Carbon Dioxide in Maturation, etc., of Seeds. 419 



germinated sporadically afterwards, but systematic observations were hot 

 made in this first series of experiments as regards the after-behaviour of 

 the inhibited seeds. A further series was, however, set in which the 

 subsequent behaviour was noted. In this it was found that the results 

 obtained with seeds inhibited in the soil closely conformed in all respects 

 to the results obtained with those inhibited in laboratory conditions. In 

 this experiment, which was conducted at a temperature of 5-7° C, and in 

 which the seeds were left in the ground for 16 days, the C0 2 content in the 

 soil over the buried grass rose from 10 per cent, on the 3rd day to 22 per cent, 

 on the 16th. No germinations occurred with the Brassica alba seeds placed 

 in the soil over the pit in which grass had been placed. All the seeds 

 placed in the soil in the control pit without grass vigorously germinated 

 within 10 days. When the inhibited seeds were removed to normal con- 

 ditions of germination, 20 per cent, germinated sporadically within the first 

 10 days. The remainder were apparently living at the end of two months. 

 None had been attacked by moulds. At this stage the testa was removed 

 from a number of the seeds, with the result that germination was imme- 

 diately induced, as in the laboratory experiments recorded above. 



It would appear, therefore, that it is possible to reproduce in natural 

 conditions, which may occur widely in the soil, the results obtained in the 

 laboratory with inhibited Brassica alba seeds. 



(e) Action of the Testa. Bare Embryos inhibited by Carbon Dioxide. — It is 

 desirable now to return to the problem in its original form, in which it was 

 indicated that germination may be due (1) to the action of a definite stimulus 

 such as would be supplied by the access of oxygen under suitable conditions 

 of moisture and temperature ; or (2) to the removal of some inhibitory agent 

 which has so far restrained the seed from entering upon the cycle of changes 

 which begins with germination ; or (3) to an inter-relation of both these 

 causes. 



In the experiments with carbon dioxide acting on the seed in germinating 

 conditions so far related, it will be seen that certain partial pressures of CO2 

 have the effect of retarding and inhibiting germination, the see^ being 

 capable of resuming growth without any apparent injury on the removal of 

 the depressant. In the cases dealt with we seem to have two classes of 

 results which must be separated. In the cases of all the seeds excepting 



after treatment. It appears from these results that caution is necessary in placing seed 

 in the ground into which green crops have been ploughed or which has been recently 

 heavily manured. In some of the experiments with pits described above the partial 

 pressure of C0 2 in the soil atmosphere over buried grass was found to be as much as 

 8 per cent, seven months after the green grass had been buried. 



