44 MINERAL NUTRIENT MATERIALS. 



wards confirmed by E. Guenther (I.), that not only calcium, 

 barium, and strontium, but also the closely allied metals beryl- 

 lium, zinc, and cadmium are unsuitable for replacing magnesium ; 

 and that, in fact, they behave as poisons when added in slightly 

 larger amount to the nutrient medium. An addition of 0.02 

 2)er mil of cadmium sulphate or cadmium chloride is sufficient 

 for Aspergillus and Penicillium ; whilst for Rhizopus nigricans 

 0.00 1 per mil is enough. According to Guenther, an addition 

 of 0.2 per cent, of beryllium chloride is necessary to restrict the 

 development of the last-named Phyromijces. 



With regard to zinc, it was observed by J. Raulin (I. and 

 III.) before 1870, in his experiments with Aspergillus niger, 

 that the mycelial development of this fungus could be consider- 

 ably facilitated by a small addition of zinc sulphate to the 

 nutrient medium. The conclusion drawn therefrom that, in 

 contradistinction to the earlier discoveries of the same worker, 

 zinc is indispensable to the structure of the fungus in question 

 was, however, unable to stand subsequent investigation. Both 

 in this fungus and in the case of Penicillium glaucum and Botrytis 

 cinerea it was found by W. Pfeffer (II.), H. M. Richards (I.), 

 and Ono (I.), that the action of zinc is stimulative (§81) in the 

 sense of H. Schulz's law (I.). Even an addition of 0.0005 per 

 cent, of zinc sulphate to a nutrient solution of, e.g. saccharose 

 and mineral salts, resulted in a considerable increase in cropping. 

 This attained double the yield (furnished in the absence of zinc) 

 when the addition reached the optimum amount of about 0.003 

 per cent, of zinc sulphate ; but, on raising the addition to 0.05 per 

 cent., a poisonous action was observed. Rhizopus nigricans seems 

 to be still more sensitive, since, according to E. Guexther (I.), it 

 will not stand more than o.oi gram of zinc sulphate in 100 c.c. of 

 nutrient solution. A noteworthy observation made by several 

 workers is that this stimulation is really a kind of fattening 

 process, the stimulative influence being confined to the develop- 

 ment of the mycelium, that is, to the vegetative portion of the 

 thallus ; whilst the production of conidia, or organs of fructifi- 

 cation, is retarded, and even entirely suppressed. Supported by 

 other experimental results (relating to copper as well as zinc), 

 Andr. Richter (I.) has pointed out that, in such a state of dilu- 

 tion, the salts {e.g. zinc sulphate) are no longer capable of acting 

 as such, but — in accordance with the theory of dissociation — 

 are more or less separated into their components, the ions Zn 

 and SO^, which are therefore the real stimulants. Moreover, 

 because the extent of the dissociation is also dependent upon 

 the nature of the solvent — in this case the nutrient solution — 

 the action exerted b}' an addition of such saline stimulants is 

 also determined thereby. The present is a suitable occasion for 

 referring to the labours of Th. Paul and B. Kkoenig (I. and II.), 

 SCHEURLEN and Spiro (I.), and others, to whom we are indebted 



