The genus Cannabis belongs to the Cannabaceae family, a small but diverse grouping of flowering vines, herbs, shrubs and trees. The 270 or so individual species of this family are distributed throughout the temperate regions of the northern hemisphere, and several of them have useful applications in medicine, textiles or food. At first glance, many Cannabaceae species appear to have little in common; the sinuous vines of the Humulus genus bear little resemblance to the tropical evergreen trees of the Trema genus, for example, and the erect herbs of the Cannabis genus are radically dissimilar to the tall, grey-hued trees of the Celtis genus. But there are various characteristics found in the genera of the Cannabaceae family that are common to many species, and from that viewpoint, their interrelatedness is less surprising.
The genus Cannabis belongs to the Cannabaceae family, a small but diverse grouping of flowering vines, herbs, shrubs and trees. The 270 or so individual species of this family are distributed throughout the temperate regions of the northern hemisphere, and several of them have useful applications in medicine, textiles or food.
What do Cannabaceae plants have in common?
At first glance, many Cannabaceae species appear to have little in common; the sinuous vines of the Humulus genus bear little resemblance to the tropical evergreen trees of the Trema genus, for example, and the erect herbs of the Cannabis genus are radically dissimilar to the tall, grey-hued trees of the Celtis genus. But there are various characteristics found in the genera of the Cannabaceae family that are common to many species, and from that viewpoint, their interrelatedness is less surprising.
Almost all plants found within the Cannabaceae family are dioecious, meaning that males and females exist as separate plants; the most notable exception to this is the Celtis (hackberry) genus, a grouping of 60-70 tree species which are all monoecious (with separate male and female flowers on the same plant). While monoecy can occur in other species within the Cannabaceae family, including Cannabis, the default is generally dioecy; thus, some botanists believe that Celtis actually belongs to its own family, the Celtidaceae.
Cannabaceae leaf characteristics
Cannabaceae plants generally possess either palmate lobed or palmate compound leaves. Cannabis itself generally has palmate compound leaves, with separate leaflets radiating outward from a central stem; however, some genotypes of Cannabis—such as Ducksfoot—express a palmate lobed phenotype, exhibiting ‘webbed’ leaves more reminiscent of Humulus than Cannabis.
Cannabaceae leaves all exhibit stipules (small outgrowths) on either side of the petiole, or the stalk that attaches the leaf to the stem—in Cannabis, the stipules are the two small spikes that appear at the internodes—as well as cystoliths in their cellular matrix. Cystoliths, which are often situated at the base of a leaf hair, are tiny deposits of calcium carbonate that build up throughout the lifetime of the plant.
While their purpose has not been fully ascertained, it is suggested that these cystoliths may absorb and store toxic heavy metals such as cadmium to prevent toxicity of more sensitive cells. Cannabis has already proven useful as a remediator for soils contaminated by industrial heavy metals; this may partly explain why, and may also indicate that other plants within the family could also have this use.
Flower characteristics of Cannabaceae plants
Cannabaceae flowers are generally small, symmetrical and non-showy (lacking bright colours, specialised shape, and strong fragrance). Flower appearance is generally adapted for wind pollination rather than insect or animal pollination, thus showy traits are not positively selected for. Instead, traits allowing more effective wind pollination have been selected for; the corolla (the main, showy whorl of petals often found in flowering plants) is usually absent and the calyxes (the outermost whorl of petal-like structures that often protects the petals as they grow) are short, allowing easy access to the stamen of the male flower and the stigma of the female flower.
In most Cannabaceae plants, the flowers are arranged in groupings known as cymes. A cyme is a flower cluster with a terminal flower at the end of the stalk that matures fastest. Once the terminal flower matures, new growth appears from the sides. This accounts for the growth structure of the female Cannabis plant, where the flowers become denser after they have ceased to grow longer. Female flowers are classed as bracteate spicate cymes—clusters of flowers surrounded by bracts (modified leaves that surround the flower), which are arranged in spikes and mature according to the cymose pattern. Male flowers are arranged in bracteate cymose panicles—loose branching clusters of flowers possessing bracts, with a cymose growth pattern.
There is mild controversy among botanists as to the correct taxonomic classification for the eleven genera comprising the Cannabaceae family. Up until recently, it was supposed Cannabaceae belonged to the taxonomic order Urticales; now, genetic testing has shown that the families making up the Urticales more properly lie within the Rosales order, which includes roses, strawberries and countless other commercially-valuable crops. Urticales is now no longer considered a family in its own right, merely an interrelated subclade of the Rosales.
However, Cannabaceae and other formerly-Urticales families are still more closely related than many other Rosales, a fact that has earned them the informal classification of Urticalean Rosids. The other Urticalean Rosids are Ulmaceae (elm), Urticaceae (nettle), and Moraceae (mulberry). Of these, the tall Celtis trees of the Cannabaceae family are seen as being more closely related to the Ulmaceae, while the similarities between nettles, mulberry, and Cannabis have often been noted.
Other notable Cannabaceae plants
Although it is commonly believed that Cannabis and Humulus are the only two Cannabaceae genera with any commercial value, there are several other species that are of great value in the regions in which they are indigenous. Various Celtis species are cultivated either for timber or as ornamentals, such as the protected South African tree C. africana, which is attractive, fast-growing and frost- and drought-resistant. Its abundant flowers and small, sweet fruits also attract a variety of birds and insects to the garden. C. sinensis from East Asia and C. australis from southern Europe are also widely cultivated for these purposes.
Trema orientalis, a common species distributed throughout the tropical and warm temperate regions of Africa, Asia and Oceania, has abundant uses. It is commonly known as the charcoal-tree by many indigenous populations, as one of its uses is to make charcoal. Its wood burns quickly and easily, and is also useful for making paper and pulp, due to the high tensile strength of the fibres. The fibres of the bark are used to make rope or twine, and the sap exuded can be used to waterproof fishing lines. It is also used as animal fodder. Interestingly, T. orientalis has various traditional medicinal uses, many of which are quite similar to those of Cannabis—it is used to treat dysentery, inflammation, sore throats, and asthma, among other things, and has shown promising results in preliminary trails as a blood-sugar stabiliser in diabetes.
Aphananthe aspera, an East Asian species also known as the Muku tree, is a highly-prized ornamental traditionally used in Chinese classical gardens. A. aspera is also useful as a source of fibre and wood, and its leaves are used as fine sandpaper for polishing wooden and similar objects. A particularly fine, seven-trunked A. aspera is listed as one of South Korea’s natural monuments; this sixteen-metre specimen is believed to be around three hundred years old.
As our knowledge of plant phylogeny, taxonomy, and pharmacology continues to grow, it is likely that more applications for the plants of the Cannabaceae family will be discovered. Simultaneously, as human activity continues to fragment ecosystems across the globe, particularly in the tropical regions, the rate at which plant and animal species are disappearing is rapidly increasing—to the point that ecologists predict a mass extinction event occurring in the near future, potentially within just a few decades. Thus, it is of the utmost importance that plants are classified and assessed for their usefulness, so that we can protect as many potentially-important species as possible.