Re: how do flowers create their unique fragrances?

In addition to attracting people with their beautiful colors and fragrant 
aromas, flowers also attract insects, usually for the purpose of moving 
pollen from one flower to another.  This is a finely tuned system with a 
combination of smells and colors (including some we can't even see) to 
attract a specific insect to visit the same species of flower.  The effort 
of producing pollen is wasted if it doesn't get to the stigma of the same 
species.  During the course of evolution, it was advantageous to those 
flowers which produced an attractant.  The attractant production was an 
accident but had the effect of increasing pollination for the species.

What kinds of molecules are the attractants?  Each fragrance is a 
combination of a number of chemicals, but most of them are from a large 
and diverse family of molecules known as terpenes.  Chemically, terpenes 
are composed of different numbers and types of 5-carbon chains.  All 
plants make some terpenoid compounds, specifically chlorophyll and the 
carotenoids needed for photosynthesis.  You are probably more familiar 
with them from your interaction with some of the volatile terpenes (these 
are ones you can smell) which are major components in the aromas of lemon, 
camphor, menthol, mint, and turpentine.  As you might guess from this 
list, terpenes can act as repellants as well as attractants, which can 
also be useful to the plant.   

The plants did not set out to make all of these fragrant molecules.  They 
are what are called "secondary products" which means that they have no 
known use in the basic metabolism of the plant.  They were probably 
intermediates in the synthesis of a larger terpene (such as chlorophyll) 
which were accumulated due to a mutation.  Sometimes these mutations had 
an advantage for the plant by attracting pollinators or repelling 
predators.  Plants which had the mutation were able to produce more seeds, 
which also had the mutation.  Thus, the change was passed on to the next 
generation.  

You can read about some of the recent research in Anthurium in the journal 
Phytochemistry vol. 49, p. 521-528 (some of this work was done at the 
Missouri Botanical Gardens, just a short ride from Mad Scientist 
headquarters) or some work on orchids in the journal Science vol.164, p. 
1243-1249.  Go to the site 
http://www.gi.alaska.edu/ScienceForum/ASF8/852.html for a short discussion 
of some terpenes or search the web under "terpenes" to find the many ways 
terpenes touch (literally) your everyday life.