CHAPTER X. 



OF THE SECRETIONS OF PLANTS. 



359. WE have seen that the elaborated sap contains the 

 materials of the various tissues of the Vegetable fabric ; and an 

 outline has just been given, of what is known of the mode, in 

 which they are converted into living structure. The principal 

 uses to Man of the various kinds of these structures, will be best 

 stated, when the chief groups of plants are described, in the 

 second division of this volume. "We have next to consider a class 

 of products, which are not of the same character ; for they serve 

 no obvious purpose in the nutrition of the plant itself, and are 

 never converted (so far as can be ascertained) into the materials 

 of its tissues. They usually make their appearance in the ela- 

 borated sap ; but not unfrequently they are afterwards separated 

 in some degree from it, and stored up (as it were) in a particular 

 portion of the plant. In Animals we find a provision of a similar 

 kind. The blood not only contains the elements of the solid 

 tissues which are to be nourished by it, but also of fluid secretions, 

 which are separated from it by special organs. Hence the term 

 secretion, which means a separation or setting-apart, is derived. 



360. In Animals, however, such secretions are usually des- 

 tined to answer some obvious purpose, either in the system or out 

 of it. Thus the secretion of saliva serves to moisten the food, 

 and that of gastric juice to digest it ; and in this process it is one 

 function of the bile to assist. Again, the secretion of milk in the 

 female for the nourishment of the young, that of poison in the 

 venomous serpent for the destruction of its prey, that of the glu- 

 tinous fluid with which the spider constructs its web, are instances 

 of the separation of certain ingredients of the blood, which are 



