72 GROWTH AND SECRETIONS. 



almost imperceptibly, and as a simple effect of 

 the porous nature of the tissue. The nourish- 

 ment of the cellular tribes appears then to be 

 thus accomplished ; the water which reaches 

 them penetrates either at given points, or by the 

 whole surface, and reaches the cells, where it is 

 elaborated by each, separately, in its own cavity. 

 50. As the blood of animals performs two 

 distinct offices, first, depositing throughout the 

 whole body the materials necessary for the 

 nourishment of each organ ; and, secondly, un- 

 dergoing in certain particular organs, named 

 glands, an operation which is called secretion, 

 and from which results the formation of par- 

 ticular juices ; so in the vegetable economy, the 

 sap, besides affording the general sustenance 

 which has been considered above, experiences a 

 peculiar action in certain organs, and furnishes 

 peculiar secretions as the result. These secre- 

 tions never form any part of the tissue of a 

 plant, and are either excrementitious, i. e. those 

 which are thrown off ; or special secretions, which 

 remain, in most cases, where they are formed, 

 and are seldom removed from one organ to 

 another ; but in others pervade the whole plant, 

 and, as in the case of Tannin, impregnate 

 the soil around them. The excretions are ex- 

 tremely various, and are probably a provision 



