GROWTH AND SECRETIONS. 73 



for the removal of some material which is use- 

 less or injurious to the plant. One of the most 

 singular is that of the fraxinella, though this is 

 probably of the same class with the volatile oils to 

 be mentioned presently. If at the close of a dry, 

 hot day, a light be held near the top of that plant, 

 the vapour which surrounds it takes fire, and 

 burns with a lambent flame, without injury to the 

 plant. This vapour appears to be of the nature 

 of an extremely volatile oil, which escapes from 

 the small glands that cover the surface of the 

 plant, for the white fraxinella, which has fewer 

 glands than the red, exhibits the phenomenon in a 

 slighter degree. Other excretions are acid, some 

 are caustic, some glutinous (such as the leaves 

 of the gum cistus). Some plants secrete a waxy 

 matter from their surface ; others saline or sac- 

 charine particles. Manna is one of these excre- 

 tions, it both exudes naturally, and is also ob- 

 tained when artificial incisions have been made 

 in the tree. It would be impossible to describe, 

 or even enumerate, all the excretions of plants in 

 this treatise, the above may convey an idea of 

 their nature. 



51. The Special Secretions are liquids secreted 

 in the bark, or some other organ. Their prin- 

 cipal characters are, 



1. That they are all composed of two or 



