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 andrcecial 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- 



