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The Botany of Cannabis

11/5/2010 1:26:45 PM

Tags: The Pot Book, From Our Bookshelf, Marijuana, Cannabis, Botany

11-5-2010-pot book coverExcerpted from The Pot Book: A Complete Guide to Cannabis, edited by Julie Holland, M.D., with permissission from Inner Traditions Bear & Company (c) 2010. The following excerpt can be found on Pages 35 to 43. 

In many locations, the plant genus Cannabis has become synonymous with the recreational drug marijuana. While Cannabis plants are grown and used for food, fiber, fuel, medicine, and shelter (Brown 1998a, Guy 2004) in different areas of the world, primary cultivation, especially in the United States, is for the psychoactive chemical constituents known as cannabinoids. These cannabinoids have been demonstrated to be effective in the treatment of an assortment of human disease conditions (Russo 2004), but they have also been deemed addictive and dangerous substances with no therapeutic value (NIDA 2007). Thus, species of Cannabis are considered to be both a botanical blessing and a scourge to society.

As a plant with a long history of cultivation and use (Russo 2004; Schultes 1970; Wills 1998), Cannabis has been dispersed from origins in Central Asia, the northwest Himalayas, and, quite possibly, China (Nelson 1996; Schultes 1970) to a number of habitats throughout the tropical and temperate regions of the world (Russo 2004; Wills 1998) by populations enthralled by the intoxicating resin and the functional applications of the fibers and the extractable oil from the fruit (achenes, commonly known as seeds) (Clarke 1993; Schultes 1970). This dichotomy of uses for Cannabis as a medicinal and recreational drug and as a fiber and oil source has continuously stimulated public and scientific interest and curiosity in the value of the plant, leading to earlier reviews on the botany and other aspects of the plant (Brown 1998b; Boyce 1912; Clarke 1993; Guy 2004; Joyce and Curry 1970; Walton 1938). Cannabis is a member of the Cannabaceae family along with the genus Humulus (hops) and the genus Celtis (hack-berry and sugarberry).

11-5-2010-cannabis
Cannabis ilustration courtesy
Wikimedia Commons 

Nomenclature 

Complete taxonomic classification within the genus Cannabis remains under considerable dispute. Some authorities (Small and Cronquist 1976; Quimby 1974) claim that all Cannabis plants grown for fiber or resin or other purposes belong to the species C. sativa, with subspecies, such as C. sativa subspecies indica, to differentiate among types. Other authorities (Schultes and Hofmann 1991) insist that morphological differentiation (narrower leaflets, thinner cortex, and more branches) and lack of cannabinoids within plants of European origin, as compared with plants in India, indicate two species, C. sativa (historically identified as the source of hemp fibers) and C. indica (historically identified as the source of canabinoid-containing resin). Additional species have been distinguished: C. ruderalis (wild/naturalized accessions) and C. chinensis (currently thought to be a subset of C. indica) have been proposed due to differentiation in phenotypic traits of the plants (Schultes and Hofmann 1991). A recent investigation on allozyme (an enzyme that differs by one amino acid from other forms of the same enzyme) variation within 157 populations of Cannabis (Hillig 2005) strongly suggests that the genus Cannabis consists of only two species, C. sativa and C. indica.

The relationships within Cannabis species and the production of fiber and cannabinoids, however, are not completely understood, making abso-lute assignment of C. sativa as the source of fiber and C. indica as the source of the resin unwarranted until more complete chemotaxonomic data is available. The movement and selection of plants by growers and others has certainly led to a number of environmental and cultivated vari-ants of Cannabis, as the plants became adapted to growth in various loca-tions and growers chose and seeded plants (accessions) with desirable characteristics. Strains of Cannabis approved for industrial hemp production in Europe and elsewhere have been selected to produce only minute amounts of psychoactive constituents, while strains of Cannabis used for medicinal and recreational use have been selected for production of can-nabinoids (Small and Marcus 2002). A study of ninety-seven Cannabis accessions (de Meijer et al. 2003) indicated that plants produced for delta-9-tetrahydrocannabidiol (THC) and cannabidiol (CBD) demonstrated a continuous variation in content of these constituents among the acces-sions with no phenotypic characteristic (physical appearance) that could accurately separate those with high THC from those with low THC con-tent. All species of Cannabis can seemingly be bred to produce fiber or cannabinoids.

Botanical History 

Historically, botanical interest in Cannabis undoubtedly began as the plant became recognized as a source of food, fiber, and medicine. Archaeological evidence indicates use of the plant in China as a fiber some twelve thou-sand years ago (Nelson 1996; Schultes 1970). Early use as medicine is documented by inclusion of the plant in the first known Chinese materia medica (treatise on medical remedies), Pen Ts’ao (accredited to Emperor Shen Nung, 2737 and 2697 BCE), in which people were advised to cultivate the female plant for its greater medicinal properties (Schultes 1970). As the plant moved from country to country in trade, first to Asian counties such as India, Korea, and Japan, cultivation was initiated by farmers as demand for the product increased. Over subsequent years, several rituals were developed for cultivation of the plant, most likely to ensure the growth of the female plant to be used for the resin produced.

After contact with the Indian subcontinent by the Indo-Europeans, cultivation of Cannabis was spread throughout the Middle East and Europe for both fiber and resin. Cannabis was a fiber crop in America in prehistoric times (Schultes 1970), and the plant was prevalent in America before the arrival of European explorers. Early cultivators of Cannabis, including George Washington and Thomas Jefferson, provided extended notes on planting, harvesting, and expected yields, indicating a familiarity with the botany of the plant (Nelson 1996). After America outlawed the cultivation and use of marijuana, it became “hidden” among other crops, forested areas, and enclosed structures to prevent discovery. Yet, despite the efforts at restriction on the growth of Cannabis by local, state, and fed-eral governments, American growers are estimated to have produced 22.3 million pounds of marijuana with a value of $35.8 billion in 2006 (Gett-man 2006).

Growing in the wild, Cannabis plants usually have limited growth, generate small seeds, and produce small amounts of oil, fiber, or resin, as com-pared with cultivated species, due to lack of soil nutrients. To maximize development and productivity, the Cannabis plant, which is known as a “heavy feeder,” needs lots of mineral nutrients, levels that can be supplied under cultivated conditions. Other environmental variables, such as tem-perature, light, water availability, and plant spacing, also affect the growth and development of the Cannabis plant, causing variations in plant appearance and productivity

(Bósca and Karus 1998; Clarke 1993; Potter 2004). To maximize quality production for use of the plant for medicinal purposes and to ensure quality and “hidden” production of the plant for recreational use, Cannabis is often produced hydroponically in a greenhouse or enclosed room where the environmental conditions can be controlled (Clarke 1993; Potter 2004). Of the current Cannabis crop grown in America, 17 percent is thought to be cultivated inside buildings under controlled conditions (Gettman 2006). To meet the need for specialized equipment for controlled growth, various equipment suppliers offer hydroponic equipment and instructions designed to produce vigorously growing plants.

Morphology 

Cannabis is a rapidly growing dioecious (male and female reproductive organs on different plants), wind pollinated, annual herb that in some plant selections can reach heights of twenty feet (six meters) (figures 4.1 and 4.2). Seeds, which readily germinate within a week, develop two seed leaves (cotyledons) that are approximately one-half inch (1.7 cm) long, slightly unequal in size, and broader at the tip than the base (Clarke 1993; Stearn 1970). The first true leaves, which form as a pair on opposite sides of the stem at right angles to and approximately an inch above the cotyle-dons, consist of two narrow, serrated leaflets (blades) two to four inches (5 to 10 cm) long connected to the plant stalk by a distinct petiole (Clarke 1993). The next pair of leaves, formed at right angles to the first pair, can be unifoliate (one leaflet per leaf) or palmate (multiple leaflets arising at a single point on the end of the petiole). The number of leaflets per leaf generally increases as new leaves form on the stem until a maxi-mum of ten or eleven leaflets per leaf is reached.

The stem, which is sometimes hollow, is angular and pubescent (cov-ered in small hair-like structures) (Stearn 1970). If the plant has adequate growing space, the axillary buds (growing points located where the leaf joins the stem) will form branches. If vegetative growth conditions are fa-vorable, the stem will increase in height by two inches per day when ex-posed to the long daylight periods of summer. While some selections of Cannabis are day-neutral (flower under any day length), most are classified as short-day plants (they need a long dark period, usually fourteen hours or more) and shift from vegetative to generative (reproductive) growth upon exposure to short daylight periods. With the change to reproductive growth, the leaf pairs change from leaves opposite each other on the stem to an alternate, spiral arrangement (a single leaf on one side of the stem and the next leaf higher on the stem is not directly above the lower leaf) (Potter 2004).

During vegetative growth, male and female plants of Cannabis cannot be distinguished from each other with any certainty, although the female plant tends to be more stocky and flower later than the male plant (Raman 1998). As the plants enter the reproductive phase, however, the density of leaves on the upper part of the plants begins to differ, with the male plant having fewer leaves than the female plant. The male plant forms flowers in long, loose clusters (six to twelve inches long) from buds within claw-shaped bracts on branches at the top of the plant, while the female plant forms flow-ers in tight, crowded clusters from buds within tubular-shaped bracts. In male flowers, the bracts are formed from five relatively short pubescent se-pals (small leaflike structures), a half-inch long or less, that are a yellowish, greenish, or whitish in color (Clarke 1993). The female flowers are borne in pairs, and each individual flower is enclosed in green colored bracts (calyx formed by sepals).
The upper leaves, unfertilized flower heads, and flower bracts of the female plant are the primary source of cannabinoids in Cannabis (Russo 2004). The cannabinoids are enclosed in tiny (just visible to the eye) glan-dular trichomes (globe-shaped structures, supported on short stalks) found on bracts and floral leaves and unstalked, glandular trichomes (peltate trichomes) found on vegetative leaves and pistillate (flower) bracts (Hammond and Mahlberg 1977; Raman 1998; Starks 1990) and produce the sticky resin containing cannabinoids and terpenes.

Maximum cannabinoid-producing trichomes occur on the flowering part of the Cannabis plant during the late flowering period. The flowering head and other vegetative tissues (lower leaves and stems) of the plant can also develop three other types of trichomes (unicellular curved, squat uni-cellular, and bulbous) that do not produce cannabinoids. The number of resin-producing trichomes is higher in female plants than on male plants, especially on the bracts.

Applications 

Pollination occurs when pollen grains move from the male to the female flower by floating in the wind or by the purposeful transfer of pollen to cre-ate specific crosses (mating the same or two different strains to develop de-sirable features) (Green 2005). Following fertilization (the fusion of the male and female gametophytes, sperm and egg, respectively), the female flower develops seeds (achenes) over fourteen to thirty-five days. The male plant usually dies after shedding pollen, but the female plant, fertilized or unfertilized, continues to mature for another two to five months. Monoecious plants (having both male and female flowers on the same plant) occasionally occur, but this is not a normal occurrence except in specially selected varieties.

In new plantings from seed, an essentially equal number of male and female plants can be expected, although extreme stress, such as that pro-duced by nutrient excesses or deficiencies, temperature extremes, altered light cycles, or mutilation may increase the number of female plants in the population (Clarke 1993). Cannabis plants can also be grown from vegetative cuttings (asexual reproduction, also known as cloning, in which a small part of a plant is used to develop a complete plant) (Clarke 1993; Potter 2004). The use of vegetative cuttings is used to preserve unique characteristics, as the new plant will have the same genotype (genetic composition) and thus the potential for the same phenotypic char-acteristics as the plant it was taken from, though a different environment may change the plant appearance and chemistry. Offspring of sexually propagated plants (those grown from seed) will have genotypes different from the parent plants, as inheritable characteristics (genes) that deter-mine the plant phenotype come from both the male and the female plant. Planned plant crosses (mating of male and female plants by trans-fer of pollen from a specific male to a specific female) are used to develop new varieties.

Cannabis plants used for fiber production are strains that produce only very small quantities of cannabinoids (Rannali 1999; Schultes 1970). To produce the best fibers, these plants are best grown in cold or temperate regions and harvested in the juvenile (vegetative) stage of growth. As the plant ages, the bast fibers (fibers that run the length of the stem, produced in the inner bark) undergo lignification (a hardening that makes the fibers brittle) (Potter 2004). Monoecious plants are preferred for fiber produc-tion because the plants all mature at the same time, enabling mechanical harvest. Seeds to be used as a fixed (fatty) oil source (nonnarcotic), food, or propagation material are harvested only after the seeds have matured. Many seeds usually fall to the ground before harvest, as the achenes held in the flower pods are loose, and the pods have a tendency to dehisce (discharge seeds) upon ripening.

Cannabis plants grown for medicinal or recreational products are har-vested for the resin produced by the leaves and bracts of the flowering tops (Nelson 1999). To maximize the number of glands and resin production, male plants are frequently removed from the production location to pre-vent flowers on the female plants from producing seeds. The lack of seed formation induces the female plants to produce more flowers, leading to increased resin production, frequently with higher cannabinoid content than resin from plants with seeds (ElSohly et al. 1984). The plant material used for recreational purposes produced from nonseed flowering tops of female plants is known as sinsemilla and is valued for high THC content, enhanced appearance, and a more intense aroma, as compared with other similar products obtained from plants allowed to form seeds (Hanrahan 2001; Rosenthal 1984). Dried, crushed flowers and small upper leaves used as the recreational drug are commonly known as marijuana, while the resin collected by brushing the glandular trichomes from the plant tissues and used as a recreational drug is commonly known as hashish.

Conclusion 

As plants closely associated with humans for several thousand years, Cannabis species have undergone morphological and chemical changes through plant selection and breeding to adapt the botany of the plant to meet the needs of the populace. The closeness and overlapping of traits among the species has made differentiation difficult and created confusion among taxonomists (Schultes 1970; Schultes and Hofmann 1991). Differ-ences among Cannabis types suggest that some were selected and improved to produce fibers, while other types were selected and improved for pro-duction of cannabinoids. Such selection of desirable types continues and has led to plants that can grow in different environments and produce more resin or fiber than wild types of Cannabis (de Meijer, van der Kamp, and van Eeuwijk 1992, 1993; Hillig 2005; Russo 2004; Schultes 1970).

Considerable efforts in breeding and selection have produced Cannabis varieties and cultivars that are uniquely suited for production of medicinal or psychoactive compounds, fibers, and fixed oils. Examination of medicinal applications of Cannabis has been renewed in the past twenty years, with botanical selections now being made to meet that need (Guy 2004). Having been in close association with humans for thousands of years, the Cannabis plant continues to be botanically adaptable to meet the requirements of the societies in which the plant is grown.



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