302 PLANT PHYSIOLOGY 



Acidity. — The degree of acidity or alkalinity plays an important 

 part in the rate of respiration. This differs for different plants. 

 Nitrosomonas, one of the nitrifying bacteria, shows the greatest 

 respiration in a medium which is slightly alkaline; while Penicillium 

 shows the greatest rate in a medium distinctly acid. With the sea- 

 weed, Laminaria, decreasing the pH value from 7.9 to 6.9 (i. e., 

 increasing the acidity) decreased the respiration, while changing 

 from pH 7.9 to 8.75 increased it (Hoffmann, 1929). In the higher 

 plants, the cell sap is generally acid while the protoplasm itself is 

 found to be slightly alkaline. Under normal conditions these 

 environments probably remain fairly constant. 



Wounding. — Injuries such as wounding increase the respira- 

 tion. Richards (1897) has shown that in potato tubers, onions, 

 carrots, etc., there is a pronounced increase in respiration for 

 about twenty-four hours after wounding, followed for four days 

 by a gradual decrease until the normal respiration is again reached. 

 In one experiment 200 g. of potatoes, which gave off 2 mg. of 

 carbon dioxide per hour, were wounded by slicing lengthwise. 

 In twenty-seven hours the potatoes were giving off about 16 mg. 

 of carbon dioxide per hour, or were respiring at eight times their 

 normal rate. Not until five days after the experiment started 

 had the respiration rate fallen to near normal. After the wounding 

 there is also an increase of sugar at the cut surface, so that a part 

 of the increased respiration may be due to the extra sugar rather 

 than to a direct stimulation (i. e., wounding) effect. Also part of 

 the extra respiration in these cases is probably due to the expos- 

 ing of a greater area, which thus facilitates the entrance of oxygen. 

 This is further indicated by the fact that merely bruising the 

 tuber without breaking the surface does not cause a marked in- 

 crease in respiration (Lutman, 1926). That respiration varies 

 decidedly with the area exposed is shown also by the fact that small 

 potatoes give a relatively higher respiration than big ones. Large 

 potatoes gave 259 mg. of carbon dioxide per kg. in 24 hours; average 

 sized potatoes, 314 mg.; and small ones, 326 mg. (Hoffmann, 1916). 



Stimulation. — Other forms of stimulation besides wounding 

 increase respiration. Pollination has been found by White (1907) 

 to increase the respiration of the carpels as much as five to eight 

 times normal in the case of Pelargonium. According to Schley 

 (1920), roots which are stimulated geotropically show an increase 

 in respiration, and he was even able to measure an increase on the 



