438 THE FOOD OF PLANTS 



Vanadium has been shown by Knop. Zinc has been detected by Forchhammer 

 in certain woods, Titanium by Salm-Horstmar in cereals. 



Arsenic has been frequently found in plants 1 . Arsenious acid is extremely 

 poisonous, whereas many, both of the higher and of the lower plants, can withstand 

 large doses of arsenic acid and can accumulate large quantities of arsenic when 

 supplied to them in this form. Gosio has observed that various fungi evolve 

 arseniuretted hydrogen in the presence of carbohydrates and arsenic compounds. 



Boron is absorbed whenever present and traces are found in many plants 

 growing in the open. Borates are rather poisonous, but according to Loew less 

 so in the case of certain algae. 



According to Knop, Telluric acid is not markedly poisonous, but selenic and 

 selenious acids are. All three acids have been detected in Phanerogams. 



Fluorine has only been found in a few cases, but traces may be frequently 

 present in plants, for fluorides are found in bones, eggs, Sic. According to 

 Tamman soluble metallic fluorides are comparatively poisonous. 



Iodine and Bromine occur in marine plants but may be found in terrestrial 

 plants as well, for the latter can absorb iodides and bromides. Both are probably 

 present as soluble salts, and Golenkin's supposition that free iodine or a coloured 

 iodine-compound may be present in the vacuoles of living cells of Derbesia 

 Lamourouxii requires further proof. 



SECTION 76. The Influence of the Quality of the Soil upon the 



Distribution of Plants. 



Climatic conditions, especially the temperature and the amount of 

 moisture, are the main factors in determining the geographical distribu- 

 tion of plants, for an autotrophic plant can obtain sufficient nutriment 

 from any fertile soil. Nevertheless, all those plants do not develop upon 

 a given area which might do so if protected from competition, for the 

 latter influences the distribution of plants to a great extent, and is often 

 responsible for the localized range of a particular species. The influence 

 of competition is admirably shown when a garden is allowed to run wild, 



1 Arsenic: Gorup Besanez, I.e.; Daubeny, Jahresb. cl. Chemie, 1861, p. 736; E. W. Davy, 

 Jahresb. d. Agr.-Chemie, 1860-61, p. 83 ; Targioni-Tozzetti, Ann. d. sci. nat., 1846, iii. sen, T. V, 

 p. 177. Arsenious acid: Nobbe, Versuchsst, 1884, Bd. xxx, p. 394. Arsenic acid: Knop, I.e., 

 p. 49 ; Loew, System d. Giftwirkung, 1893, p. 19 ; Molisch, Sitzungsb. d. Wien. Akad., 1896, Bd. cv, 

 Abth. i, p. 10 ; cf. Stoklasa, Zeitschr. f. landw. Versuchsst. in Oesterreich, 1898, p. 154; Gosio, 

 Jahresb. liber Gahrungsorganismen, 1893, p. 83. Boron: Hotter, Versuchsst., 1890, Bd. xxxvil, 

 p. 435 ; Bechi, Bot. Jahresb., 1891, p. 30; Brand, Bot. Centralbl., 1894, Bd. LX, p. 189; Loew, 

 Flora, 1892, p. 37. Fluorine: Salm-Horstmar, Jahresb. d. Chemie, 1860, p. 540; Wilson, cf. Ad. 

 Mayer, Agr.-Chemie, 1895, 4. Aufl., p. 292 ; Tamman, Zeitschr. f. physiol. Chemie, 1888, Bd. XII, 

 p. 323. Iodine and Bromine: Dircks, Ber. cl. Sachs. Ges. d. Wiss., 1869, P- 2O '> Knop, ibid., 1885, 

 p. 44 ; Golenkin, Algol. Notizen, Bull. d. 1. Soc. Imp. d. Naturalistes, Moscow, 1894, p. i. 



