HORTICULTURAL JOURNAL. 185 
cumstances mentioned, and the apparent suspension of their emission 
have not been satisfactorily determined. It has been supposed that the 
heat of mid-day, under the direct action of the sun's rays, produces so 
much evaporation as to empty, in a great measure, the cells, and that the 
stomatas close and prevent the emission of the odorous substance; again, 
it is thought that the excessive evaporation would carry off more of 
the odors than the plant could generate, and thus the supply becomes 
exhausted during these hours of heat, and it requires the coolness of even- 
ings, when aqueous evaporation is nearly suspended, for the plant to regain 
its supply ; but a more probable reason (were we disposed to attribute it to 
any one alone) we conceive to be, that the excessive heat of mid-day, pro- 
ducing upward currents of vapor, the ordorous emissions are carried with 
them beyond our notice ; but, as night comes on, the currents cease, and 
the fragrant exhalations accumulate near the earth. A shower plainly 
would produce the same effect, cooling the surface of the earth, and 
reversing, in some degree, the atmospheric currents. 
In the production of odors the direct light of the sun is necessary ; 
hence, after long rains, flowers become comparatively scentless-; and this 
circumstance adds weight to the reason given above, and shows that the 
emission, so far from depending on the absence of light, as would seem at 
first view, from the fact of their becoming more sensible at the approach of 
night, and ceasing as the light becomes more intense ; a long continuance 
of even cloudy weather prevents the emission entirely, shoAving that the 
generation of the fragrant fluid is dependent, as above observed, on the 
direct action of the sun's rays. And it is well known, that most of the 
secretions of Phanerogamous vegetables require the same action, and the 
more volatile products especially. From the extreme volatility of the 
substances producing fugitive smells, and the necessity of the direct solar 
rays for their secretion, we could not be led to suppose that any loss of the 
secretions could take place under the influence of the mid-day sun, or that 
they could be detained in tissues which were continually emitting watery 
exhalations. 
Intermittent odors are such as are given off at particular times, and the 
plants which yield them are entirely destitute of such odors at other times. 
Many Orchidacese are perfectly scentless during the day, but during the 
night are fragrant. A remarkable example of this class of odors is 
exhibited by the Cacalia septentrionalis, which, when exposed to the direct 
rays of the sun, emits a strong aromatic odor, but by merely interposing 
a screen between it and the sun, its fragrance vanishes. The Cereus gives 
