6i 



Every organism, however, is limited in its power to withstand the 

 effects of these external forces. The range of susceptibility is repre- 

 sented by what is known as a minimum, optimum, and maximum con- 

 dition. Whenever this range is disregarded, or in other words 

 whenever the minimum or maximum conditions of the organism are 

 passed, death results, but what constitutes the minimum, optimum, or 

 maximum condition for one organism does not necessarily constitute 

 the same for another and hence arise specific forms of susceptibility 

 or powers of response in organisms. 



EFFECTS OF HEAT. 



We have already shown the effects of heat upon nematodes. A 

 temperature of about 140'' F. kills them and destroy the eggs, but 

 they appear to thrive at those temperatures of the greenhouse soil 

 which may vary anywhere from 45'^ F. to 75° F. The optimum 

 temperature for Heterodera is probably not far from 60° to 70° F. 



EFFECTS OF COLD. 



Undoubtedly most, if not all, of the non parasitic forms of nema- 

 todes found here are indigenous to our northern climate, as their eggs 

 will stand our severest winter temperatures. The adult worms, how- 

 ever, are easily killed by freezing as we have frequently seen in our 

 experiments. That the eggs of these species can stand low temper- 

 atures is shown by an observation on old squashes which we have 

 examined after they had lain upon the ground most of the winter and 

 been subjected to alternate thawing and freezing even at a tempera- 

 ture equal to 20° F. below zero. When the squashes were brought 

 into the laboratory no nematodes could be found, but when moist- 

 ened with sterilized water and examined again after having remained 

 in a warm room a week or ten days they were swarming with nema- 

 todes. We have observed the same thing in cultures of nematodes 

 which we purposely allowed to freeze. This, however, does not 

 apply to the parasitic species such as Heterodera which attacks 

 cucumbers, tomatoes, violets, etc., inasmuch as this species is not 

 native and freezing always kills the adult worms and their eggs. 

 We have repeatedly shown this to be the case by allowing badly 

 infested nematode soil to become frozen and on making thorough 

 examinations of the soil afterwards have never found nematodes. 



