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Humulus lupulus, the common hop or hops, is a species of flowering plant in the hemp family Cannabaceae, native to Europe, western Asia and North America. It is a perennial, herbaceous climbing plant which sends up new shoots in early spring and dies back to a cold-hardy rhizome in autumn. It is dioecious (separate male and female plants). Hops are sometimes described as bine plants rather than vines because they have stiff downward facing hairs that provide stability and allow them to climb. These shoots allow H. lupulus to grow anywhere from 4.6 to 6.1 metres (15 to 20 ft). Hops have fragrant, wind-pollinated flowers that attract butterflies. The female cone-shaped fruits from H. lupulus are used by breweries to preserve and flavor beer, and so H. lupulus is widely cultivated for use by the brewing industry. The fragrant flower cones, known as hops, impart a bitter flavor, and also have aromatic and preservative qualities. H. lupulus contains myrcene, humulene, xanthohumol, myrcenol, linalool, tannins, and resin. The genus name Humulus is a medieval name that was at some point Latinized after being borrowed from a Germanic source exhibiting the h•m•l consonant cluster, as in Middle Low German homele. According to Soviet Iranist V. Abaev this could be a word of Sarmatian origin which is present in the modern Ossetian language (Ossetian: Хуымæллæг) and derives from proto-Iranian hauma-arayka, an Aryan haoma. The specific epithet lupulus is Latin for “small wolf“. The name refers to the plant’s tendency to strangle other plants, mainly osiers or basket willows (Salix viminalis), like a wolf does a sheep. Hops could be seen growing over these willows so often that it was named the willow-wolf. The English word hop is derived from the Middle Dutch word hoppe, also meaning Humulus lupulus. Humulus lupulus is a perennial herbaceous plant up to 10 meters tall, living up to 20 years. It has simple leaves with 3-5 deep lobes that can be opposite or alternate . The staminate (male) flowers do not have petals, while the pistillate (female) flowers’ petals completely cover the fruit. The cones found on female plants are called strobili. The fruit of H. lupulus is an achene, meaning that the fruit is dry and does not split open at maturity. The achene is surrounded by tepals and lupulin-secreting glands are concentrated on the fruit. Humulus lupulus grows best in the latitude range of 38°-51° in full sun with moderate amounts of rainfall. It uses the longer summer days as a cue for when to flower, which is usually around July/ August. Humulus lupulus can cause dermatitis to some who handle them. It is estimated that about 1 in 30 people are affected by this. Humulus lupulus is first mentioned in 768 CE when King Pepin donated hops to a monastery in Paris. Cultivation was first recorded in 859 CE, in documents from a monastery in Freising, Germany. The chemical compounds found in H. lupulus are main components in flavoring and bittering beer. Some other compounds help with creating foam in beer. Chemicals such as linalool and aldehydes contribute to the flavor of beer. The main components of bitterness in beer are iso-alpha acids, with many other compounds contributing to the overall bitterness of beer. Until the Middle Ages, Myrica gale was the most common plant used for brewing beer. H. lupulus took off as a flavoring agent for beer because it contains preserving agents, making the beer viable for longer. Predicted genes in homologous primary contigs have been identified as accounting for various traits expressed via variation in the growth, flowering, and stress responses in the plant. These homologous primary contigs correspond to regions with large amounts of sequence variation. Genes in the hop that contain higher rates of sequence divergence in homologous primary contigs (overlapping DNA sequences inherited by a common ancestor) have been attributed to the expression of flowering, growth and responses to (both antibiotic and biotic) stress in the plant. The responses to stress are thought to manifest in the distinct differences and difficulties in the cultivation processes between geographically popular varieties of the hop plant. Outside environmental stress, such as changes in temperature and water availability has also been shown to significantly alter the transcriptome and incite reductions in genes known to be involved in the synthesis of secondary metabolites (including bitter acids), which are organic compounds produced that do not impact development or reproduction of hops. Environmental stress has also been shown to reduce expression of the valerophenone synthase gene, which is known to be an essential genetic component in the regulation of bitter acid production. This shows that impacts of outside stress on Humulus lupulus likely has a direct implication of the expression of the bitter flavor that remains an essential component of the popularity of the plant
