H2 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



In certain cases resting organs may be prematurely awakened to 

 renewed activity by means of chemical stimuli. Thus treatment with 

 chloroform induces a resumption of growth and a development of the 

 buds in plants during the winter resting period. In addition, special 

 chemical stimuli are requisite for the development of many spores, and for 

 that of the seeds of a few Phanerogams. The chemotropic movements 

 shown by so many freely motile organisms, as well as those shown by the 

 leaves of carnivorous plants, such as Drosera or Dionaea, afford good 

 instances of responses produced by special chemical substances. 



Chemical stimuli may also exercise a retarding action. Thus 

 Winogradsky and Omeliansky 1 have found that the development of the 

 nitrate and nitrite bacteria is retarded or suppressed by the presence of very 

 small traces of substances, such as peptone, glucose, and asparagin, which 

 have a high nutritive value for most bacteria and fungi. In this way, the 

 process of nitrification is suppressed until the growth of those denitrifying 

 organisms, which consume nitrates and nitrites with loss of nitrogen, has 

 ceased owing to the consumption or decomposition of the glucose, peptone, 

 &c., which they also require. Further, the' nitrite bacteria which oxidize 

 ammonia can withstand the presence of large quantities of ammonium 

 salts, which, even when in considerable dilution, retard or inhibit the 

 development of the nitrate bacteria which oxidize nitrites to nitrates. 

 Similarly the development of most bacteria is prevented by the presence 

 of small amounts of acid, even when produced by the organism itself, 

 whereas some species, and most fungi, are highly resistant to acids 2 . If, 

 however, the acid is neutralized, or if in the case of the nitrate and nitrite 

 bacteria the sugar or peptone is removed, the organisms resume their 

 growth, the removal of the retarding agency acting as a favourable stimulus 

 to their development. 



The following table by Winogradsky and Omeliansky gives the percentage 

 of certain substances which retards and inhibits the development of nitrite and 

 nitrate bacteria. The ammonia was presented in the form of the sulphate. 



Glucose 

 Peptone 

 Asparagin 

 Ammonia 



1 Winogradsky and Omeliansky, Centralbl. f. Bact., 1899, II, Bd. v, p. 436. Duggar (quoted 

 by Pfeffer) has found that i % peptone retards the germination of Uslilago avenarum. 



a Cf. Fliigge, Mikroorganismen,i896, 3.Aufl.,Bd.i, p. 456; Zumstein. Jahrb. f. wiss. Bot., 1899, 

 Bd. XXXIV, p. 174 (Euglena). Concerning the plants which grow on alkaline soils see Davy, Investi- 

 gation of the Native Vegetation of Alkali Lands, 1898 (Report of the University of California). 



