426 THE FOOD OF PLANTS 



the periodic system, their atomic or molecular weights, their tendency to 

 dissociation, their osmotic properties and the power of their salts to exchange 

 ions, could the acquirement of this physiological importance have been 

 predicted l . The fact that none of the elements with high atomic weights 

 are essential may simply be due to their rare occurrence and restricted 

 distribution. Indeed if caesium can replace potassium, it affords an example 

 of an element with a high atomic weight acquiring important functions in 

 the vital mechanisms of certain organisms. The atomic weights of the 

 essential, and commonly present but non-essential elements, lie between 

 that of iron (56), and of hydrogen (i). 



Iron. The absence of iron is at once perceptible in chlorophyllous 

 plants, for they do not become green, but it seems to be of importance 

 in many other ways, and is also essential to fungi. Moreover, since 

 according to MolisCh 2 the molecule of chlorophyll contains no iron, the 

 non-formation of chlorophyll may be merely a pathological phenomenon, 

 for it is well known that even in the presence of iron the formation of 

 chlorophyll may be partially or entirely suppressed when the plant is in 

 an unhealthy condition. A small portion of the iron is apparently held 

 by the plant in the form of organic compounds, and may thus take part 

 in the integral structure of the plasma, and hence of the chloroplastids as 

 well. It appears that dilute hydrochloric acid dissolves the iron from all 

 proteids except nuclein compounds, and hence the iron present in seeds, &c. 

 seems to be mainly combined with nucleins. Moreover it is from plants 

 that animals obtain the organic iron-compounds which they require 3 . 



The necessity of iron for Aspergillus, Penicillimn and other fungi was 

 first conclusively proved by Molisch, although in spite of all precautions 

 the development of the fungi could not be completely suppressed by its 

 withdrawal. This is, however, readily explicable, owing to the fact that the 

 merest trace of iron may suffice to permit growth, and hence various 

 authors have concluded that it was not essential. When more than the 

 optimal amount is present the growth of fungi may be accelerated owing to 



1 Cf. Errera, Malpigbia, 1886; Sestini, Versnchsst., 1886, Bd. xxxn, p. 197 ; Benecke, Ber. d. 

 Bot. Ges., 1894, Generalvers., p. 115. Nageli's attempt (.Bot Mitth., 1881, Bd. Ill, p. 466) to 

 explain the physiological difference between the alkalies and alkaline earths as due to the different 

 relationships of the salts to water hardly requires discussion, for the ideas then current as to the nutiitive 

 value of the different elements were paitly erroneous, and on these Nageli's conclusions were based. 



a Molisch, Die Pflanzen in ihren Beziehungen zum Eisen, 1892, p. 81. 



3 Neumeister, Physiol. Chemie, 1893, I, p. 311, and on nucleins containing iron, I.e., p. 40; 

 Petit, Compt. rend., 1892,!'. ex IV, p. 246. The reagents Molisch employed caused him to erroneously 

 believe that he could detect organic iron-compounds universally in plants, and hence it is doubtful 

 whether the presence of organic iron-compounds in the plant has as yet been detected by micro- 

 chemical means. Cf. Molisch, Ber. d. Bot. Ges., 1893, p. 73, and C. Muller, ibid., p. 259. [Macallum's 

 observations seem, however, to be trustworthy (see Quar. Journ. Micr. Soc., Vol. xxxviil, p. 175 ; 

 Bot. Centralbl., 1893, Bd. LV, p. 138), and on organic haematogen-like compounds in plants cf. 

 Compt. rend., 1898, rxxvm, p. 282.] 



