46 THE VITAL FUNCTIONS. 



quality. Some, on the contrary, are found to 

 have an excess of oxygen ; and this is the con- 

 dition of most of the acid secretions; while 

 others, again, appear to have acquired an addi- 

 tion of nitrogen. 



All these substances have their respective uses, 

 although it may frequently be difficult to assign 

 them correctly. Some are intended to remain 

 permanently inclosed in the vesicles where they 

 were produced ; others are retained for the pur- 

 pose of being employed at some other time ; while 

 those belonging to a third class are destined to be 

 thrown off from the system, as being superfluous 

 or noxious : these latter substances, which are 

 presently to be noticed, are specially designated 

 as excretions. Many of these fluids find their 

 way from one part of the plant to another, with- 

 out appearing to be conducted along any definite 

 channels; and others are conveyed by vessels, 

 which appear to be specially appropriated to this 

 office. 



The following are examples of the uses to 

 which the peculiar secretions of plants are ap- 

 plied. Many lichens, which fix themselves on 

 calcareous rocks, such as the Patellaria immersa, 

 are observed, in process of time, to sink deeper 

 and deeper beneath the surface of the rock, as if 

 they had some mode of penetrating into its sub- 

 stance, analogous to that which many marine 

 worms are known to possess. The agent appears 



