CHAPTER III. DESTRUCTIVE METABOLISM. 24I 



the plant, do not return at once to the mineral kingdom from 

 which they were derived, but their return is by successive steps 

 and by various roads. The products of destructive metabolism 

 are, therefore, numerous and varied. They range in complexity 

 from bodies like the alkaloids to those as simple as carbon-diox- 

 ide and water. It is not easy to draw the line between products 

 of metabolism which are still serviceable to the plant, and those 

 which are not. Some products of the disintegration of proto- 

 plasm, being of the utmost nutritive value, are hardly to be 

 classed as products of destructive metabolism. This is the case 

 with starch and other carbo-hydrates, fixed oils, and the amides. 

 But here we must class the organic acids, for, though some of 

 them are certainly to some extent nutritive, they are on the 

 whole much less valuable. In the ripening of acid fruits, some 

 of the acids are apparently converted into sugar, a nutritive 

 carbo-hydrate ; some do a service in bringing mineral matters of 

 the soil into solution, so that it may be taken up by the root- 

 hairs, and some are of use in decomposing mineral salts. The 

 glucosides, which are probably produced by the breaking down 

 of proteid matter, have not been proved to possess nutritive 

 value beyond being a source of sugar. When decomposed by 

 ferments or organic acids, the glucose they yield may, of course, 

 be applied to nutritive uses. The alkaloids are probably purely 

 waste products, as are also the resins and volatile oils, so far at 



M 



least as nutritive purposes are concerned ; they may, however, 

 as has already been suggested, be of some slight use, by afford- 

 ing protection against animals and destructive fungi, or, in the 

 case of volatile oils, they may indirectly aid in cross-fertilization. 

 While in a general way the products of destructive change 

 resemble each other in different plants, in fact, in many cases are 

 identical, this is not always the case. There are numerous 

 instances where a compound is produced by a single species or 

 genus, and is not found elsewhere. The valuable alkaloid, quinine, 

 for example, is, so far as we know, confined to the genus Cin- 

 chona ; the bitter principle Aloin is not known to occur elsewhere 

 than in the genus Aloe, and the very poisonous glucoside, digi- 

 toxin, has never been met with in any plant except the Foxglove. 

 Moreover certain species under some conditions develop 

 products which, under different conditions are never produced. 



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