijuana) that affects the brain and that is usually dangerous and illegal." And, in a medical context defines a "narcotic" as "a drug (as codeine, methadone, or morphine) that in moderate doses dulls the senses, relieves pain, and induces profound sleep but in excessive doses causes stupor, coma, or convulsions" or "a drug (as marijuana or LSD) subject to restriction similar to that of addictive narcotics whether in fact physiologically addictive and narcotic or not." According to these definitions drug Cannabis could be termed "narcotic" because it "affects the brain" but there is little if any evidence to support its being "usually dangerous." And, although Cannabis remains "illegal" in many jurisdictions where it is "subject to restriction," calling cannabis a "narcotic" is a defamatory artifact of the legal system, rather than a choice backed by scientific observation.
The online Free Dictionary (http://www.thefreedictionary.com. Accessed September 2, 2015) gives the modern popular usage of "narcotic" as "a drug, such as moiphine or heroin, that is derived from opium or an opium-like compound, relieves pain, often induces sleep, can alter consciousness, and is potentially addictive" or as simply a "controlled substance." Although cannabis is a "controlled substance" often attributed with some of the same medicinal qualities of opiates, its use much more rarely results in addiction. The online Urban Dictionary (http://www.urbandictionary.com. Accessed September 2,2015) provides a more popular definition, "Narcotics are opium-based pain chugs such as heroin, morphine, codeine, etc." while making the distinction that, "marijuana and psychedelic chugs are not considered narcotics, whereas heroin is." The Urban Dictionary (http://www.urbandictionary.com. Accessed September 2, 2015) also points out that the term "narcotic" is, "too often misused by propagandists and the ignorant as a word to describe any illegal drug."
"Intoxication" is defined by the Oxford English Dictionary Online (http://www.oxforddictionaiies.com. Accessed September 2, 2015) as "the state of being intoxicated, especially by alcohol" and the Merriam-Webster Unabridged Dictionary (http://www.merriam-webster.com. Accessed September 2, 2015) tells us that "intoxication" means "an abnormal state that is essentially a poisoning" or "the condition of being drunk." The online Free Dictionary (http://www.thefreedictionary.com. Accessed September 2, 2015) states that to intoxicate is "to stupefy or excite by the action of a chemical substance such as alcohol" or "to poison." Cannabinoids are extremely non-toxic, poisonings are very rare, and the effects of cannabis use are markedly different from those of intoxicant alcohol.
While in an antiquated, often biased, and unscientific context Cannabis has sometimes been referred to as a "narcotic" or "intoxicant" plant, in modern usage these terms are both inaccurate and pejorative as they equate cannabis use with use of addictive and considerably more dangerous hard drugs. As Small (pages 280-281) states in his Postscript; "Scientific research on Cannabis has been suppressed for most of the last century, a victim of the sometimes observed tendency to avoid examination of sensitive or sinister subjects. Ignorance, however, generally exacerbates problems, and has likely contributed to worsening the substantial harm that has become associated with Cannabis." In light of the increasing public outcry for legalization, a growing realization that in relative terms cannabis is a soft drug and not nearly as dangerous as many legal drugs, and continuing testimonials to the medical efficacy of cannabis preparations, it is illogical and detrimental to continue using such antiquated terms. Researchers should represent Cannabis plants and their products in a scientifically accurate and unbiased context. A simple solution would be to use the terms "drug Cannabis" and "psychoactive Cannabis" which are accurately descriptive and carry no innuendo.
Concerning the linguistic origins of the word "cannabis" Small casually states (page 219) that, "During the age of sailing ships, Cannabis was considered to provide the very best canvas, and indeed this word, as well as the genus name Cannabis, are derived from an Arabic word for hemp." According to the Oxford Dictionary Online (http://www.oxforddictionaries.com. Accessed September 2, 2015) the name "cannabis" is from Greek kannabis, via Latin cannabis', and the Online Etymology Dictionary (http://www.etymonline.com. Accessed September 2, 2015) tells us that "cannabis" was originally a Scythian or Thracian word. There is no mention of derivation via Arabic.
Small (2015) thoroughly reviews the recent taxonomic history of Cannabis and allows direct comparison of differing taxonomic treatments (summarized in Table 2, page 261). The table illustrates, for instance, that Clarke and Merlin (2013) clearly incorporated both the anatomical and chemotaxonomic research of Hillig (2004a, 2005) as well as the nomenclature of McPartland and Guy (2004, which was in turn adapted largely from McPartland et al., 2000). Previous taxonomic interpretations of Cannabis biodiversity are presented in chronological order: Small & Cronquist (1976), Hillig (2004a, 2005), McPartland and Guy (2004), Clarke and Merlin (2013) and most recently Small's revised taxonomic system (2015), and this order is preserved herein when discussing various naming systems, with the omission of Small and Cronquist (1976) which is considered below.
All of these authors agree that there are four groupings of Cannabis extant today, all of which have served as building blocks for modern hybrids bred for fiber, seed or drug production. Populations traditionally utilized for hemp fiber and seed production (and their ruderal relatives) are subdivided into European populations (variously called C. saliva "hemp biotype," C. saliva ssp. sativa, C. sativa ssp. saliva "narrow-leaf hemp (NLH)" or "Group 1" by different researchers); and eastern Asian populations (variously called C. indica "hemp biotype," C. indica ssp. chinensis, C. indica ssp. chinensis "broad-leaf hemp (BLH)" or "Group 2"). And, populations traditionally utilized for drug production (and their ruderal relatives) are subdivided into an indigenous southern Asian (and historically widespread) marijuana group (variously called C. indica "narrow-leaflet drug variety", C. indica ssp. indica, C. indica ssp. indica "narrow-leaf drug (NLD)" or "Group 3"); and a much more recently disseminated hashish group native to the region of present-day Afghanistan (variously called C. indica "wide-leaflet drug biotype", C. indica ssp. afghanica, C. indica ssp. afghanica "broad-leaf drug (BLD)" or "Group 4"). Present-day fiber and seed varieties (called either NLH/BLH or Group 5 hybrids) result from crosses between the traditional European (NLH or Group 1) and East Asian (BLH or Group 2) fiber and seed landraces; and present-day drug varieties (called either NLD/BLD or Group 6 hybrids) result from crosses between traditional South Asian (NLD or Group 3) and Afghan (BLD or Group 4) drug landraces. The basic four groupings circumscribing all cultivated Cannabis, European narrow-leaf hemp (NLH), Asian broad-leaf hemp (BLH), South Asian narrow-leaf drug (NLD) and Afghan broad-leaf drug (BLD) were recently published by Clarke and Merlin (2013). Small (2015) added the ruderal relatives to each grouping, and simply renamed them Groups 1 through 4. He also assigned numbers to the hybrid hemp (NLH/BLH or Group 5) and hybrid drug (NLD/BLD or Group 6) gene pools also previously described by Clarke and Merlin (2013). Assigning numbers to plant groups strips them of their descriptive names, weakens our perception of them as gene pools, and divorces them from their evolutionary past. We fail to see how this constitutes any improvement in our understanding of Cannabis evolution and its taxonomic ramifications.
Remaining taxonomic disagreement revolves around how to assign scientific names to circumscribe each of these four widely recognized groups, as well as the modern hybrid strains. Hillig (2004a, 2005), McPartland and Guy (2004) and Clarke and Merlin (2013) accept a two (or three) species interpretation; while Small (2015) defends the single species system generally accepted 40 years ago and published by Small and Cronquist (1976). In his recent review, Small invokes two different systems of plant nomenclature to support two different taxonomic schemes for Cannabis. There are several systems regulating the naming of plants and their cultivated varieties. Small (page 269) informs us that, "The latest is The International Code of Nomenclature for Algae, Fungi and Plants (ICNAFP; McNeill et al., 2012). This is the most respected and universally applied way of determining plant names." and that, "There is no impediment to treating groups that are completely or partly domesticated under this code." Small's (2015) four-group taxonomic subdivision under ICNAFP follows Small and Cronquist (1976). Small (2015) divides the whole of Cannabis into two groups based on potency (THC content) and then each half is divided again based on whether the populations are "cultivated" or "wild." This approach is somewhat surprising as the author is hesitant to recognize a taxonomic distinction between wild and cultivated populations due to perceived extensive crossing and blending of the wild and cultivated genomes, and indeed doubts that truly "wild" populations exist (see below).
Another naming system, the International Code of Nomenclature for Cultivated Plants (ICNCP; Brickell et al., 2009) allows more latitude in the use of partly non-Latinized names for strains of domesticated plants. As Small tells us (pages 271-272), "A key feature of the ICNCP provides for recognition of "groups" of cultivars, allowing considerable flexibility in their formation ("Criteria for forming and maintaining a Group vary according to the required purposes of particular users."), while insisting that "All members of a Group must share the characters) by which that Group is defined. ... The group concept is flexible in choice of characters serving to define membership (of course, there may be disagreements among specialists about which characters should be the basis for group recognition)." And also that, "The group concept provides a simple, sound, alternative way of labeling variation of domesticated forms in genus Cannabis. It eliminates the need to consider rank; what various authors may have treated as species, subspecies, or varieties can be reduced to the same level. The four domesticated assemblages noted in Table 2 can simply be recognized as groups. There is considerable hybridization in Cannabis, which often makes identification problematical, but the same is true of most important domesticated plants. Groups that are hybrids between other groups can simply be recognized as separate groups."
Following the ICNCP system Small (2015) recognizes the four groups previously adopted by Hillig (2004a, 2005), McPartland and Guy (2004) and Clarke and Merlin (2013), by splitting Small & Cronquist's (1976) C. sativa subsp. sativa var. sativa into the European (Clarke & Merlin's NLH, Small's Group 1) and Asian (Clarke & Merlin's BLH, Small's Group 2) "fiber and oilseed" groups, and by splitting C. sativa subsp. indica var. indica into two "narcotic" groups, one THC-dominant (Clarke & Merlin's NLD, Small's Group 3) and one with more balanced THC/CBD ratios (Clarke & Merlin's BLD, Small's Group 4). Small (2015) correctly circumscribes two hybrid groups; "fiber and oilseed hybrids" (Group 5) and "narcotic hybrids" (Group 6). These hybrid groups were thoroughly discussed by Clarke and Merlin (2013) who accepted them as recent gene pools created by plant breeders, but did not assign them group numbers. The use of "groups" removes some hierarchical classification, and may simplify the taxonomic issues surrounding Cannabis, but the assignation of group numbers to gene pools could also result in oversimplifying and obscuring deeper evolutionary relationships.
Concerning use of the term "cultivar" Small (page 214) tells us that, "Article 2.2 of the [ICNCP] forbids the use of the term "strain" as equivalent to "cultivar" for the purpose of formal recognition" and explains that, "Although Cannabis strains are conceptually identical to Cannabis cultivars, in this review the strain names are not denoted in single quotes, the convention for cultivar names. In fact, very few Cannabis strains satisfy the descriptive requirements for cultivar recognition." According to Small the rules of nomenclature indicate that Clarke and Merlin (2013) as well as all other published specialists assigning cultivar names to Cannabis varieties, are in error concerning the characterization and naming of domesticated Cannabis populations. Considering only this narrow interpretation of ICNCP guidelines Small is indeed correct, but upon viewing the bigger picture it turns out there is justification for applying cultivar names to Cannabis, and by other standards Cannabis strains may be eligible for cultivar status.
Small's hemp hybrids (Group 5) circumscribe present-day sexually reproduced hybrid industrial hemp "cultivars", and drug hybrids (Group 6) circumscribe present-day vegetatively reproduced hybrid marijuana "strains". As he informs us (pages 270-271), "Since the middle of the 20th century, domesticated selections of plants satisfying certain descriptive and publication requirements and termed 'cultivars1 have been the subject of [the ICNCP]. The ICNCP provides the following definition: A cultivar is an assemblage of plants that (a) has been selected for a particular character or combination of characters, (b) is distinct, uniform, and stable in these characters, and (c) when propagated by appropriate means, retains those characters." And also that, "Cultivars as defined by the ICNCP can be of quite different nature ... they may be hybrids, clones, ... but frequently many of the cultivars within a given species differ very little genetically." Many industrial hemp seed varieties as well as vegetatively reproduced hybrid marijuana varieties (strains) certainly satisfy these criteria. However, Article 9.1, Note 1 of the ICNCP restricts the meaning of "cultivar" as follows: "No assemblage of plants can be regarded as a cultivar ... until its category, name, and circumscription has been published." In essence, this restricts the use of the "cultivar" classification only to plants with published "category, name, and circumscription" data. In Europe and other regions where industrial hemp cultivation is legal, varieties have been given registry status just like any other crop, and as Small states, "There are a hundred or more recognized cultivars of non-narcotic forms of Cannabis, grown for fiber and/or oilseed." However, drug Cannabis remains illegal in most jurisdictions, and cultivar registries hesitate to include plant varieties that are proscribed. Therefore, modern NLD/BLD hybrids have only rarely been granted cultivar status.
There are, however, limitations to the "group" concept. As Small tells us (page 271), "Because the group concept of the cultivated plant code has only a single rank (really no rank), it does not provide for using taxonomic rankings as an indication of phylogenetic history." However, Small (page 277) states that artificial classifications are "based on selected similarities of particular (practical) interest to people" and that, "It is often claimed that restricting the character base to only certain economic considerations means that the resulting classification is not based on evolution, and so not an acceptable basis for biological taxonomy. However, characteristics of domesticated organisms are the result of evolution, and when they are produced by strong selective pressures they may merit special taxonomic consideration." Small continues, "Characters or character complexes that are selected by humans are adaptive for domesticated plants, at least in the context of cultivation, and using such characters in recognizing taxa does constitute evolutionary classification." Problems arise when choosing which characters to include or exclude during classification. When determining evolutionary relationships between cultivated plants and their ruderal relatives the continuum of their physical characteristics causes considerable confusion, and we may be better able to understand Cannabis evolution once we have access to genome data.
One of Small's key points supporting his single species taxonomy for Cannabis is that cross pollination between differing "wild" as well as cultivated populations has led to a continuum of traits which do not lend themselves to being divided and circumscribed. On page 204 we read that, "Because of widespread clandestine cultivation, the pollen can be found, at least in small concentrations, over most of the planet. While the inverse square law dictates that the probability of pollen distribution decreases rapidly with distance, it is likely that there is frequent genetic interchange among populations." Since most Western "clandestine cultivation" is of seedless marijuana there are no males present and therefore there is no pollen, however, in traditional growing regions most hashish crops and some marijuana crops are seeded and pollen spread is more likely. Farther on (page 262) we read, "Because both domesticated and wild Cannabis populations are extremely widespread, interbreed spontaneously over vast distances (Small & Antle, 2003), with a common diploid chromosome number (2n = 20) and no biological barriers to interbreeding (Small, 1972), wild-growing and domesticated plants exchange genes easily and extensively. In nature, one finds a complete spectrum of intermediate forms, demonstrating continuity of variation between wild and domesticated forms (Small, 1975)." And, again on page 274, "In Cannabis, hybridization between the most distinctive variations has largely obliterated populational differences, especially between the two kinds of non-narcotic (fiber) forms and between the two kinds of narcotic forms."
Small (2015) places much emphasis on the ability of Cannabis populations to freely interbreed, while Clarke & Merlin (2013) found little evidence for introgressive hybridization between cultivated and ruderal populations. It may be easy to visualize that cultivated and escaped-ruderal populations freely interbreed and exchange genes through introgression (simply because all Cannabis plants are inter-fertile and male plants produce copious pollen that is carried by breezes for great distances) but in actuality there is little practical nor theoretical evidence for introgression between ruderal and cultivated (or vice versa) Cannabis populations (see Clarke & Merlin, 2013 for a detailed analysis of introgression in Cannabis). In addition, although Cannabis is often found growing as a weed in regions where it was previously cultivated, it likely did not evolve from proto-weed status growing amongst other crops (see Clarke & Merlin, (2013) for an in-depth discussion of Cannabis origin from weeds).
Small (page 275) also observed that, "Hillig (2004b) concluded that most Cannabis accessions in the Vavilov Research Institute [VIR] (St. Petersburg) germplasm bank (most of these are fiber land races), by far the world's largest such collection, are of hybrid origin." But, on page 235 he explained that "land race germplasm in the Vavilov Research Instutute ... has been extensively hybridized (Small & Marcus, 2003, Hillig, 2004b) due to inadequate maintenance." The VIR Cannabis seed collection was established in the 1970s and the hybridization "due to inadequate maintenance" occurred over the years as each accession was reproduced without adequate pollen isolation, and their genomic purity was compromised by intercrossing. The VIR's error in seed bank multiplications should not be interpreted as evidence for evolutionarily significant introgression amongst naturally distributed ruderal populations, landraces, or intentionally bred and selected cultivars.
Hybridization between hemp fiber and seed populations (i.e., NLH and BLH or Groups 1 and 2), as well as between marijuana or hashish populations (i.e., NLD and BLD or Groups 3 and 4) was likely very limited except during certain brief and relatively recent historical episodes. Before the middle 20th century most Cannabis landraces remained in geographical isolation from each other. European and East Asian hemp fiber landraces were intentionally crossed by European and North American hemp breeders several times during the 20th century, and since that time none of the hybrid (NLH/BLH or Group 5) offspring have had an opportunity to interbreed with any of their predecessors (except through back-crossing in breeding programs). Through years of group selection hemp hybrids have become relatively true breeding hemp strains maintained as reproducible seed lines, year after year, through selection and breeding. Commercial seed is sold under varietal trade names, and therefore they are recognized as cultivars. European and New World ruderal populations were founded by feral escapes from cultivation and are a subset of "Group 5" hybrids. Many East Asian hemp (BLH or Group 2) landraces certainly still exist in situ (Clarke, 1995, Clarke, 2006, Clarke & Gu, 1998), without ever having been hybridized with European hemp (NLH or Group 1) plants. A limited number of Chinese landrace seeds were used by Western hemp breeders, but there are few if any records of Western landraces or commercial cultivars incorporated into Asian hemp gene pools.
Hybrids between individuals from marijuana (NLD or Group 3) and hashish (BLD or Group 4) landraces have a more recent and altogether different domestication history beginning in the 1970s. During this time many different NLD landraces from Asia, Africa and the New World (and their hybrid cultivars) were already being grown by Westerners, and they were crossed for the first time with BLD landraces from Afghanistan. The resulting NLD x BLD = NLD/BLD hybrids (or Group 3 x Group 4 = Group 6 hybrids) spread rapidly, and after several generations covered the entire genetic spectrum from pure NLD landraces, through a continuum of slightly differing hybrids, to pure BLD landraces. Although the hybrid offspring did share some beneficial traits, they were generally not true-breeding enough to be considered "cultivars" on a par with European hemp varieties. Phenotypically diverse hybrids provided a myriad of combinations of favorable traits, but the genes controlling these traits often reassorted in subsequent generations, and many unique genotypes/phenotypes were lost. Concurrent with the proliferation of novel drug phenotypes was a spreading awareness that cuttings can be taken from select female plants, induced to grow roots, and kept indefinitely under long photoperiod in a vegetative state, until cuttings are required for flowering crop production (Clarke, 1981). The favorable traits of asexually reproduced clonal populations are genetically stable, uniform and reproducible, and as such they should be accepted as cultivars.
Vegetative propagation essentially freezes evolution, and makes gene transport much more difficult than the diffusion of seeds. Present-day clonal cultivars have remained geographically and functionally isolated from any gene exchange with parental populations in Afghanistan or other regions of origin. However, many NLD/ BLD hybrid seeds were produced in Western countries and some did interbreed with original ancestral populations. Well-meaning travelers visited many regions where NLD landraces were still growing, gave modern hybrid seeds to local farmers, who hoping for economic benefit, crossed them with their traditional landraces. Original pure NLD landraces have become rare in all traditional marijuana producing countries (e.g., Jamaica, Mexico, Morocco, and Thailand). The four key evolutionary events differentiating drug cultivars from fiber and seed cultivars are (1) rampant and continued blending of the modern domestic hybrid drug gene pool; (2) exportation of "improved" seed; (3) gene flow of modern drug hybrid traits back into traditional drug populations; and (4) the establishment of unique, asexually reproduced clonal cultivars. Recent stages in the evolution of modern drug varieties have no parallel in the breeding and distribution of hemp cultivars.
During its diffusion across a wide latitudinal range Cannabis has undergone a natural adaptation to seasonal changes in day length. On page 209, Small states that, "Although Group 4 [("indica-type")] strains originate from relatively southern areas of the Northern Hemisphere, they seem to mature earlier than Group 3 ("sativa-type") strains because of adaptation to a shorter season due to drought." BLD or Group 4 plants originated within and near present-day Afghanistan about 33[degrees] north latitude, while most NLD or Group 3 populations originated from 28[degrees] north latitude (e.g., Nepal) south to the equator. NLD landraces came from regions with less variable day length, more favorable climates, and therefore much longer growing seasons. In addition, Afghanistan and many surrounding regions are naturally arid, agricultural lands and must be regularly irrigated, and it is much more likely that early maturation of Group 4 plants results from a natural adaptation to the longer summer day lengths and shorter seasons associated with more northern latitudes, rather than adaptation to drought.
When describing phenotypic changes in stalk architecture resulting from domestication for drug use, Small (page 217) refers to Asia (likely the Himalayan foothills) where:
... one method of preparing hashish involved using hands or leather to collect (by adherence) sticky resin from the inflorescences at the top of the plants (alternatively and more conventionally today, hashish is prepared by filtering techniques ...). Accordingly, strains suitable for hashish collection based on stickiness should not be too tall. As Bouquet (1950) recorded: 'The cultivators, dressed in leather, moved about through the plantations. The resin sticks to their clothes, which are scraped from time to time with a blunt curved knife. This method of collection shows clearly that in those regions the plant does not grow to any great height.'
Although Afghan BLD or Group 4 plants may be shorter than NLD or Group 3 plants, BLD populations often average 2 to 3 m tall, and therefore they were likely not selected for stature shorter than a human. Perhaps shorter internodes and wide leaflets provide more wind and sun resistance, but this evolutionary adaptation is possible only because BLD populations originated in an arid climate, otherwise inflorescences with dense compact structure would be attacked by fungi. NLD plants make up the majority of drug varieties and are used throughout the Himalayan Mountain foothills to collect hand-rubbed hashish. NLD landraces evolved in humid climates where dense inflorescence architecture would invite fungal infection, and they often reach 3 or more meters in height. When resin is collected by hand from ruderal plants the branches and tops of the plants are simply bent over or broken so the inflorescences can be reached, and when collected from cultivated plants they are harvested and brought back to home for processing (Clarke, 2007) so there was never a reason to select for shorter stalks. BLD landraces may have been selected for increased leaf surface, encouraging resin production for sifted resin collection. NLD landraces were not selected for enhanced resin production because the resin collecting technology in the Himalayan region never developed beyond hand-rubbing fresh plants, and again there would have been no human selective pressure for shorter and more compact stem architecture.
In his review of glandular trichomes, Small (page 239) tells us that:
Hot conditions seem to favor release of the resin, but apparently there has been selection for strains that retain resin within the gland heads so that when fabric sieves are used to prepare hashish, they will not become clogged with sticky resin. However, strains that produce extruded sticky resin have been favored when hands or leather are used to rub off the resin for hashish preparation (McPartland & Guy, 2004; Clarke, 1998). Clarke and Watson (2002) state that there is an 'abscission layer' at the base of the head, although there seems no reason why dropping the heads is adaptive from the plant's perspective.
However, on page 243 Small reminds us that, "Cannabis sativa has minor allelopathic properties (Inam et al., 1989, McPartland 1997, McPartland et al., 2000), and chemicals leached into the soil may inhibit competing plants, as suggested by Haney and Bazzaz (1970)." Furthermore, on page 259 he tells us that, "Technologies have been created to collect and concentrate the THC-rich heads of the glandular trichomes, and this development seems to have resulted in the selection of strains in which the THC-rich heads abscise readily."
The abscission layer in Cannabis glandular trichomes is best observed in sifted hashish preparations in which gland heads and gland stalks appear as separate entities with the head cells broken away cleanly at the abscission layer. It is the abscission layer of glandular trichomes that allows resin glands to be collected by rubbing or sifting. The effectiveness of the abscission layer varies significantly between strains, and may well be enhanced in hashish varieties, both BLD and NLD. The most important criterion when selecting a candidate strain for hashish production is how easily and efficiently the intact resin glands are released and can be isolated from the plant material, and variations in readiness to abscise are important economic determinants in selecting hashish cultivars. There are two evolutionary advantages to resin gland abscission; natural allelopathic competition with other plants, and increased dispersal of its seeds by humans.
Small (page 243) continues his discussion of the function of glandular trichomes:
"Touch-sensitive glandular trichomes" rupture when touched by an arthropod, rapidly releasing a sticky exudate which can discourage, even kill herbivorous insects (Krings et al, 2002). In living (but not dried) Cannabis glands, the resin head readily ruptures when touched, suggesting that the released resin is indeed anti-herbivorous.
Once released from gland heads the aromatic principles (primarily terpenoid compounds) could be quite effective deterrents. However, glands do not rupture unless physically disturbed, and even then there is no evidence that resins flow onto the surface of the bracts and leaflets. The waxy cuticle of each gland encapsulates the sticky resin until it is needed for protection from herbivory (or to attract humans who disseminate it) and abscise readily as a convenient way of delivering its evolutionarily common allelopathic aromatic constituents, in addition to its evolutionarily unique cannabinoids.
Hillig's modern chemotaxonomic work (2004a, 2004b and 2005) rekindled scientific interest in the two (or possibly three) species debate, rather than the long dead 1970s spurious species defense as Small suggests (pages 264-265). Since the 1970s, for more than thirty years, cannabinoids had provided the only scant chemotaxonomic clues to Cannabis evolutionary and taxonomic relationships. More than ten years ago, Karl Hillig's work with allozyme and teipenoid characterizations gave us greater insight into Cannabis evolution. Presently, the sequencing of DNA nucleotides is providing scientists with more accurate ways of distinguishing between evolutionary and taxonomic groups. Analyses of archeological specimens, landrace seeds and herbarium accessions from the 1800s through the 1950s will allow characterization of populations prior to the rapid intensification of widespread human travel, and may provide the time perspective needed for evolutionary study. Ten years from now, we will routinely refer to the findings of scientists exploring the Cannabis genome, and the ramifications of their findings will impact the way we view the evolution of Cannabis and ultimately how we classify Cannabis biodiversity. Four evolutionarily distinct, geographically restricted, and culturally determined population groups have existed throughout history to the present day, and are deserving of taxonomic recognition. Yet, is it really so important if we agree on one species, two or three? As Shakespeare might have interjected, the Cannabis taxonomic debate has become "much ado about nothing," yet Cannabis specialists and evolutionary scientists await further data. Ernest Small has added his latest perspectives on the evolution and taxonomy of Cannabis and we doubt we will see any new proposals in the near future. We all agree on the fundamental points, and we must "agree to disagree" on the fine points, until the fresh flowering of genomic data provides us with richer food for thought.
Published online: 30 November 2015
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Robert C. Clarke (1,3) * Mark D. Merlin (2)
(1) International Hemp Association, Amsterdam, Netherlands
(2) University of Hawai'i at Manoa, Honolulu, Hawaii, USA; e-mail: firstname.lastname@example.org
(3) Author for Correspondence; e-mail: email@example.com
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|Author:||Clarke, Robert C.; Merlin, Mark D.|
|Publication:||The Botanical Review|
|Article Type:||Letter to the editor|
|Date:||Dec 1, 2015|
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