76 AERATION AND AIR-CONTENT. 



Kolkwitz (1901 : 285) has shown that the respiration of barley with 

 a moisture content of 10 to 11 per cent is very weak, averaging less 

 than 1 mg. CO2 per kilogram per day. It rises rapidly with increas- 

 ing moisture and reaches 2,000 mg. per day for each kilogram at 33 

 per cent, while if the temperature and oxygen-content of the air are 

 raised, it reaches astounding amounts. When seeds are cut into 

 halves, the part containing the embryo respires three times as much 

 as the one with endosperm alone. If the grains are ground in a coffee- 

 mill, respiration increases 50 per cent, due either to wounding or to 

 easier access of oxygen. Respiration does not stop if the grains are 

 ground to meal. 



Duval (1904 : 79) kept the seeds of cabbage and onion in a vacuum 

 for 182 days and found that they germinated 75 and 73 per cent 

 respectively, in comparison with 81 and 74 per cent for controls. 

 During the period no evolution of gases occurred. 



Takahashi (1905 : 439) found that peas could not germinate in 

 the absence of air, although intramolecular respiration was carried 

 on for a number of weeks. Rice grains were able to germinate in 

 plain water and in the absence of air. This was explained by the 

 fact that rice grows naturally in places where germination must occur 

 in the presence of very little oxygen. In the experiment, however, 

 growth seemed to stop when the young plumule reached the length 

 of 3 cm. 



Crocker (1906 : 273) has shown that delayed germination in the 

 cocklebur is due to the exclusion of oxygen by the seed-coats. While 

 the coats of both upper and lower seeds reduce the amount of oxygen, 

 reduction is greatest for the upper. With the coats removed, lower 

 seeds absorb 1.6 to 1.7 times as much oxygen as with them intact, 

 while the upper take up 2 to 2.4 times as much. Seeds of Xanihium 

 canadense soaked for 12 to 18 hours and then kept at 21° to 23° C. for 

 6 days showed complete germination in pure oxygen and none in air. 

 He further demonstrated (1907 : 378) that delayed germination of 

 the seeds of water-plants is due to the exclusion of water by the seed- 

 coats. Oxygen does not seem to be concerned, as increased pressures 

 do not effect germination with the coats intact, and Httle oxygen is 

 needed, moreover. Seeds with intact coats showed no germination 

 after 10 days, while those with the coats broken gave the following 

 percentages at the end of 1 and 10 days respectively: Alisma 

 plantago, 86, 98; Eichhornia, 96, 98; Polygonum amphibium, 81, 85; 

 Potamogeton natans, 42, 51; P. pectinaius, 47, 53; Sagittaria variabilis, 

 92, 92; Typha latifolia, 85, 89. 



Shull (1911 : 475) has found a marked difference in the demand for 

 oxygen by the embryos of the upper and lower seeds of Xanihium. 

 In seeds without coats, the minimal oxygen pressure at 21° C. is 

 about 9.5 mm. for the lower seeds and 12 mm. for the upper. A rise 

 of 10° in temperature lowers these minima to 3 mm. and 7 mm. 



