138 
JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
Modern researches have amply proved that ozone is 
developed when the sun shines on most kinds of fragrant 
plants, such as flowers, fir and pine trees, and sweet herbs 
generally. 
What is Odour or Perfume ? 
Now let us ask ourselves what odour or perfume really is. 
I asked a very celebrated chemist this question the other day, 
and he said frankly that odour, like electricity and many other 
things, was a very subtle and “ unknown quantity,” and that no 
one knows absolutely and precisely what it is, nor why one odour 
should please us, and actually invigorate or stimulate us, while 
another disgusts us so much that we sometimes call it by 
another name. Odour seems a product given off by the action 
of oxygen on essential oils—a vapour being evolved under 
certain physical conditions of heat, moisture, or pressure, and 
even light and darkness now and then have some share in its 
evolution. 
Leaf Odours versus Floral Odours. 
When we compare leaf odours with flower odours we find a 
considerable difference between them. Thus in the case of the 
orange there is a difference between the essential oils of the 
flowers and of the leaves, and of that of the rind of the fruit, 
which afford three different kinds of perfume. 
Then floral odours are generally positive, being exhaled by 
most flowers spontaneously as it were, so that you must inhale 
floral odours whether you like them or not. 
Leaf odours, on the other hand, are latent or negative, and 
are rarely to be detected except after the leaves have been 
touched, pressed, or bruised. Both leaf and flower perfume 
depends on the same essential oil being in different states or 
conditions. 
Floral odours again are emitted only at particular times, 
that is to say, just when the androecial whorl attains maturity, 
and the flowers are quite fresh ; and even then, in the case of 
many Orchids and other flowers, their scent is intermittent, and 
only to be perceived at different times of the day or night—this 
time, as we suppose, having some connection with the diurnal 
or nocturnal visits of the insects that act as marriage priests in 
their native wilds. But, on the other hand, leaf odours are per- 
