GROWTH AND SECRETIONS. 77 



their flavour to it ; and that they are volatilized 

 by heat without decomposition. The fixed oils, 

 on the contrary, are inodorous and insipid, sup- 

 port two or three hundred degrees of heat with- 

 out volatilizing, and are decomposed at a higher 

 temperature. In a physiological point of view 

 their difference is equally striking. The volatile 

 oils are found in the leaves or in the cortical 

 system, the usual place of the secretions ; the 

 fixed oils are either situated in the seeds them- 

 selves, or more rarely in the tissue of the peri- 

 carp. 



52. There are many local secretions, of which 

 a detailed account belongs more properly to a 

 chemical treatise than to one that, like the pre- 

 sent, is only physiological, and also too brief to do 

 more than glance at the other sciences immedi- 

 ately connected with the subject : it will there- 

 fore only be possible to notice these secretions 

 slightly here. They consist of acids, such as 

 citric, malic, acetic, &c. prussic acid (remarkable 

 by the absence of oxygen) which is found in 

 peach and laurel leaves, &c. of Gluten, Albumen, 

 Tannin and Colouring matter, of which indigo 

 is one of the most important, and a variety of 

 other secretions or principles, each confined to 

 the particular vegetable in which it is found, 



