THE FUNCTIONS OF THE ESSENTIAL ELEMENTS 427 



the stimulatory action already mentioned (Sect. 73), and the same occurs 

 when in the presence of a minimal trace of iron a little cobalt, zinc, or 

 manganese is added, showing that this minimal amount suffices for all 

 requirements ! . None of these metals can replace this minimal trace of iron, 

 nor can they either in chlorophyllous plants, which do not turn green in 

 the absence of iron although they may be supplied with salts of manganese, 

 nickel, cobalt or aluminium -. 



E. Gris discovered that iron was essential to the higher plants, and it is easy 

 to show that the formation of chlorophyll is dependent upon its presence. Thus 

 if seeds containing but little iron, such as maize or buckwheat, are grown in 

 a culture-solution free from iron, the first three or five leaves turn partially green, 

 but the next one remains quite white (Fig. 61). If a few drops of a solution of an 

 iron-salt are added and transpiration is active these leaves begin to turn green in 

 two to three days, the colour appearing first along the veins and thence spreading 

 over the whole leaf. If the chlorotic leaves are painted with a dilute solution 

 of iron they may also turn green, as was first shown by Gris. The colourless 

 plastids which are at first formed soon undergo deformation, and when this occurs 

 the power of recovery may be lost 8 . 



So far as is known, a plant can make use of any compound of iron it can 

 absorb, even potassium ferrocyanide, but since humus retains and decomposes 

 soluble iron-salts a large quantity must be added to remove chlorosis. When 

 growth is rapid and iron is supplied but slowly, as for example when plants 

 in pots are forced to rapid development, the young shoots may pass through 

 a chlorotic condition 4 . During this condition the chloroplastids are functionally 

 inactive and exhibit no power of assimilating carbon dioxide. This is not entirely 

 due to the non-formation of chlorophyll, for the power of evolving oxygen may 



1 Raulin did not therefore succeed in proving that iron was essential (Ann. d. sci. nat., 1869, 

 v. ser., T. xi, p. 224), for he only showed that fungi grew more rapidly when the supply of iron was 

 increased. On the other hand, Wehmer (Beitrage z. Ktnntniss einheimischer Pilze, 1895, Heft 2, 

 p. 159) erroneously supposes that the sole action of iron is a stimulatory one. Cf. Richards, Jahrb. 

 f. wiss. Bot., 1897, Bd. xxx, p. 674. 



3 Mn : Sachs, Experimentalphysiol., 1865, p. 144; also Birner und Lucanus, Versuchsst., 1866, 

 Bd. via, p. 140 ; Wagner, ibid., 1871, Bd. xm, p. 72 ; and Bertrand, Compt. rend., 1897, T. cxxiv, 

 p. 1032. Ni : Risse in Sachs, Experimentalphysiol., p. 145. Al : Knop, Kreislauf d. Stoffes, 1868, 

 p. 614. 



3 E. Gris, De Faction d. composes ferrug. s. 1. vegetation, 1843 and 1844, ar >d ' n Compt. rend., 

 1844-7. Confirmatory results by Salm-Horstmar, Vers. iiber die Ernahrung d. Pflanzen, 1856, 

 pp. 8, 17 ; A. Gris, Ann. d. sci. nat., 1857, iv. ser., T. VII, pp. 201, &c. Completely white leaves 

 can only be obtained in Phaseolns inultiflonis when the cotyledons are removed (Molisch. 1892, I.e., 

 p. 92\ Zimmermann, Beitrage z. Morph. u. Physio!., 1893, I. p. 53. The action of Fe in aiding 

 the development of blue flowers in Hortensia is still uncertain ;cf. Hoffmann. Bot. Zeitung, 1875, 

 p. 622 ; Unters. iiber d. Variation, 1877. P- 2O ! Molisch, Bot. Zeitung. 1897, p. 41) . 



4 Sachs, Arb. d. Bot. Inst. in Wur/burg, iSSS, Bd. Ill, p. 433. On the injurious effect of an excess 

 of iron salts in the soil, cf. Sachsse, A^r.-Chem., iSSS, p. 505 ; Thomson, Beibl. z. Bot. Centralbl., 

 1893, in, p. 497 ; Petit, Bot. Centralbl., 1894, Bd. LIX, p. 146. K,Cfy : Knop, Ber. d. Sachs. Ges. 

 d. Wiss. zu Leipzig, 1869, Bd. XXV, p. 8; Wagner. Versuchsst.. 1870. Bd xm, p. 74. 



