VEGETABLE SECRETION. 39 



chemical action, which effect still greater changes. The na- 

 ture of the. agents by which these changes are produced are 

 unknown, and are therefore referred generally to the vital 

 energies of vegetation; but the process itself has been termed 

 Secretion^ and the organs in which it is conducted, and 

 which are frequently very distinguishable as separate and 

 peculiar structures, are called Glands. When the products 

 of secretion are chemically analyzed, the greater number 

 are found to contain a large quantity of hydrogen, in addi- 

 tion to that which is retained in combination with oxygen 

 as the representative of water: this is the case with all the 

 oily secretions, whether they be fixed or volatile, and also 

 with those secretions which are of a resinous quality. Some, 

 on the contrary, are found to have an excess of oxygen; and 

 this is the condition of most of the acid secretions; while 

 others, again, appear to have acquired an addition of nitro- 

 gen. 



All these substances have their respective uses, although 

 it may frequently be difficult to assign them correctly. Some 

 are intended to remain permanently enclosed in the vesicles 

 where they were produced; others are retained for the pur- 

 pose of being employed at some other time; while those be-^ 

 longing to a third class are destined to be thrown off from 

 the system as being superfluous or noxious: these latter sub- 

 stances, which are presently to be noticed, are specially de- 

 signated as excretions. Many of these fluids find their way 

 from one part of the plant to another, without appearing to 

 be conducted along any definite channels, and others are 

 conveyed by vessels, which appear to be specially appro- 

 priated to this office. 



The following are examples of the uses to which the pe- 

 culiar secretions of plants are applied. 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 substance, analogous to 

 that which many marine worms are known to possess. The 



