EFFECT OF TEMPERATURE 83 



tubers, after an exposure to a temperature of 40 C. for several 

 hours and then transferred to room temperature, show a gradual 

 increase in respiration intensity which reaches its maximum 

 after a lapse of twenty-four hours. Exposure to higher 

 temperatures, e.g. 44 C., results in a permanent increase in 

 the intensity of respiration which indicates a permanent change 

 in the organism. To obtain this stimulation the temperature 

 must be sufficiently high, no effect being observed for tempera- 

 tures below 38 C. This is the minimal temperature for the 

 phenomenon described in the potato, but the value probably 

 varies in different plants. In the instance of the potato, the 

 difference between the maximum intensity of respiration of 

 heated tubers and that of unheated tubers is proportional to the 

 difference between the minimal temperature and the increased 

 temperature to which the tubers were exposed. Thus it was 

 observed that after exposure to 44 C. the respiration was two 

 and a third times as great as after an exposure to 40 C. These 

 authors consider that the effect of the high temperature is 

 adversely to affect the protoplasm in much the same way as 

 does old age : as a result of this decadence, less starch is formed 

 by the leucoplasts ; also the enzymes are less active so that 

 there is less invert sugar for the leucoplasts to fix. For 

 instance, at 18 C. previously heated potatoes contain a 

 relatively large amount of sugar ; at o C., which temperature 

 inhibits starch formation in normal tubers,* the much slower 

 formation of sugar indicates that the increased temperature also 

 has had an injurious effect on sugar formation although the 

 amount of diastase apparently is unaltered. The weakening 

 effect of high temperature also is seen in the respiratory activity 

 of wounded potatoes ; the general effect of wounding (see p. 78) 

 is to increase the intensity of respiration, which increase is 

 maintained. After wounding there is no immediate respiratory 

 response in previously heated tubers and even when recovery 

 from the heating effect had apparently been made the maximum 

 intensity of respiration due to the traumatic stimulus never 

 reached so high a degree as in unheated wounded tubers. 



Dormant structures were the subjects of this investigation ; 

 the subjects for the work of Kuijper,f on the other hand, were 



* See Vol. I., p. 132. 



t'Kuijper: " Extr. Trav. Bot. Norland.," 1910, 7, 130. 

 6* 



