512 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



lowered, it requires a certain interval before the plant accommodates 

 itself to the new conditions by diminishing its respiratory activity (Sect. 100), 

 and the same would probably be the case if the supply of nutriment was 

 suddenly reduced to an amount insufficient to satisfy the previous metabolic 

 requirements. As the plant accommodated itself to these changed condi- 

 tions it would probably not only grow more slowly but also perform all its 

 different vital activities in a more economical manner than when fed to 

 excess (cf. Sect. 73). A return to the original conditions is as a general 

 rule followed by the gradual assumption of the previous equilibrium, 

 provided no permanent displacement of the latter has been produced. 



Temperature. Since a certain temperature is one of the essential conditions for 

 life, it follows that the action of temperature is mainly a stimulatory one, and that 

 a change of temperature affects metabolism in a manner corresponding to the 

 reactive power of the organism itself, and modified only to a slight extent by any 

 direct action upon the chemical processes involved in metabolism. The physio- 

 logical curve and the curve of chemical reaction hence do not coincide, for the latter 

 usually commences at a lower temperature than the former, which again soon falls 

 above a certain optimum, whereas the chemical curve continues to rise until a very 

 high temperature is reached at which molecular dissociation occurs '. 



A rise of temperature may affect antagonistic processes differently, as for 

 example, when potatoes kept at oC. to 6C. become sweet owing to the produc- 

 tion of sugar, which above 10 C. is again converted into starch 2 . The change is 

 still more rapid in the so-called starch-trees (Tilia, Betula, &c.), for when brought 

 into a warm room in winter, starch appears in the cortex in a few hours, and is 

 rapidly reconverted into sugar when returned to a low temperature : \ a phenomenon 

 which may be frequently repeated. It seems very commonly to be the case that 

 when exposed to cold, plants contain a larger proportion of sugar, and perhaps also 

 of other soluble substances, than when at a higher temperature 4 . 



The action of different substances. Metabolism is influenced and regulated not 

 only by the quality and quantity of the nutrient material but also by the accumula- 

 tion of the metabolic products (Sect. 93). Non-nutritive substances may exercise 

 quite as pronounced an influence, and it has already been mentioned that sub- 

 maximal doses of many and perhaps all poisonous substances accelerate respiration, 

 growth, &c. Indeed chemical stimuli may play a much more important part in 

 metabolism than appears possible at present. 



Reciprocal and antagonistic actions. The entire organic world and all the 

 friendly and antagonistic relationships of different organisms are primarily regulated 

 by the necessity of obtaining food (cf. Sects. 51, 64, 65, 76). Fungi, bacteria, and 



1 Details in text-books of physical chemistry. On changes in chemical reactions at very low 

 temperatures, cf. R. Pictet, Compt. rend., 1892, T. cxv, p. 814. 



2 H. Miiller-Thurgau, Landw. Jahrb., 1883, Bd. xi, p. 751 ; 1885, Bd. xiv, p. 851. 



3 A. Fischer, Jahrb. f. wiss. Bot, 1891, Bd. xxir, p. 112 (cf. Sect. 109). 



4 For examples, see H. Miiller-Thurgau, I.e., 1883, p. 787; Copeland, Einfluss von Licht u. 

 Temp. a. d. Turgor, 1896, p. 5 ; Rosenberg, Bot. Centralbl., 1896, Bd. LXVI, p. 337; Lidforss, ibid., 

 1896, Bd. I. xvi ii, p. 33. 



