The morphology of Cannabis sativa L. Achenes
Cannabis sativa L. has been domesticated for fibre, oilseed, and marijuana; it also occurs as ruderal plants. “Marijuana” refers to plants selected for high concentrations of the chemical tetrahydrocannabinol (THC), while “hemp” refers to plants low in concentration of THC and which are domesticated for either stem fibre or oilseed. In my first chapter I review the botany of cannabis, taxonomy and origins, ethnobotany, and crop ferality. In my second chapter, I performed a study where achenes (“seeds”) from herbarium collections representative of these classes of C. sativa were assessed for variation in morphological characters and pericarp resistance to fracture. Multivariate analysis of the data revealed significant divergence among the groups. In contrast to ruderal plants, domesticated plants (hemp or marijuana) possessed achenes that are significantly longer, heavier, covered with a less adherent perianth, and lacking a pronounced basal attenuation. These characteristics reflect traits that are advantageous in domesticated plants, and are consistent with the "domestication syndrome”. Marijuana achenes, in comparison with hemp achenes, are shorter and darker. Achenes of fibre cultivars are larger than the achenes of oilseed cultivars. Achenes of dioecious oilseed cultivars are larger than the achenes of monoecious oilseed cultivars. We propose several mechanisms by which this phenotypic divergence may have occurred, including potential differences in outcrossing rate and the evolution of life history strategies among C. sativa groups that deserve further exploration. While only one species of Cannabis merits recognition, we postulate these phenotypic differences in C. sativa are a result of domestication for different purposes. In my final chapter I discuss the limitations and future studies. This work contributes a more complete understanding of cannabis morphology to the greater body of literature on plant domestication.