866 ODOURS OF PLANTS. 
such origin is found, and the source of the odour is unknown, 
whilst its nature defies analysis. It is generally considered 
that smell is due to the giving off of minute particles into the 
air; Morren, however, from observations on the flowers of 
Orchids, was led to the inference that in some cases it depended 
on a physiological cause. He observed that the aromatic odour 
of Mawilluria, which continued to be exhaled so long as the 
flowers were unfertilised, was lost a little while after pollen was 
applied to the stigma. 
Though chiefly developed under the influence of solar light, 
there are not a few plant-odours which are given off in the 
evening or at night. Several Orchids, Cestrwm nocturnwm, 
Hesperis tristis, Lychnis vespertina, and Cereus grandiflorus are 
examples. In the Jast-named plant, the odour is given out in 
intermittent pufts. 
There seems to be a connexion between the colour and odour 
of flowers; thus it has been observed that white flowers are 
very frequently fragrant, whilst brown and orange ones have 
often a foetid smell—the so-called Carrion-flowers (Stapeliz), 
certain Aroids, some Balanophoracez, and the Rafflesiz being 
examples. The flowers of Monocotyledons are more often 
odorous than those of Dicotyledons. 
