456 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



if wholly composed of plastic material, can only consume a certain portion 

 of its own substance. The proteids of which the protoplastic framework 

 is constructed seem for the most part to be comparatively stable and 

 aplastic, but many bacteria and fungi are able to consume the cell-walls 

 and plasma of dead individuals of their own kind. Auto-digestion is no 

 doubt prevented during life not merely by the chemical nature of the living 

 substance, but also by its structural arrangements and by the proper regu- 

 lation of its metabolic activity. 



Even in an individual plant no hard and fast line of demarcation 

 can be drawn between plastic and aplastic constructive materials, and 

 a good nutrient substance for one organism may be useless to another, 

 especially in the case of fungi and bacteria. It is, however, of the utmost 

 importance for the maintenance of the balance of nature that almost every 

 metabolic product of both plants and animals shall be assimilated or 

 decomposed by some heterotrophic organism or other. This is the case 

 with such typical aplastic products as tannin, wax, alkaloids, pigments, &c., 

 and even resin appears to be attacked by certain bacteria. 



Many plastic materials are found in but few plants or only in small 

 amounts, but the non-formation of a particular substance affords no sure 

 sign that it may not be available in metabolism when presented to the 

 plant. As might however be expected, the majority of the special products 

 formed by a restricted number of plants, usually in limited amount, 

 belong for the most part to the class of aplastic substances. These 

 may function as aids to metabolism (enzymes) or may have a biological 

 importance (pigments, ethereal oils, alkaloids, &c.), and it is of manifest 

 importance that a small quantity should suffice for the attainment of 

 the required end. Why certain aplastic products, such as tannin, are 

 commonly produced in large amount cannot as yet be satisfactorily 

 answered. 



It is always important to know whether a substance is produced as a 

 direct means to a certain end, or only as an accidental by- or end-product 

 (Sects. 50, 77). When an aplastic substance is formed in limited amount 

 it is safe to assume that the former is the case, whereas continuous 

 production and excretion characterize a waste end-product, such as carbon 

 dioxide, or in ferment-organisms, lactic or butyric acids, alcohol, hydrogen, 

 sulphuretted hydrogen, ammonia, &c. The formation of such excreta can, 

 however, be regulated and controlled, so that under different circumstances 

 a particular waste product may cease to be formed or may even be con- 

 sumed by the same organism which produced it (cf. Sects. 50, 77). 



The metabolic activity must necessarily correspond to the needs of 

 the organism and the conditions under which it exists, so that in cor- 

 respondence to the adaption to different modes of life, the plastic and 

 aplastic products of closely allied plants may differ widely in character 



