438 
PHYSIOLOGY. 
BOOK II. 
CHAPTER XVII. 
OF ODOURS. 
All that relates to the cause of odour in plants is enveloped 
in great obscurity, and will remain so till chemists shall have 
examined the subject much more carefully than hitherto. 
Our senses are daily gratified by the sweet perfumes exhaled 
by the leaves and flowers that surround us ; and art exhausts 
its skill to preserve them by means which enable us always to 
have them present for our use ; but as to the reasons why one 
kind of flower is odoriferous and another scentless, or why 
that which is perfumed in the evening is scentless at noon, 
we are still more in the dark than in what relates to colour. 
The facts connected with the enquiry have been well stated 
by De Candolle. 
All odours are owing to the disengagement of volatile 
matter; and as there are few organised bodies in which, in 
their natural state, there is not some volatile constituent part, 
so neither are there many organic bodies absolutely destitute 
of smell. But it is only to cases in which the scent is very 
perceptible to our senses that we apply the idea of odoriferous, 
and it is consequently to those that we here confine ourselves; 
dividing them mto permanent^ fugitive, and intermittent. 
Those odours are the mo?,l permanent in which the volatile 
matter is so enclosed in cells and concentrated as to disperse 
slowly. Of this many instances are afforded by wood and 
bark, which, being in truth the only permanent parts of 
vegetation, will of necessity be the receptacle of durable 
odours. Such parts are not scented, because of their own- 
proper nature, for all the tissue of plants is originally scent- 
less, or nearly so ; but they owe their property to the fragrant 
secretions imprisoned in their cavities, and the permanence of 
their odour will be proportioned to the difficulty the volatile 
