1 34 ODORS. 



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 oti' more of 

 the odors than the plant could g<^nerate, and thus the supply 

 becomes exhausted during these hours of heat, and it requires 

 the coolness of evenings, when aqueous evaporation is nearly 

 suspended, for the plant to regain its supply ; but a more prob- 

 able reason (were we disposed to attribute it to any one alone) we 

 conceive to be, that the excessive heat of mid-day, producing up- 

 ward 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 shower 

 plainly would produce the same effect, cooling the surface of the 

 earth, and reversing, in some degree, the atmospheric currents. 



253. 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, showing 

 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 espe- 

 cially. 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 con- 

 tinually emitting watery exhalations. 



254« Intermittent odors are such as are given off at particular 

 tipies, and the plants which yield them are entirely destitute 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 exhibited by the 

 Cacalia sejytentj'ionalis, which, when exposed to the direct rays 

 of the sun, emits a strong aromatic odor, but by merely inter- 

 posing a screen between it and tlie sun, its fragrance vanishes. 

 The Cereus gives out flashes or puffs of perfume, as its inter- 

 mittent odors are called. "Morren observed in one case of a 



253. What is necessary for the production of odors ? How do long rains 

 affect odors ? — 254. What are intermittent odors ? What of the Cacalia! 

 CereiiB '. 



