410 Mr. F. Kidd. The Controlling Influence of 



considerable class of cases the seeds appear to be capable of remaining 

 indefinitely in the ground without germinating, while preserving latent their 

 power of growth under certain conditions, the nature of which does not 

 appear to be clearly understood. We find the embryos of these latent seeds 

 to be apparently in good germinating conditions, that is, supplied with 

 sufficient water, in an atmosphere containing the normal percentage of oxygen, 

 and at a temperature sufficient for germination. 



In a large number of cases of this phenomenon quoted by Nobbe and 

 Hanlein (6), sporadic germination over periods of months, and even years, is 

 a marked feature. In natural conditions Brassica nigra is an example of 

 these cases of delayed germination. In Sussex it is locally called Kelke, and 

 every farmer and labourer along the northern slope of the South Downs will 

 give examples from his experience of the seeds sprouting in newly ploughed 

 land after they have lain dormant for years, while the land has been under 

 pasture or hay. 



In certain of these cases of delayed germination in germinating conditions, 

 non-germination has been shown by Ewart to be accompanied by a lack of 

 water in the embryo due to the impermeability of the testa to water. These 

 cases do not bear upon our problem. It is with the range of cases in which a 

 full water supply is demonstrated that interest lies. So far as explanations 

 based on experiment have hitherto been forthcoming for non-germination in 

 these seeds, they have been mainly directed to elucidate this somewhat 

 striking phenomenon from the point of view that the testa is shielding the 

 embryo from a sufficient supply of oxygen. 



Crocker (5) has reached this conclusion from his work upon the upper seeds 

 of Xanthium burrs, which normally do not germinate till after they have lain 

 over one year in the soil. He found that while at a temperature of 19° C. 

 these seeds would not germinate — though containing a sufficient supply 

 of H 2 and though lying in a normal atmosphere {i.e. with a partial 

 pressure of oxygen equal to 150 mm.) — germination could nevertheless be 

 immediately induced by removal of the testas. Recently Shull (4), working 

 upon these same seeds, has given us the actual minimum values of oxygen 

 necessary for the germination of the naked embiwos. At a temperature of 

 21° C. the minimum partial oxygen pressure required by them is not more 

 than 12 mm. If we are to adopt Crocker's view, therefore, that the non- 

 germination of these seeds with the testa intact is due simply to the fact that 

 only a subminimal quantity of oxygen can reach the embryo, we shall have to 

 say that the wet testa is able to reduce the pressure of oxygen in its passage 

 through it from 150 mm. pressure to less than 12 mm. 



It is conceivable that in the maturation of the seed and in delayed 



