CARDINAL TEMPERATURES 91 



nite periods or whether one activity is favored by a given tempera- 

 ture whereas another is adversely affected by the same tempera- 

 ture is none too well known at present. The investigator is led 

 to suspect, however, that physiologic unbalance results if tem- 

 peratures are maintained, because at a constant level a single 

 temperature may not necessarily be optimum for the germination, 

 the mycelial development, and the reproduction of all species. 



The duration of exposure of a fungus to a given temperature 

 should also be taken into consideration. This factor becomes im- 

 portant in a study of the rate of growth, which varies within the 

 culture period, there being a lag at the initiation of growth, fol- 

 lowed by a period of acceleration and eventually terminated by a 

 period of deceleration. These facts are expressed in the well- 

 known sigmoid growth curve, characteristic of all organisms. 



Another difficulty that presents itself, as has been indicated in 

 Chapter 1, is the inadequacy of methods for measuring growth. 

 In nearly all reports use is made of the diameter of colonies or of 

 the amount of surface area of colonies, when as a matter of fact 

 growth is three-dimensional. These two criteria are of value 

 in comparing the growth of an organism at different constant tem- 

 peratures on the same medium, but they largely lose their value 

 when a comparison is made of the same or different organisms 

 grown on different media. In studying the rate of growth of 

 Verticillium albo-atrum, Chaudhuri (1923) was led to conclude 

 that the "rate of spread" on different media may be associated 

 with extremely different rates of mycelial production. 



CARDINAL TEMPERATURES 



Each fungus may be presumed to possess a minimum, an opti- 

 mum, and a maximum temperature. The minimum and the maxi- 

 mum limit growth at low and high temperatures, respectivelv. 

 These values are difficult to fix absolutely and usually are oniv 

 closely approximated. The optimum temperature is that which 

 permits greatest metabolic activity; it is usually based upon meas- 

 urement of the greatest increment of growth during some defi- 

 nite time interval. Respiratory activity and mycelial extension, 

 however, may be correlated with one optimum, whereas, as has 

 been indicated and will be discussed subsequentlv, conidial pro- 

 duction may occur at a different optimum. Observations by 



