114 ODOUR. 



upon the other hand, some plants have been shown to 

 lose them when vegetating in a warm atmosphere. 

 Nocca says a Calendula lost its odour in a warm green- 

 house ; and Link observed the Marrubium vulgare to be 

 odourless in Portugal. 



155. Many plants will not afford their odour until 

 bruised or broken ; in others they are so volatile, that 

 they are continually flying off from the plant. The Dic- 

 tamnus fraxinella secretes an essential oil in great 

 abundance ; and in warm weather this exudes and vola- 

 tilizes, so that the air becomes highly impregnated with 

 it, and it is said that if a candle be brought near, the oily 

 vapour will take fire. 



156. The most curious circumstance connected with 

 this subject, is a certain periodicity observed in some 

 plants in regard to the evolution of their odour. Hes- 

 peris tristis is destitute of odour in the day, but is 

 fragrant at night. Pelargonium triste evinces by day no 

 trace of scent, but near sunset, and generally at a stated 

 hour, it begins to be odorous. Cactus grandiflorus 

 developes its delicious odour at the expansion of its 

 flower, which takes place an hour after sunset ; and 

 Morren observed the odour evolved at intervals of half 

 an hour. 



157. According to Chevallier and Bouillay, a great 

 number of odorous plants, both agreeable and dis- 

 agreeable, exhale a considerable quantity of Ammo- 

 niacal gas. 



158. It is a well-known physiological fact, says an 

 author, that powerful odours often affect the nervous 

 system strikingly, and hurtfully ; even flowers of the 

 most delightful odours do it when they are collected to- 

 gether in great number. The delicate scent of Lime 

 trees when they are in a state of bloom, causes faint- 

 ness and insensibility to those sleeping undar them ; and 

 Violets have produced the like results in healthy females, 

 when too many had been placed in the apartments they 

 slept in. The influence of vegetable odours upon hys- 

 terical females is yet more striking ; but in all such cases, 



