WATER AND GROWTH 



25 



Two related types of study concern us here: the effect of relative 

 humidity on growth and the effect of osmotic pressure on growth. 

 Both presumably measure the ability of the organism to draw water 

 from its environment. Spore germination per se is considered else- 

 where (Chapter 12); but it should be noted that experimental designs 

 used in growth studies do not fully discriminate between spore germi- 

 nation and mycelial development. 



Relative humidity effects must, of course, be determined by grow- 

 ing the organism on a substrate which is in equilibrium with the 

 atmosphere. Agar and other solid substrates — bran, wood blocks, 

 fabrics, etc. — have been used. Unfortunately, quantitative determina- 

 tion of growth on such materials is difficult, and the data are rather 

 imprecise. This imprecision makes it hard to decide whether rela- 

 tive humidity itself or the moisture content of the equilibrated sub- 

 strate is the more important factor in fungus growth. Data can be 

 cited for each view (21, 277). Probably the amount of water in the 

 substrate is the more important: thus if cotton and wool are incu- 

 bated at 92 per cent relative humidity only the wool, which absorbs 

 more water at this humidity than does cotton, supports growth of 



fungi (21). 



The influence of moisture on fungal deterioration of economically 

 important materials has been studied intensively and reviewed often 

 (13, 184, 271, 277). Fungi tolerant of low humidity (85 to 90 per 

 cent) include Steream frustulosum (13), Schizophyllum commune 

 (131), and Aspergillus spp. (163, 276). A few fungi attack substrates 

 at as low as 65 per cent relative humidity (276), but most are limited 

 to much higher humidities, 95 per cent or more. 



Tolerance to low relative humidity appears, as expected, to be 

 associated with tolerance to high osmotic pressure (131, 157); this 

 relation is especially clear in spore germination experiments (Chap- 

 ter 12). Species in which spore germination occurs at low humidity 

 also tolerate a low humidity during growth (321, 323), and it is 

 reasonable to infer that spore germination is more affected by 

 marginal water supply than is mycelial growth (329). 



Studies on the role of osmotic pressure are always complicated by 

 possible non-osmotic effects of the solute. Sodium chloride is dis- 

 tinctly inhibitory to some fungi (207); other complications are dis- 

 cussed by Brancato and Golding (29) and Stuart (290). Tolerance 

 of high osmotic pressure is specific. Thus, a mutant of Neurospora 

 crassa is inhibited at 0.17 M glucose (139); at the other extreme, species 

 of Aspergillus can grow in media with an osmotic pressure of as high 



