CONDITIONS OF LIFE OF THE MOULD FUNGI. 509 



obtained under these circumstances, and the production 

 of fruit-hearing mycelium and spores has never heen 

 observed ; hence the relation of the fungi to oxygen may 

 be stated by saying that normal growth with fructifi- 

 cation can only occur in an atmosphere containing free 

 oxygen, while on the other hand, only a formation of 

 mycelium occurs where (as in many animal bodies) 

 there is no free oxygen at their disposal. This view 

 corresponds also on the whole with the facts as to the 

 occurrence of the parasitic mould fungi in the lower 

 animals ; the pathogenic species of empusa, cordyceps, 

 botrytis, and isaria form in the bodies of the caterpillars 

 and insects which they attack well-developed mycelium, 

 and ultimately the so-called cylindrical gonidia ; true 

 fructification, however, only takes place by means of 

 fruit-bearing hyphae, which have burst through the 

 surface and come in contact with the air. 



Thus far we have discussed the question of the quality Quantity of 

 of the nutrient materials which are required by the mould material. 

 fungi. In addition to the quality we must now allude 

 to their quantity. It is evident that either too small or 

 too large a quantity of any of the nutrient materials 

 will have an unfavourable effect, and that there must be 

 an optimum of quantity as regards each individual com- 

 ponent at which nutrition goes on best ; this optimum, 

 however, will depend on the general composition of the 

 nutrient mixture, and will vary according to the amount 

 of the other substances which are present at the same 

 time. On this subject very little is as yet known, and 

 on]y as regards two points are the experiments suffi- 

 ciently numerous and precise to admit of their discussion 

 at the present time namely, in the first place, the 

 amount of water which is necessary for a suitable nutri- 

 tive mixture, or, in other words, the concentration of the 

 mixture ; and, in the second place, the quantity of free 

 alkali or free acid, i.e., the reaction of the nutrient mixture. 



As regards the concentration or the amount of water Limits within 

 in the nutriment, it may vary very considerably without 



completely preventing the growth of the mould fungi; the water ma y 

 mould fungi are in this respect much less sensitive than 



