Article in Critical Reviews in Plant Sciences · November 2016 -Robert C. Clarke & Mark D. Merlin
Humans and the Cannabis plant share an intimate history spanning millennia. Humans spread Cannabis from its Eurasian homelands throughout much of the world, and, in concert with local climatic and human cultural parameters, created traditional landrace varieties (cultivars resulting from a combination of natural and farmer selection) with few apparent signs of domestication.
Cannabis breeders combined populations from widely divergent geographical regions and genepools to develop economically valuable fiber, seed, and drug cultivars, and several approaches were used with varying results. The widespread use of single plant selections in cultivar breeding, inbreeding, and the adoption of asexual reproduction for commercial drug production, reduced genetic diversity and made many present-day cultivars susceptible to pathogens and pests. The great majority of drug Cannabis cultivars are now completely domesticated, and thus are entirely dependent on humans for their survival. Future ramifications remain to be realized.
I. Cannabis botany and ecology
The ecological requirements and genetic inheritance of plants determine where they grow naturally. Cannabis plants require well-drained soils, adequate sunlight, warmth, and moisture, so most naturally growing populations are found seasonally across accommodating northern temperate latitudes. Under natural conditions, Cannabis grows well along exposed riverbanks, lakesides,the margins of agricultural land, and other areas disturbed by humans (Merlin, 1972; Clarke, 1977, 1981;
Clarke and Merlin, 2013; Small 2015).
Based on ecological constraints, Cannabis evolved somewhere in temperate latitudes of the northern hemisphere, and Eurasia is
favored as its primary region of origin (Clarke and Merlin, 2013). Seminal uses, early cultivation, worldwide dissemination, and eventual domestication of Cannabis all began within this natural biogeographical range.
Cannabis plants are usually dioecious and produce either male (pollen) flowers or female (seed) flowers. An individual Cannabis plant’s gender is determined by X and Y chromosomes, and monoecious plants of this
genus producing flowers of both sexes occur only rarely in nature. Cannabis also relies on air currents to spread pollen grains from male plants to female seed plants.Wind pollination, dioecious sexuality, and X/Y sexual inheritance are each relatively rare in plants, yet all three
are characteristics of Cannabis.
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