132 ODORS. 



step further than had before been taken in the explanation of 

 these common phenomena. 



We can only give in few words the results to which his ex- 

 tended observations have led him. He concludes that these 

 various changes are owing to a derangement or suspension of 

 functions of the organs of nutrition. This point he strengthens 

 by the consideration that the puncture of an insect will cause an 

 organ to pass through all the steps to maturity, giving all the 

 hues belonging to its species, whether of fruit or leaves. Also, tho 

 cold of autumn and winter produces a similar derangement : 

 although the agent is different, yet the result is the same. 

 Many evergreen leaves become tinged with red in winter from 

 the influence of cold, but, with the return of summer, assume 

 their accustomed greenness ; also, the leaves of the extremities 

 of the branches being most exposed to atmospheric influences 

 are changed to red, while those nearer the trunk continue green. 

 If one half of a leaf be protected from the cold it will remain 

 green, while the other half will change to red. But in the case 

 of fruit, heat is the agent in producing similar effects to those 

 above ascribed to mechanical injury and cold. 



Section 9. — Odoi-s. 



248. Much of the importance attached to flowers by people 

 generally, is owing to the odors they exhale. The rose has long 

 been cultivated by amateurs, no less for its grateful fragrance, 

 than for its beauties of form and color; and those which com- 

 bine these properties, are the most favored objects of the Florist's 

 care. The cause of the odors of plants is, no doubt, the dis- 

 engagement of a volatile oil, which, in some cases, is easily ob- 

 tained, and made subservient to the use of man ; in others it 

 entirely eludes every effort to confine or preserve it, being as 

 evanescent as the light, which is the agent of its production. 



249. Odors are distinguished into permanent, fugitive, and 

 intermittent. Permanent odors are such as are inclosed in the 

 tissues of the wood and bark of plants in a concentrated form ; 

 and either from being but slightly volatile, or contained in close 

 vesicles which prevent exhalation, they remain for a long time, 

 giving to the organs in which they are contained their peculiar 

 odor. There is probably no part of a vegetable absolutely desti- 

 tute of permanent odor. Every variety of wood, under certain 



247. How does Mohl explain the change of color in ripening fruit? 

 Autumn leaves and evergreens in winter ? — 248. What is the cause of odor 

 in plants ? — 249. How are odors distinguished ? What are permanent 

 odors ? Give examples. 



