368 
PHYSIOLOGY. 
BOOK II. 
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. Cestrum 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 anything is inter- 
posed between it and the sun its odour disappears, but is 
renewed as soon as the interference is removed," 
