io INTRODUCTION [ch. 



there are many indications that certain fungi grow better in impure than in 

 pure cultures. This, however, may merely indicate that in a mixed culture 

 the waste products of one organism are used up by the others, whereas in a 

 pure culture they tend to accumulate and inhibit growth. Thus Dodge found 

 that spores of Ascobolus Winteri failed to germinate on the agar in which 

 the parent ascocarps were growing. 



I n the case of Mucor and R/iizopus, Baden found that the presence of 

 bacteria prevented the germination of the spores. It may be worth con- 

 sidering whether the progressive development of bacteria in the dung may 

 be a factor in the succession of fungi which appear on it. 



Another factor of some importance in the development of coprophilous 

 and perhaps of other fungi is the action of direct sunlight. Cultures which 

 remain sterile in a darkened room can often be induced to fruit by placing 

 them in a sunny window; reference has already been made to the occasional 

 action of sunlight on spores. Many coprophilous fungi are moreover posi- 

 tively heliotropic; this is well shown by the sporangiophores of Pilobolus 

 and the perithecia of Sordaria and its allies. The ejection of the spores into 

 an open space is in this way ensured. 



Fungi on Fatty Substrata. It is probable that all or most fungi are 

 able to utilize fats and oils; such substances are a common form of food 

 reserve in the spores (zoospores, oospores, uredospores, etc.), in the mycelium, 

 and especially in the sclerotia where, in the case of Claviceps purpurea, the 

 proportion of fat reaches as much as 35 per cent.; in several cases the fat- 

 splitting enzyme, lipase, has been extracted. 



It is therefore not surprising that man}' fungi grow readily on a fatty 

 substratum, some, such as Empusa and species of Cordyceps, on animal re- 

 mains, some on other fungi and some on oil-containing fruits and seeds 

 and on cotton, rape and other oil-cakes which are made from the waste re- 

 maining after such seeds have been crushed ; they may reduce the oil content 

 of the cake from over 10 per cent, to between 1 and 2 per cent, in two years. 



Eurotium and Penicillium occur on the layerof sweet oil placed over bottled 

 fruits to prevent decomposition and together with other genera are concerned 

 in the "ripening" of cheese. The related J\fonascus heterosporus does con- 

 siderable damage in parts of Australia and New Zealand if it is allowed to 

 get a footing on stored tallow. 



Fungi producing alcoholic Fermentation. A number of fungi obtain 

 nutriment from solutions consisting largely of soluble carbohydrates and 

 they may also obtain energy by directly breaking up certain of these sub- 

 stances without the intervention of oxygen and with the formation of ethyl 

 alcohol, carbon dioxide and small quantities of other substances. This reaction 

 is due to the presence of the enzyme zymase and is known as alcoholic 

 fermentation. 



