Erica's Original Notes Scents are part of a plant's advertising campaign; scented flowers may be less colourful, using scent rather than colour for attraction, as Nature does not waste energy. Odour comes from: a) anthers - pollen b) nectaries c) scent glands - essential oils.
Scent can be classified as aminoid (hawthorne); indoloid (rotting meat); heavy (jasmine); and aromatic (sweet and spicy). (See note 1 below)
Reasons for scent: to attract pollinators and discourage predators, as in marigolds.
Uses: added to foods, drinks and cosmetics.
Bees: sense of smell is a hundred times better than ours.
Forest scent: due to the terpenes emitted by the trees.
Loss of scent is noted to occur sometimes after pollinization has occurred.
Cleopatra: the sails of her boat fetching Mark Anthony were scented by having her slaves fan incense smoke into them!
Religions: many use scent, such as incense.
Perfume industry: flowers are picked at the peak of their scent output, jasmine before sunrise, carnations during the heat of the day. Some essences cannot be extracted and have to be made chemically, such as lilac and lily of the valley.
Extraction of scent is by: a) distillation: already practiced 7,000 years ago; the collected petals are boiled and the resulting gas collected in glass tubes, cooled into a liquid; b) enfleurage and extraction: an old, laborious method; glass plates covered in fat are spread with petals (enfleurage) and then the extraction consists of putting these plates in alcohol and heating. The essential oil is dissolved by the alcohol and rises to the top to be skimmed off. A ton of petals would result in 10 to 16 ounces of essential oil, quite a labour! c) expression is the method for obtaining the oils from lemon and orange rinds.
Some other scents: Mint in the leaves and stems; cinnamon in the bark; rosewood, cedar and sandalwood in the wood itself; cloves, nutmeg and anise in their seeds. Orris root of the iris smells of violets.
In Erica’s notes there is a mix of chemical and common classification of scents – which is not surprising as plant scents are categorised in various ways both biochemically and then by the perfume trade.
It may be useful to expand on her chemical classification – which may be based on the writing of Anton Kerner von Marilaun referenced at the end of this note. He distinguishes five groups of floral scents: the indoloid, aminoid, paraffinoid, benzoloid, and terpenoid scents.
Indoloid – as Erica comments is associated with the smell of rotting meat. Some flowers emitting these smells are known as carrion flowers e.g Titan arum (Amorphophallus titanum) which has the world’s largest flower head and draws crowds of people to see (and smell!)it
Aminoid - It has been shown that the odour of the hawthorn is due to trimethylamine. It is very probable that other flowers with similar scent such as pear, medlar and mountain ash, have trimethylamine or the related chemical ammonia as the major component.
Benzoloid – Examples of these compounds are eugenol (or oil of cloves) which gives the flowers of many pinks (Dianthus spp.) their fragrance, while Cinnamyl- alcohol smells like hyacinths, and the vanilla-like scent from Heliotrope flowers comes from another benzoloid compound.
Paraffinoid - the best known of these compounds are Valerianic acid producing the Valerian scent in the flowers of numerous Valerians, especially of Valeriana officinalis and Pelargonic acid, which is closely connected with Rose scents (but is named as such because it was originally extracted from Pelargonium leaves).
Terpenoid - The last group consists of scents produced from terpenes. As Erica mentions this scent is associated with 'forest scent' and is more usually produced by stems and other parts of the plant than the flower. The best-known terpenoid scent occurring in flowers is Neroli oil produced from the blossom of the bitter orange tree (Citrus aurantium subsp. amara). Its scent is sweet, honeyed and somewhat metallic. Also Lavender oil found in the leaves and flowers of Lavendula species and the citron scent from Oil of Citron which occurs in the flowers of some species of Thyme (Thymus citriodorus)
Flower scent is complex – not only can the scent of a flower be a mixture of several different chemicals, but these may be released in different balances at different times of day. E.g. Daphne odora, is suggested to have a blend between rosy notes and neroli oil elements, plus a strong lemon element.
It is interesting that many of the same scents are found in species of very different plant families, suggesting they have developed for similar purpose at several separate times in plant evolutionary history.
Conversely, similar and closely-related species of plants often have very different scents. E.g. species of the genus Daphne - Daphne alpina has a vanilla scent, Daphne striata a lilac scent, and Daphne blagayana a clove scent.
RP References: For more information about scent glands (osmophores):
Peter K Endress (1994), Diversity and Evolutionary Botany of Tropical Flowers, Page 166-170
Classification of scents in plants: Anton Kerner von Marilaun (1895-96) The natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2)