40 THE VITAL FUNCTIONS. 



All these substances have their respective uses, 

 although it may frequently be difficult to assign 

 them correctly. Some are intended to remain 

 tpermanently 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 pre- 

 sently to be noticed, are specially designated 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 appropriated to this 

 office. 



When the plant is in full vegetation, a quantity 

 of fecula is, as we have seen, contained in the form 

 of minute granules, enclosed in the globules of 

 chlorophyllite ; the amount of nutritive substance 

 thus accumulated, when the extent of the leaves is 

 taken into account, must be very considerable : 

 and it appears to be deposited in reserve for future 

 occasions, when extraordinary supplies of nourish- 

 ment are wanted for the developement of the fruit. 

 In those plants which lose their leaves in autumn, 

 this fecula is absorbed and carried into the interior 

 of the stem, where it remains deposited until the 

 ensuing spring, when it furnishes materials for the 

 evolution of the buds.* 



The following are examples of the uses to which 

 the peculiar secretions of plants are applied. Many 

 lichens, which fix themselves on calcareous rocks, 



* See Mohl, Ann. cles Sc. Nat. Bot. serie 2, ix, 166. 



