CHAP. XVII. 
ODOURS. 
441 
eminently favourable to the eliciting of vegetable perfumes, a 
continuance of wet and gloomy weather, without much sun- 
shine, is as greatly unfavourable. This latter circumstance is 
explicable upon the general law of physiology, that secretions 
cannot be readily produced without the direct assistance of 
the sun’s light. 
With regard to what we call intermittent odours, no expla- 
nation seems possible in the present state of our knowledge. 
A few examples of them will therefore be all that we can 
give. All dingy-flowered plants, such as botanists call tristes^ 
belong to this class ; such as the Pelargonium triste, Hesperis 
tristis. Gladiolus tristis, which are almost entirely scentless 
during the day, but become deliciously fragrant at night. 
Great numbers of Orchideous plants have flowers possessing 
the same property : the Catasetums have a fine aromatic 
odour at night, none in the day, except C. purum ; Cymbi- 
dium sinense is also chiefly fragrant at night ; and so with a 
great many more. Oestrum nocturnum is another plant of 
the same nature; in the day it has no odour, at night its 
perfume is extremely powerful. One of the most singular 
instances of exceptions to all rules appears to be referable to 
this class : Cacalia septentrionalis exhales an aromatic odour 
if exposed to the direct rays of the sun; and if any thing is inter- 
posed between it and the sun its odour disappears, but is 
renewed as soon as the interference is removed. 
The best observations upon intermittent odours, that I know 
of, are those of Morren ( Observations sur V Anatomie et la Phy~ 
siologie de la Fleur du Cereus grandijlorus) , He states that in 
this plant the fragrance is not traceable to any glandular ap- 
paratus, or to some reservoir of secretions, but that it is strictly 
functional, a vital action of the organs of fructification. The 
fragrance is evidently formed in the organs that part with it ; 
for when an unexpanded flower was cut in two in the morning, 
being at that time scentless, it became fragrant towards 7 
o’clock in the evening. The odour is undoubtedly formed 
in each cell of parenchyma by a particular process.” It is the 
property of the Cereus flower to part with its fragrance at 
intervals only. Morren observed in one case of a cut flower, 
that it gave off* puffs of odour every half hour, from 8 to 12 
