ASH, I 85 



quite distinct from that in the absence of potassium or magnesium. During 

 the first few weeks the growth appears healthy and the newly formed leaves 

 are of normal size ; then they — but not the older leaves — begin to die off after 

 the appearance on them of brown spots. Schimper has shown that these 

 brown spots are due to poisoning by oxalic acid, which cannot be neutralized 

 owing to the absence of calcium, and he has drawn the conclusion that, in 

 general, the neutralization of this acid is the function of calcium. Pfeffer 

 (Phys. I, 427) points out, on the other hand, that those plants which do not 

 form calcium oxalate, and they are by no means few in number, are as much 

 injured by free oxalic acid or potassium oxalate as the others. More recently, 

 PoRTHEiM (1901) has succeeded in demonstrating that beans grown in a calcium- 

 free soil become diseased, but that they showed the presence neither of oxalic 

 nor of any other strong free acid. Schimper' s hypothesis may be correct in 

 individual cases, but it does not explain the general necessity for calcium, and 

 we are bound to admit that its function has not as yet been discovered. In look- 

 ing for a substitute for calcium, strontium would naturally first occur to us, but 

 the researches of Haselhoff (1893), undertaken to determine the capabilities of 

 replacement of calcium by this metal, are not very convincing. Benecke (1895, 

 p. 521) found that strontium was poisonous to Fungi. 



Our knowledge of the function of iron has been long believed to be much 

 better grounded than that of the other constituents of the ash. Absence of 

 iron in Phanerogams brings about the highly characteristic appearance known 

 as chlorosis. Chlorosis consists in the young organs taking on a pale yellow 

 or bleached appearance, as a consequence of which they soon die, owing to 

 their not possessing chlorophyll, which we shall find (Lecture IX) plays so 

 important a part in nutrition. For the development of the chlorotic condition 

 in plants water-culture solutions from which every trace of iron has been care- 

 fully excluded are necessary. Even then chlorosis appears at first only gradually. 

 The first leaves of the seedhng are always green, because there is enough iron 

 present in the reserves to supply what is wanted. As a matter of fact some 

 plants with large cotyledons, such as the bean, are especially unsuitable for such 

 experiments, because the amount of iron present in them is sufficient for the 

 whole plant ; good results may, on the other hand, be obtained by using such 

 plants as maize, buckwheat, or sunflowers. Pea-seedlings grown in iron-free 

 nutritive solutions develop, according to Molisch (1892), three or four green 

 leaves first of all, then one yellowish -green leaf; the remaining leaves, as well as 

 the tendrils, are white. Such chlorotic plants may be again made green, as E. 

 Gris (1843) first showed, by permitting them to absorb an iron salt through 

 the root, or by applying it directly to the chlorotic leaf. The cuticle of the leaf 

 must, for the success of the experiment, be possessed of a certain degree of per- 

 meability, such as is exhibited, according to Molisch (1892) by Helianthus. 

 Further, if greening is to take place the chlorotic organ must be young ; appli- 

 cation of an iron salt to old chlorotic leaves has no effect. 



For a long time chlorophyll was held to contain iron, and the appearance 

 and disappearance of chlorosis in these experiments was accounted for by the 

 presence or absence of that metal. The question as to whether or not chloro- 

 phyll contains iron has been reinvestigated by Molisch (1892), who has con- 

 firmed an older research of Raulin's (1869), showing that Fungi which have 

 no chlorophyll, cannot do without iron ; Benecke (1895) draws attention to the 

 same fact. Another function, therefore, must be found for iron. It seems 

 probable that, like potassium and magnesium, iron is necessary to the formation 

 of protoplasm, and that its absence is followed by chlorosis in the higher plants 

 as a secondary effect. Further, iron cannot be replaced by any other related 

 metal such as manganese. 



One element present in the nutritive solution alone remains to be con- 



