VEGETABLE SECRETION. 45 



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 

 converted into gummy, saccharine, amylaceous, 

 or ligneous products ; there are other celhdar 

 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 are unknown, and are therefore 

 referred generally to the vital energies of vege- 

 tation ; 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 combi- 

 nation with oxygen as the representative of 

 water : this is the case with all the oily secre- 

 tions, whether they be fixed or volatile, and also 

 with those secretions which are of a resinous 



