84 METABOLISM 



found to be quite unsuitable, and, save sodium, all are poisonous. Sodium, it is 

 true, may to a certain extent act as a substitute for potassium, e.g. if the latter be 

 present only in small amount. Under such circumstances the plant thrives 

 better if sodium be provided than if it be not ; a partial replacement of potas- 

 sium by sodium may be conceded, but we must not forget that this does not 

 apply in other cases, and certainly never when the principal functions are under 

 consideration. The case appears to be otherwise in the lower organisms. The 

 Cyanophyceae (according to Benecke, 1898, p. 96) get on just as well with 

 sodium as with potassium ; in the lower Fungi potassium cannot be replaced 

 by sodium or lithium, but Benecke found a marked increase in dry weight 

 when rubidium only was used, an increase which, when the rubidium was 

 present in a certain concentration, was as great as when potassium was present 

 in the nutritive solution. It is true that in this case there was a development 

 of vegetative organs only, and no spores were formed, hence one may conclude 

 that potassium cannot be entirely replaced by rubidium. Caesium behaves 

 like rubidium. It is possible that traces of potassium were present with these 

 metals as impurities, and so might influence the result; another explanation 

 of this remarkable result will be given later (p. 88). Apart from such doubtful 

 cases, we may say that potassium is absolutely essential. The function of 

 potassium in higher plants may be deduced from the effect of its exclusion 

 from water-cultures. Schimper (1890) observed that in Tradescantia new 

 organs containing potassium were still produced at the growing point, although 

 potassium was excluded from the culture fluid. This is explained by the fact 

 that the new leaves obtained it from the older leaves, which died off when 

 potassium was removed. The new leaves were in every case, however, smaller 

 and thinner, and in the end attained only minute dimensions. As the amount 

 of potassium available from the dying tissues became less and less the growing 

 point began to die also. This research proves conclusively that potassium is 

 essential to the formation of the primordia of organs, whose size depends 

 within certain limits on the amount of potassium available. Although not 

 based on actual evidence it is still very probable that potassium plays an im- 

 portant part in the construction of the principal compounds which occur in 

 protoplasm, more especially the proteids. The evidence for this belief is not 

 as yet forthcoming in physiological chemistry, still any day may produce it. 



What has been said of potassium is true also of magnesium. This metal 

 cannot be replaced by any related alkaline earth ; it is itself essential to every 

 member of the vegetable kingdom. Water-cultures, from which magnesium 

 has been excluded, give results similar to those which have no potassium. In 

 this case, also, we are driven to believe that magnesium takes part in the con- 

 struction of proteid, more especially since, according to Schmiedeberg (1877), 

 the proteid crystals of the brazil-nut consist of a magnesium salt of vitellin, and 

 since Grubler (1881) has proved that the crystallizable proteid of the gourd 

 contains magnesium in no inconsiderable amount. Magnesium appears always 

 so to be a constituent of chlorophyll. 



The case is quite otherwise with calcium. Though most Algae and Fungi 

 can do without it (according to Benecke, 1898, it is essential in the cases of 

 Spirogyra and Vaucheria), we must not jump to the conclusion that it is an 

 important constituent of the proteids of protoplasm, although various im- 

 portant chemical compounds in individual groups of plants contain it. There 

 are other grounds, however, against this view as to the significance of calcium 

 in Phanerogams. To begin with, according to Schimper, calcium is absent 

 from regions where protoplasm is being formed and where potassium and mag- 

 nesium are prominently present, e. g. at growing apices ; on the other hand, 

 it occurs in large quantities in older organs and especially in leaves. The ap- 

 pearance of Tradescantia when grown in a water-culture without calcium is 



