146 ' ODORS. 



sive heat of mid day producing upward currents of vapor, the 

 odorous 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 shovver 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 necessa- 

 ry, hence, after long rains, flowers become comparatively scent- 

 less, 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, showing 

 that the generation of the fragrant fluid, is dependent, as above 

 observed, on the direct action of the sun's rays. And it ie 

 well known , that most of the secretions of Phenogamous vege- 

 tables 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. 



211. Intermittent odors n.Ye such as are given oft" at partic- 

 ular times, and the plants which yield them, are entirely des- 

 titute of such odors at other times. Many Orchidaceae are 

 perfectly scentless, during the day, but during the night are 

 fragrant. A remarkable example of this class of odors, is ex- 

 hibited by the cacalia septenirionalis, 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 out fljishes or puffs 

 of perfume, as its intermittent odors are called. " Morren ob- 

 served in one case of a cut flower, that it gave off* puffs of odor 

 every half hour, from 8 to 12 P. M., when it faded, and the smell 

 became very slight. On another occasion, when the flower was 

 left on the plant, it began to expand at 6 P. M., when the first 

 fragrance was perceptible in the green house. A quarter of an 

 hour afterwards, the first puflTof odor took place, after a rapid mo 

 tion of the calyx ; in rather less than a second quarter of aL 

 h-our, another powerful emanation of fragrance took place . 

 by 35 minutes past 6, the flower was completely open ; and 

 at a quarter to 7, the odor of the calyx was the strongest, but 



