Wayne Handlos

1. 2. 3.

1. human anatomy  2. aging & olfaction  3.English language of odors

4.  5.

4. Variation in citronellol concentration in S. Africa

5. Array of different essential oils in "Rose" geranium


6. Changes in levels of menthol & isogeraniol during vegetative stages of P. sp.

7.  8.  9.  10.

7. & 8. Floral scented P. gibbosum (flowers & leaf)

9. & 10. Coconut scented P. grossularioides (flowers & leaf)

11. 12. 13. 14. 15.

11. & 12. P. reniforme (flowers & leaf)  13. & 14. P. exstipulatum (flower & leaves)

15. 'Chocolate Peppermint'

Olfaction or the sense of smell is of importance when it comes to talking (or writing) about

scented geraniums/Pelargoniums. We have noted that the scents produced by various plants

can vary from time to time based on various environmental factors. More exhaustive studies show

that among the scented geraniums there are very significant differences in which scented compounds

are produced by different species. Local population studies (particularly in P. capitatum in South

 Africa) have shown that there are great differences among the populations as to which set of scented

chemicals and what quantity of each of them is produced (Fig. 4). The primary commercial cultivars

of the "rose geranium" yield very different arrays of essential oils and produce them in different quantities

 (Fig. 5). The essential oils are not produced in equal quantities on all plant parts � the leaves being

much more productive than the stems. In addition younger plants produce less oil than older plants.

Various aspects of the climate and weather, including sunlight, rainfall, temperature, humidity and time

of harvest (Fig. 6) affect the quantity and quality of rose oil produced by commercial cultivars.

Furthermore, such physical conditions as soil fertilization, shade, weeds, diseases and irrigation also

have an effect on the oil being produced.

But we haven't noted the extent to which individual humans vary in their ability to detect scents.

Humans (and many other animals) have special sensory cells in the skin (epithelium) lining their

nasal cavities. The sensory cells are connected by nerves to the part of the brain called the

olfactory bulb which is located above the nasal cavity and below the frontal lobes of the brain

(Fig. 1). Volatile, air-borne, scented chemicals get attached to the olfactory receptors and a

signal is sent to the olfactory bulb where it is interpreted as a scent. The relationship between

the scent molecule and the receptor is thought to be very precise, like a lock and key, and only

the appropriate scent receptor and scent molecule will trigger a signal to the olfactory bulb.

It has been suggested that there are a thousand genes that code for odor reception in animals.

Each person has only a few of these genes functioning � so the human ability to recognize different

scents is much less than many other animals. Studies have determined that humans have as many

as 368 different olfactory receptors. However, not everyone has the same set of receptors which

are under genetic control. Each person has his own distinct/unique combination of receptors. So

we then end up with a situation where we are trying to recognize the same scent but we don�t have

the same genetic equipment to recognize the scented chemicals.

Humans are estimated to have about 10 square centimeters of olfactory epithelium while a dog may

have 170 square centimeters of such epithelium � in addition to as many as 100 times more receptors

on each square centimeter of epithelium. Hunting dogs� have the ability to smell one million to ten

million more times acutely than humans, while bloodhounds are 10 to 100 million times more acute

in their ability to detect scents. Mice may have 1000 different olfactory receptors putting them at the

top of the list of creatures with different abilities to detect different odors.

Studies have shown that humans' peak ability to detect scents is between the ages of 20 and 40.

In the general population this ability then declines with time (Fig. 2). So while 20-40 year olds can

identify 50 to 75% of odors, 50 to 70 year olds can identify only 30-45% of odors. (D. Purves et

al., 2001, Neuroscience 2nd ed.)

Women's sense of smell is more acute than men's (Fig. 2). The decline in ability to detect different

scents is more pronounced in people with Parkinson's disease or Alzheimer's syndrome. Some

commonly prescribed drugs, especially anti-cholesterol and anti-hypertension drugs are known to

reduce olfactory ability.

Classical studies of people of German and Japanese cultures show significant differences in their

ability to identify various scents. The European languages have very few abstract words to identify

different scents (unlike their capacity to identify different colors with abstract terms). Europeans

most often describe scents in relation to some event in life. On the other hand certain ethnic groups

on the Malay Peninsula have numerous abstract words for recognizing different scents (Fig. 3).

However, it is possible to develop the sense of smell and an appropriate vocabulary through training

as is the case with perfumers and sommeliers.

Different chemical compounds can be detected in different concentrations so there is no one threshold

level at which odors become detectable.

As an aside, about one in a thousand humans cannot detect butyl mercaptan, the scent produced by

skunks. And one in 10 cannot detect hydrogen cyanide (a poisonous gas) or ethyl mercaptan (the

chemical added to natural gas to make it detectable).

But to reiterate. Any scented geranium/Pelargonium produces essential oils that give that plant its

distinctive odor. Chemical studies show that each plant produces a variety of different scent compounds

at the same time. (The quantity and proportions of each compound varying through time and in reaction

to environmental and genetic factors.) Some of these compounds are judged to be pleasant while others

are less so. The number of chemical compounds produced by each plant may include as many as 120

different substances. When a human nose/brain registers these constituents each of us gets different

messages depending on which receptors we have in our repertoire. So, it is not surprising that some of

us like a given plant's smell while others of us find nothing of interest (maybe no smell at all) and yet

others of us find the plant to smell unpleasant or even repulsive.

Think of the implications of all this on the beverage (wine, brandy, etc.) and scent (perfume, cologne,

deodorant, soap) industries.



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