RESISTANCE TO TEMPERATURES 101 



hyphae and duration of exposure were found to be important 

 factors. Twenty-four-hour-old cultures were more easily killed 

 than 48-hour-old cultures. Aerial hyphae were more easily killed 

 than submerged hyphae. 



Lipman (1937) employed 12 species of fungi, cultured for 24 

 hours on synthetic agar or on potato agar. After gradual cooling 

 he immersed them in sealed tubes for 48 hours in liquid air; he 

 then gradually warmed them. Of the 12 species, belonging in 

 Aspergillus, Penicillium, Rhizopus, Mucor, Absidia, Mortierella, 

 Rhizoctonia, Armillaria, Trichoderma, Pythium, and Fusarium, 

 8 survived. As an explanation Lipman hypothesizes that this ex- 

 traordinary tolerance may be causally related to the tiny spaces 

 that exist between the colloidal micelles, which because of their 

 small size prevent dehydration through ice formation. 



Not all fungi are capable of tolerating the extremes of tempera- 

 ture which have been mentioned. Gehenio and Luyet (1939) ex- 

 posed the plasmodium of Aethalhun septimm so as to study the 

 effect of cold on vitality and the influence of the duration of ex- 

 posure, as well as to determine whether cold per se or the sud- 

 denness of the temperature change is responsible for injury. They 

 found, first of all, that there may be marked injury at tempera- 

 tures of freezing or slightly above if the plasmodia are cooled 

 abruptly, whereas with slow cooling the injury may not be ap- 

 preciable until the temperature descends to about —2.5° C. They 

 also noted that the plasmodia may be killed after exposure of only 

 5 seconds to temperatures of —1° or — 2° C. This sensitivity to 

 cold finds support in the observations that some tropical species 

 of seed plants are killed if exposed to temperatures above 0° C. In 

 these cases death cannot be attributed to the formation of ice 

 crystals. Here the mechanism of death, as postulated by Gehenio 

 and Luyet, consists of gelation of the protoplasmic sol under the 

 action of cold, the gelation being accompanied by syneresis. The 

 squeezing out of the dispersion medium, if gelation is complete, is 

 not a reversible process and hence is lethal. 



The problem of the causes of death by low temperatures in 

 fungi, in other plants, and in animals is summarized in the mono- 

 graph by Luyet and Gehenio (1940). Their summary indicates 

 that death from cold has been attributed to the following causes: 

 (1) bursting of the cells by expansion in ice formation, with con- 

 sequent mechanical injury; (2) destruction of the fine structure 



