VEGETABLE SECRETION. 39 



tioii of the leaves : their foot-stalks, more especially 

 suffering from this obstruction, perish, and occasion 

 the detachment of the leaves, which thus fall off at 

 the end of each season, making way for those that 

 are to succeed them in the next. 



§ 6. Secretion in Vegetables. 



While the powers of the simpler kinds of cells are 

 adequate to produce in the returning sap the 

 modifications above described, by which it is con- 

 verted into gummy, saccharine, amylaceous, or 

 ligneous products ; there are other cellular organs, 

 endowed with more extensive powers of chemical 

 action, which effect still greater changes. The 

 nature of the agents by which these changes are 

 produced is unknown, and is 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 analysed, the 

 greater number are found to contain a large 

 quantity of hydrogen, in addition to that which is 

 retained in combination with oxygen as the repre- 

 sentative 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 nitrogen. 



