WORK OF LEAVES: RESPIRATION 113 



the bottle moist. Place a piece of wet filter paper on the peas. 

 In ten or twelve hours after some of the oxygen has been used, 

 on this paper rest three or four of the germinated peas. Close 

 the bottle tightly. The peas in the jar will use up so much 

 oxygen and give off so much carbon dioxide that ordinary 

 respiration cannot take place in the seedling peas on the filter 

 paper. Fit up a similar bottle with folds of wet paper and 

 some free water in the bottom. Place a piece of wet filter paper 

 on the folds of paper and on this place three or four of the 

 seedling peas which have radicles of a length equal to those in 

 the other bottle. Close the bottle tightly. In 24 to 48 hours 

 compare the length of the radicles of the seedlings in the two 

 bottles. 



These experiments teach us that all plants, with very few 

 exceptions,* breathe, or respire by absorbing oxygen and giving 

 off carbon dioxide. 



191. Respiration in the absence of oxygen. This some- 

 times occurs. It can be demonstrated in germinating seeds by 

 placing them, when the root is just emerging from the seed coats, 

 in an inverted test tube filled with mercury, the lower end of 

 which is immersed in mercury placed in another vessel. In the 

 course of a day carbon dioxide is given off from the seedlings, and 

 being lighter than the mercury displaces some of it and occupies 

 the upper part of the inverted tube. 



The carbon dioxide here comes from the living substance of 

 the peas, just as it does in ordinary respiration. There is a 

 loss or waste of substance by the peas which passes off as car- 

 bon dioxide. This waste is formed by a breaking down of 

 some of the living substance of the peas. The same breaking 

 down of the living substance occurs in ordinary respiration, but 

 oxygen is absorbed from the air to replace that taken from the 



* The exceptions are found in some of the low minute plants, for example, 

 some of the bacteria which cannot grow in the presence of oxygen. These 

 are called anaerobes, because they cannot carry on their life processes in the 

 presence of ordinary air. 



