1 88 LECTURE XT. 



But it is with the processes of destructive metabolism that 

 we are now especially concerned, the processes by which 

 complex substances are decomposed into others of simpler 

 composition. The principal factor in destructive metabolism 

 is doubtless what Pfliiger terms the " self-decomposition " of 

 the living protoplasm. According to this view living pro- 

 toplasm is constantly undergoing spontaneous decomposi- 

 tion, and one important use of the various complex organic 

 substances present in the organism, such as proteids, fats, 

 and carbohydrates, is that they serve as plastic material for 

 the reconstruction of living protoplasm. The metabolism 

 of the protoplasm thus consists in unceasing construction 

 and decomposition, the constructive and the destructive 

 processes being intimately connected. Of the products of 

 decomposition some can be again used in the constructive 

 processes, whereas others are of no nutritive value. 



This view of Pfliiger's, though, from the nature of the case 

 somewhat hypothetical, is supported by some direct obser- 

 vations. In treating of the formation of starch-grains 

 (pp. 145, 1 80) it was pointed out that the starch is a product 

 of the dissociation of molecules of what we must regard as 

 living protoplasm, and the same holds good of the cellulose 

 produced in the formation of cell-walls (p. 15). Again, 

 Sachs has observed that in the autumn the cells of deciduous 

 leaves become entirely emptied of their protoplasmic con- 

 tents ; protoplasm, nucleus, chlorophyll-corpuscles, all dis- 

 appear : they are decomposed into soluble and diffusible 

 substances which are conveyed away to the persistent parts 

 of the plant. 



But the destructive metabolism of an organism is not by 

 any means confined to the decomposition of protoplasm : 

 the various complex organic substances in the cells may 

 undergo chemical change quite independently of their enter- 

 ing into the metabolism of the protoplasm. We have already 

 learned that various substances are decomposed by means of 

 certain bodies which have been termed unorganised ferments 

 in order to distinguish them from the so-called organised 

 ferments such as Yeast and Bacteria. These unorganised 



