132 RESPIRATION 



same water content, 14 per cent. ; with less water than 14 

 per cent., there is little or no difference between the two kinds 

 of grain. The effect of respiration in bulked grain is a rise 

 in temperature ; this is cumulative and causes an acceleration 

 in respiration up to about 55 C, which temperature also has 

 the effect of stimulating diastatic activity and hence rendering 

 available more immediately respirable material. On the other 

 hand, the resulting accumulation of carbon dioxide has a nar- 

 cotic effect and reduces the rate of respiration. 



Some reference may now be made to lower plants, many of 

 which, from the nature of their habitat, have to withstand 

 long periods of desiccation. Long has it been known that 

 dry mosses show a very low rate of respiration, which rises 

 with the increase of the water content of the plant.* Accord- 

 ing to Mayer and Plantefol,f the respiratory quotient of mosses 

 varies according to the degree of imbibed water, and may be 

 higher than unity when the water content is low. They also 

 came to the interesting conclusion that the respiration of 

 dry moss is anaerobic, as is possible also in dry seeds, since the 

 yield of carbon dioxide in vacuo is as much as it is in air. 



In the lichens, the increase in respiration also depends on 

 the amount of water contained in the plant. Jumelle % found 

 that on addition of equal amounts of water to material having 

 respectively a low water content and a high water content, 

 the rate of respiration was greater in the former than in the 

 latter. The addition of water beyond a certain point is with- 

 out effect on the respiration rate ; there is, moreover, a period 

 of lag shown by lichens subjected to prolonged drought, 

 some time elapsing after the addition of water before the res- 

 piration rate is equal to that of fresh material of equal water 

 content. Fraymouth § likewise observed that the lichen 

 Parmelia physodes showed no great increase in the rate of 

 respiration until a water content of 60 per cent, was reached ; 



* Bastit : " Rev. gen. Bot.," 1891, 3, 255. Jonsson : " Comp. rend.," 

 1894, 69, 440. 



f Mayer and Plantefol : " Ann. physiol. physichim. biol.," 1925, 1, 

 361. 



J Jumelle : " Rev. gen. Bot.," 1892, 4, 359. 



§ Fraymouth : " Ann. Bot.," 1928, 42, 75. 



