46 THE VITAL FUNCTIONS. 



quality. Some, on the contrary, are found to 

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

 tion of most of the acid secretions ; while others, 

 again, appear to have acquired an addition of 

 nitrogen. 



All these substances have their respective uses, 

 although it may frequently be difficult to assign 

 them correctly. Some are intended to remain per- 

 manently inclosed in the vesicles where they were 

 produced ; others are retained for the purpose 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 applied. 

 Many lichens, which fix themselves on calcareous 

 rocks, such as the Patellaria immersa, are ob- 

 served, 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 



