THE FUNCTIONS OF THE ESSENTIAL ELEMENTS 431 



has a distinctly injurious effect upon the plant 1 , it remains for precise 

 researches to determine whether a minimal amount is essential, or whether 

 chlorine simply favours growth under special cultural conditions. The latter 

 is quite possible, for either chlorides may be specially adapted for certain 

 metabolic processes, or the presence of chlorine may aid in the maintenance 

 of the acidity of the nutrient solution (Sect. 23), or some accelerating action 

 may be exercised, such as many non-essential or even poisonous substances 

 are capable of exciting (Sect. 73). Traces of sodium chloride, though 

 they exercise no poisonous action, may increase the activity of lactic 

 fermentation. Metallic chlorides may favour the growth of fungi by 

 diminishing the production of oxalic acid, which again exercises a retarding 

 action only under certain cultural conditions 2 . Hence it is possible that the 

 higher plants when grown in humus may dispense with chlorine, or that any 

 favourable influence it has may be replaced by that of another element, 

 while it has not yet been determined whether bromine and iodine may 

 produce the same effect as chlorine. If sufficiently diluted, potassium 

 bromide is not deleterious, nor is potassium iodide, although the latter 

 is more apt to injure plants 3 . 



It is still uncertain whether the action of salt upon the typical plants 

 of the seashore is merely due to its osmotic power, or whether its influence 

 is a specific one, and, if the latter is the case, whether the sodium or 

 the chlorine is responsible for the changes of shape and of assimilatory 

 activity which salt may produce, not only in strand-plants, but also in 

 other plants as well. 



The alkaline earths. Salm-Horstmar first proved the necessity of calcium 

 and magnesium for Phanerogams, and this observation has since been confirmed 

 by several investigators. Molisch and Benecke have recently shown that fungi 

 and certain algae require magnesium, but not calcium. Before their researches 

 A. Mayer had developed yeast, Raulin Aspergillus, and Winogradsky Mycoderma 

 vim in the absence of calcium, but owing to Nageli's erroneous assumption that in 

 fungi the alkaline earths may replace one another, these results were capable of 

 a different interpretation. The experiments on which Nageli's conclusions were 

 based must have been performed with impure salts, for when proper precautions 

 are taken, the fact that magnesium is essential may easily be demonstrated 4 , and 



1 Nobbe, Versuchsst., 1865, Bd. vn, p. 371, and 1870, Bd. xin, p. 394; Beyer, ibid., 1869, 

 Bd. XI, p. 262; Wagner, ibid., 1871, Bd. xm, p. 128; Aschoff, Landw. Jahrb., 1890, Bd. XIX, 

 p. 113. According to Knop and Dworzak (Ber. d. Sachs. Ges. d. Wiss., 1875, p. 61), maize 

 develops normally in a nutrient solution free from chlorine. 



2 Wehmer, Bot. Zeitung, 1891, p. 374. Lactic fermentation: Richet, Compt. rend., 1892, 

 T. cxiv, p. 1494. 



3 Dircks, Ber. d. Sachs. Ges. d. Wiss., 1869, p. 20; Knop, ibid., 1885, p. 44; Loew, Flora, 



i89 2 > P- 374- 



* Ad. Mayer, Unters. iiber d. Alkoholgahrung, 1869, p. 44; Raulin, Ann. d. sci. nat., 1869, 



v. ser., T. XI, p. 224; Winogradsky, Bot. Centralbl., 1884, Bd. xx, p. 167; Nageli, Bot. Mitth., 

 1881, Bd. in. p. 461. 



