FUNGI: EXTEKNAI, INFLUENCES. 175 



lating action on their growth, provided that certain conditions 

 are observed, and the supply of nutrient substances is main- 

 tained. 



These observations naturally lead to a consideration of 

 the question of external influences, and their action on fungi 

 in general. 



An essential condition of growth of all organisms is the 

 presence of water in such quantity as to create and maintain 

 in the cells a condition of turgescence i.e., a hydrostatic 

 pressure within the cell which keeps the protoplasmic lining 

 in direct contact with the cell-wall. They require, however, 

 only a minute quantity of water during the resting stage 

 i.e., in the state of spores and frequently these organisms 

 can withstand complete dehydration. It has already been 

 mentioned that yeasts can be preserved in a dry state for a 

 very long time, and spores of some of the moulds for several 

 years. The concentration of the food, which is dependent 

 on the amount of water present, plays an important part in 

 the development of individual species. It is well known that 

 temperature is also an important factor. 



Growth is only possible above a certain minimum tempera- 

 ture, and it is accelerated with a rising temperature until at a 

 certain optimum the organism attains its greatest activity. 

 From this point on, if the temperature is still allowed to rise, 

 the growth becomes less and less pronounced, and, finally, at 

 a certain maximum temperature, ceases altogether. Different 

 species exhibit very varied behaviour in regard to these three 

 critical points, as is shown by the following examples : 



A number of yeast species have their minimum at 0-6 C., 

 optimum at 28-30 C., and maximum at 34-40 C., whilst 

 the corresponding figures for 



Minimum. Optimum. Maximum. 



Penidllium glaucum are 1 '5 C. 25 to 27 C. 31 to 36 C. 



Bac. subtilis, . . 6 C. approx. 30 C. 50 C. 



Acetic bacteria, . . 8 C. 18 to 33 C. 30 to 36 C. 



The limits of temperature may, moreover, vary in regard 

 to the different organs of one and the same species (instances 

 are found amongst the yeasts), and finally the results obtained 



