22 THE COEFFICIENT OF MUTATION IN OENOTHERA BIENNIS L 



for years. It is not an uncommon case that the proportion of the 

 rapidly germinating seeds is a very small one, and in this case 

 a large quantity of seed is necessary to secure a small number of 

 seedlings. Moreover, in those cases where the seeds do not pro- 

 duce a uniform progeny, but a mixture, as, for example, with twin 

 hybrids or in hybrid splitting, the possibility cannot be denied that 

 the numerical proportion of the components of the mixture may 

 be different for the rapidly germinating seeds as compared with 

 the others. In other words, percentage figures may be influenced 

 to some degree by the occurrence of a more or less considerable 

 proportion of dormant seeds. 



In order to ascertain the value of this objection, I have made 

 from time to time cultures in which the rapidly germinated seed- 

 lings were planted out separately from the slower ones. As a 

 matter of fact, I have not found as yet any essential differences 

 between the two groups; but the doubt remained that such might 

 still be discovered if it were possible to bring to germination all, or 

 almost all, the slow seeds of a given sample. For a number of 

 years I have tried various means to reach this end, but only of 

 late have I succeeded. 



It is a well known fact that many kinds of hard seeds may be 

 induced to germinate by means of filing. Filing machines, espe- 

 cially for the smaller leguminous seeds, are now often used in 

 agricultural practice, the best known one being the Swedish type, 

 constructed by Hjalmar Nilsson, the Director of the Swedish 

 Agricultural Experiment Station at Svalof. It files the seeds in 

 a continuous current by throwing them against a rapidly revolving 

 filing disk. Unfortunately, in the seeds of the evening primroses, 

 the hard layer is not the external tissue, but that of the inner 

 integument. The outer coat thus prevents the filing, and experi- 

 ments which Professor Nilsson has had the kindness to make for 

 me with his apparatus did not give the desired result. 



In the soil the water is imbibed into the seeds through micro- 

 scopic and very narrow slits in the hard layer. It is assumed that 

 these slits are filled with air which, in the narrower ones, is a power- 

 ful obstacle against the penetration of the water. So long as this 

 only reaches the cuticularized parts of the walls of the slits, no 

 moisture can reach the embryo and this remains dormant. The 

 question, therefore, is to compel the water to penetrate into the 

 deeper parts of the slits so as to reach the spots which can be 

 moistened. 



