58 PHYSIOLOGY. 



The twelve peas were selected so that six for each lot showed the same 

 length of root, which varied from 15 to 25 mm long. Fig. 50 shows the 

 apparatus just after the experiment was started. The peas in tube a (the 

 right-hand tube) are nearly hidden by the mercury. 



At 2 p.m. the accumulation of gas had caused the lowering of the 

 mercury in this tube so that the upper pea was entirely uncovered. At 4 

 p.m. another pea was uncovered. By this time it was evident that the roots 

 of the peas in tube b (left-hand one) were elongating, while no increase could 

 be detected in the roots of the peas in a. At 6 p.m. three peas in a were 

 uncovered. At 10 p.m. all six peas were uncovered. The roots of the peas 

 in b were still longer than when noted at 4 p.m., but in a no elongation was 

 perceptible at that time. At 9 a.m. on the following day the mercury had 

 lowered so that it was nearly level with the mercury in the dish, while that 

 in tube b was below the level of that in the dish. There was no perceptible 

 elongation of the roots in a, while the roots in b measured about 5 mm 

 longer than when the experiment was started. 



139. Intramolecular respiration. — The last experiment is also an excel- 

 lent one to show what is called intramolecular respiration. In the tube filled 

 with mercury so that when inverted there will be no air, it will be seen after 

 24 hours that a gas has accumulated in the tube which has crowded out some 

 of the mercury. With a wash bottle which has an exit tube properly curved, 

 some water may be introduced in the tube. Then insert underneath a small 

 stick of caustic potash. This will form a solution of potash and the gas will 

 be partly or completely absorbed. This shows that the gas was carbon 

 dioxide. This evolution of carbon dioxide by living plants when there is no 

 access of oxygen is called intramolecular respiration. It occurs markedly in 

 oily seeds and especially in the yeast plant. 



