GROWTH 35 



ture is sometimes an important factor limiting the attack of plant 

 pathogens. 



The cardinal temperatures of a few fungi are given in Table 6. A more 

 extensive compilation is given by Wolf and Wolf (1947). The character- 

 istic effect of different temperatures on the rate of growth of two fungi is 

 shown in Fig. 5. Further examples may be found in the work of Lindgren 

 (1942). 



Most reports on the effect of light on the fungi have been concerned 

 with reproduction rather than vegetative growth. However, Elfving 

 (1890) found strong diffuse daylight to depress the growth of Penicillium 

 glaucum and a species of Briarea. The amount of inhibition was least 

 when the culture medium contained complex nutrients such as peptone. 

 Greater inhibition resulted when the media contained glucose, mannitol, 

 and malic acid. Scattered observations indicate that the depressing 

 effect of strong light may be rather common. In the old literature some 

 mention is made of the favorable effect of light on red yeasts. The 

 sporangiophores of Phycomyces hlakesleeanus attain a greater length in 

 darkness than in intense light. The role of light in the sporulation of 

 some fungi is discussed in Chap. 14. 



Conclusive evidence that light affects the amount of growth of Karlingia 

 (Rhizophijlctis) rosea, one of the lower Chytridiales, was presented by 

 Haskins and Weston (1950). This fungus when grown in liquid glucose- 

 nitrate medium produced twice the amount of dry weight of cells when 

 cultured in light than when the cultures were kept in total darkness. 

 With the exception of the factor of illumination, the experimental con- 

 ditions were the same. Approximately twice as much glucose was 

 utilized by cultures exposed to light as those kept in darkness. On the 

 other hand, when K. rosea was grown in a liquid cellobiose-nitrate 

 medium, more growth resulted in total darkness than in light. The 

 explanation for this behavior of K. rosea is not known. 



The moisture requirements of fungi differ. Most species in nature live 

 on substrates which are not saturated with water. The low moisture 

 content of a substrate is often a factor which limits the growth of fungi. 

 Particularly is this true of the species which live on wood or in soil. As a 

 general rule, wood which contains less than 20 per cent moisture is immune 

 to fungus decay. A difference of a few per cent in the moisture content 

 may determine whether a species will be able to grow or not. Lindgren 

 (1942) has reported that Ceratostomella pilifera, a wood-staining fungus, 

 does not grow in pine wood having a moisture content of 23 per cent but 

 develops in wood containing 24.5 per cent moisture. The maximum rate 

 of penetration was attained on wood having a moisture content of 29 per 

 cent or more. Jute sacking is subject to fungus attack only if the mois- 

 ture content exceeds 17 per cent. 



