THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 339 



The Ascomycete Neurospora has provided very interesting information 

 about the synthesis of amino-acids. When ascospores of the Fungus are 

 treated with X-rays or with ultra-violet light, then germinated and crossed 

 with the opposite heterothallic strain, mutant forms are obtained which 

 lack the synthetic ability to form certain amino-acids, due to the destruction 

 of the controlling gene. By using mutants in which a particular stage in a 

 synthesis is blocked, it has been possible to trace the course of some synthetic 

 processes such as the formation of the essential amino-acid arginine from 

 ornithine. 



Fungi differ from the higher plants in some respects regarding inorganic 

 nutrients. For example, the metals Calcium, Sodium, and Iron do not 

 appear to be essential, though the anions phosphate and sulphate are necessary, 

 as in green plants. The latter ion is sometimes absorbed in supra-optimal 

 quantities (" luxury consumption ") and subsequently excreted. It has 

 often been stated that certain of the heavy metals, such as Zinc, are essential 

 to fungal growth, and that in this respect Fungi are peculiar, but recent 

 work indicates that they do not differ significantly in this respect from the 

 higher plants, many of which also require Zinc and other heavy metals. The 

 fact is that the usual technique of cultivating Fungi on synthetic media 

 revealed their nutritional demand for small quantities of certain elements, 

 the so-called " trace elements," before the facts were appreciated in other 

 groups. We now know that most, if not all, plants require for their complete 

 nutrition a number of elements in extremely small amounts, among them 

 being Boron, Manganese, Copper and Zinc. The absence of these elements 

 is followed by depression or failure of growth. Fungi share this requirement. 

 Thus the presence of Zinc in a culture medium favours sporulation and 

 increases the dry- weight of mycelium formed, but it also increases the amount 

 of sugar consumed per gram of dry-weight formed, or, in other words, the 

 metabolic efl^ciency is lessened. 



In addition to the inorganic trace elements Fungi require certain organic 

 accessory substances or nutrilites. This was first observed by Wildiers in 

 1902 in the case of yeast, which he found required the presence of an unknown 

 organic substance, derived from the yeast cells, which he called " bios." 

 This is now known to be a complex of substances, among which the most 

 important is Thiamin, usually called Vitamin B^. This vitamin has been 

 shown by Schopfer and others to be essential to the growth of many Fungi 

 and beneficial in the majority of cases where it is not essential. These 

 accessory substances are required in such minimal quantities that thev cannot 

 function as ordinary nutrients. Thiamin, for example, is active in amounts 

 of no more than one part per billion of the culture fluid. 



Fungi often produce excretory materials of a complex organic nature which 

 affect the growth of others with which they are associated. The action may 

 sometimes be stimulatory, as in the case of the bios substances, but is more 

 often antagonistic or inhibitory, a condition which has been called antibiosis. 

 The allied phenomenon of " staling " is usually attributed to an antagonistic 

 action of the products either of one Fungus on its own growth or of the 



