514 CONSTRUCTIVE AND DESTRUCTH'E METABOLISM 



as is the case with regard to certain metabolic products of lichens, or the special 

 products formed by mixed cultures of bacteria 1 . 



The complicated relationships involved in these organic interactions have 

 hitherto mainly been studied in connexion with the diseases of animals 2 , and 

 no doubt a close study of this general problem will reveal facts of the utmost 

 importance. Indeed the existence of toxins and anti-toxins has already been 

 demonstrated in the case of animals, and it has been shown that a production 

 of substances for attack and defence may be induced in a variety of ways. 

 Similarly, induced immunity is simply the temporary or permanent after-effect of 

 a physiological reaction (cf. Sect. 3). 



SECTION 93. Self-regulation. 



Self-regulation is attained by the interactions between the different 

 organs both of the plant and of the protoplast, and each organ, however 

 minute, has its own specific reactive power. The most obvious example 

 of self-regulation lies in the fact that the course of normal vital activity 

 is such as to provide for its own continuance, while the absorption and 

 selection of a particular nutrient substance is determined by the needs of the 

 organism and the character of its metabolism. Similarly all the phenomena 

 of correlation show that the different organs of the plant and of the proto- 

 plast work in harmony, and mutually influence and regulate one another 

 (cf. Sect. 4). 



A thorough comprehension of all these relationships can be gained 

 only when a perfect knowledge of the intimate nature of the vital mechanism 

 has been attained, but on certain points sufficient is known to render dis- 

 cussion profitable. Thus self-regulation is possible because potential powers 

 arc exercised in varying degrees and in some cases not at all, while any 

 increased or diminished functional activity may more or less markedly 

 influence all other directly or indirectly connected processes, as for example 

 when a change in one or more partial functions ultimately affects metabolism 

 as a whole. 



During metabolism certain excretory products are necessarily pro- 

 duced which must be removed, as otherwise their gradual accumulation 

 would u'timately prevent further activity. Substances destined for building 

 purposes or for further use in metabolism are however not excreted, and 

 as a general rule are produced in definite amount ; so that even when 

 the consumption of such products is inhibited, no injurious accumulation 



1 Cf. Nencki, Centralbl. f. Bact., 1892, Bd. XI, p. 225; Burri u. Stutzer, ibid., 1895, Abth. ii, 

 Bd. i, p. 354; Lafar, Techn. Mykologie, 1896, p. So. 



- Cf. Fliigge, Mikroorganismen, 1896, 3. Aufl., Bd. i, p. 271 ; Buchner, Centralbl. f. Bact., 1893, 

 Bd. xiv, p. 235. 



