122 RESPIRATION 



observed for temperatures 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 potato, 

 the difference between the maximum intensity of respiration 

 of heated tubers and that of unheated tubers is propor- 

 tional 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 res- 

 piration 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 de- 

 cadence, 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 due to high temperatures also 

 is seen in the respiratory activity of wounded potatoes : the 

 general effect of wounding (see p. 115) is to increase the in- 

 tensity of respiration, which increase is maintained ; after 

 wounding there is no immediate respiratory response in pre- 

 viously heated tubers, and even when recovery from the 

 heating 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 

 active seedlings of the lupin, pea, and wheat. The seeds were 

 germinated at about 20° C. and then exposed to various 

 temperatures, the carbon dioxide being estimated hourly for 

 six hours (Fig. 6). At temperatures not exceeding io° C. the 

 output of carbon dioxide is constant ; from io° C. to 20° C. 



* See Vol. I., p. 177. 



I Kuijper : " Extr. Trav. Bot. Neerland.," 1910, 7, 130. 



