253 BIOLOGY. [BOOK u. 



spread over the surface of certain plants ? But we do not find them 

 in all plants 1 yet all plants must excrete. Must we admit, with 

 certain botanists, that the products of disassimilation are borne 

 along with the descending sap, and expulsed by the roots them- 

 selves? But all this is very improbable. In reality vegetals 

 disembarrass themselves ill and imperfectly of their disassimi- 

 lated products. Water indeed is exhaled on the surface of the 

 leaves and of the stalk. Certain semifluid substances are also 

 expulsed by simple exosmosis. But considerable residua, notably 

 a great quantity of mineral substances, remain in the vegetal 

 tissues, encumber them, and determine the death of the ana- 

 tomical elements which they contain. This is probably the 

 cause of the death and the fall of the leaves, of the decay and 

 disappearance from within to without of the ligneous elements 

 in the centre of the dicotyledonous trees. Also the blackish 

 cadaveric detritus which often bathes the internal wall of hollow 

 trees contains products of decomposition, ulmine and its 

 derivatives. 



It is in the animal kingdom alone that we find organs of secre- 

 tion numerous and perfectionated. Here the life is more intense, 

 and special organic agents are necessary to regulate the expulsion 

 of the disassimilated products, and form humours or substances 

 indispensable to the accomplishment of physiological acts of the 

 first order. 



As we have already remarked, a distinction is made between 

 the secretion which fabricates new principles, and the excretion, 

 which merely furnishes passage to the materials of disintegra- 

 tion. But the secretion itself is effected according to two 

 principal modes. It is operated either by glands without excretory 

 conduits, or by glands with excretory conduits. 



The difference between these two glandular types is however 

 purely . morphological. In the glands, with excretory conduits, 

 the product of the secretion is poured by a special canal on the 

 surface of the body, or of a mucous membrane (Fig. 35). 

 On the contrary, the closed glands act on the composition of the 



