CHAP. II.] CHEMISTRY, ETC., OF PLANT LIFE. 89 



swollen roots, bulbs, and the trunks of trees also contain 

 reserve material for future use. During tlie process of 

 metabolism a great many products are formed that are 

 of no known use in plant economy, and may be termed 

 by-products of metastasis; as examples may be men- 

 tioned several volatile oils, acids, alkaloids, resinous 

 matters, etc. Degradation products, as the various kinds 

 of gum, are produced by the more or less complete 

 disintegration of cell-walls, and cannot be re-converted 

 back into plastic or formative material. 



