VISIBLE LIGHT 



427 



Figure 2. The influence of 

 the rate of warming on viabil- 

 ity (germinability) of conidia 

 of Aspergillus flavus frozen at 

 — 70 °C. Each curve represents 

 a separate experiment. Re- 

 drawn from Mazur (103), by 

 permission of the Rockefeller 

 Institute for Medical Research. 



1 10 100 1000 



Rate of subzero warming, °C per minute 



the individual crystals (105). It follows that thawing must be equally 

 rapid, in order to prevent crystal growth. This is shown especially well 

 by the data of Mazur (103), part of which is reproduced in Figure 2. 

 The lethal effects of slow, as compared to rapid, warming were par- 

 ticularly evident in the range —20 to 0°C; i.e., the rate of subzero warm- 

 ing, not the rate of thawing of ice, is the critical factor. 



The possibility of freezing material so rapidly as to prevent crystal 

 formation entirely has been considered at length by Luyet and Ge- 

 henio (91). The experimental difficulties are formidable, and there 

 is some problem in determining whether or not success has been 

 achieved (105). In any event, it is clear that prevention of crystal 

 formation is not essential to survival at low temperature. 



We may conclude that fungi, especially vegetative mycelium and 

 certain types of spores, are relatively susceptible to freezing injury, 

 particularly to the slow freezing which occurs in nature; repeated 

 freezing and thawing are even more damaging (115). The principal 

 factor is almost certainly the moisture content of the material; it fol- 

 lows that preservation in the frozen state will be most successful if ap- 

 plied to dry spores or other cells which can withstand a preliminary 

 desiccation. Although it is unlikely that freezing can eliminate all cells 

 of a given species, cold injury must be an important ecological factor 

 at least for soil fungi and plant pathogens (22, 39). 



3. VISIBLE LIGHT 



Light of the visible range, 400-800 m/x, exerts undoubted, but so 

 far ill-defined, influences on mycelial growth, spore germination, and 

 other phenomena; effects on reproduction have been discussed in 



