18 PATHOLOGICAL MYCOLOGY. 



the nutritive medium will increase the crop by 800 grammes. 

 Notwithstanding this resemblance, the functions of zinc and iron are 

 quite different. Zinc enters the plant as a constituent of its tissues. 

 The only use of iron appears to be to destroy, or suppress pending 

 production, a poison which the plant secretes, and which, were it to 

 accumulate, would end by killing the plant. It is one of those 

 secretions which are common to all living organisms, and which they 

 should get rid of at any cost. This is the service iron renders to the 

 aspergillus. Zinc is a physiological aliment ; iron is a physiological 

 antidote." 



15. Though it be so closely allied morphologically to aspergillus, 

 the Penicillium glaucum is found to differ from it in its relations to 

 some of these chemical substances. It is true that it will also flourish 

 in Raulin's fluid, but its growth is rendered more luxuriant by the 

 addition of a little sulphate of lime ; and its relations to antiseptic 

 substances differ much from those of the aspergillus, for it can grow 

 in solutions of nitrate of silver and perchloride of mercury, the least 

 trace of which would be instantly fatal to the former. From the 

 close similarity of the functions of the cells of the body and of those 

 of the invading organism, it might have been anticipated that what is 

 food for the one would be food for the other. But, following the 

 analogy just cited, we may infer that this is not the case. One class 

 of nutrient substances will promote the growth of the normal cells, 

 and consequently health ; another the growth of parasitic cells, and 

 consequently disease. Similarly one form of medication may destroy 

 the disease, and another the patient. 



Mutability of Species. 



16. Some first principles are to be grasped before proceeding to the 

 practical points which have to be considered. We have to deal with 

 a confessedly large number of species separated from one another, as 

 it would at first sight appear, by wide differences, both of form and 

 of function, by paying attention to which these species might be 

 arranged and classified. A question has arisen as to how far such an 

 apparently natural classification may be looked on as reliable and 



