484 Physiology 



„ . 4.6 (log k2 - log ki) 



1_1 

 Ti To 



in which Tj and T2 are absolute temperature values. The [x value repre- 

 sents the heat of activation, or the number of calories required to trans- 

 form one gram equivalent of "inactive molecules" of the reacting sub- 

 stance into "active" ones. There is a close relationship between [j, and 

 Qio values, and the latter may be derived from the former for short 

 temperature ranges. A O^y value of 2.0 corresponds to a [x of about 13,200; 

 a Qio of 10, to a [ji, of about 44,000 calories. Unfortunately, the biological 

 significance of thermal coefficients and thermal increments is uncertain. 



Light and darkness 



A source of light is obviously important for chlorophyll-bearing 

 flagellates, in which the relation to photosynthesis doubtless accounts 

 for various effects on growth. However, Dusi (106) has reported that 

 under constant illumination Euglena gracilis grew well in peptone me- 

 dium but poorly in inorganic mediimi, whereas E. klebsii grew well in 

 inorganic medium under the same conditions. E. viridis, on the other 

 hand, failed to grow under constant illumination. Temperature as the 

 significant factor, rather than illumination, apparently was not com- 

 jiletely excluded in these cases. 



Light and darkness also may influence the effects of other factors on 

 growth. Thus the thermal optimum for Euglena gracilis in peptone 

 medium is about 10° in darkness and 25° in light (243). Accelerating 

 effects of certain organic acids are relatively greater in darkness, while 

 oxalate is slightly stimulatory in light and without effect in darkness 

 (244). Plant auxins also have accelerated growth of E. gracilis in light 

 but not in darkness (119). 



Even less is known about growth of higher Protozoa in relation to 

 light. Richards (481), in analyzing data on growth of several ciliates, 

 noted that the seasonal rhythms reached a peak in July. On this basis, 

 he suggested that temperature is less important than sunlight when 

 both are variables. On the other hand, light of high intensity is lethal 

 to pigmented Blepharisvia undiilans, the effect being attributed to a 

 photooxidation of the pigment with irreversible damage to protoplasmic 

 components (159). Indirect effects have been reported for Plasmodiimi 

 cathcmeriiun. Exposure of the hosts to artificially prolonged periods of 

 "day" and "night" lengthen the cycle of merogony (26). 



Effects of certain toxins and venoms 



Bacterial exotoxins are relatively inactive against Protozoa. Ex- 

 posure of Paramecium aurelia, P. calkinsi, and P. caudatum to diphtheria 



