36 SCIENCE PROGRESS 



months in their experiments, during which period the seeds 

 lay on moist sihca sand) after the removal of the seeds to air. 

 This condition of secondary dormancy in White Mustard seeds 

 can generally be terminated by redrying the seed or by remov- 

 ing the seed-coats without drying. No evidence was found 

 in support of the hypothesis that changes in the seed-coats had 

 occurred during the period of primary inhibition ; on the 

 other hand, it was shown that if the testa were removed with 

 extreme care the naked embryos remained dormant, and it 

 was suggested that a more stable condition of the tissues of 

 the embryo had become established during the period of 

 primary inhibition under the influence of carbon-dioxide. 

 Embryos in this stable condition do not respond to the ordi- 

 nary environmental factors under which germination of 

 normal embryos takes place. A definite stimulus, chemical or 

 mechanical, appears to be necessary to initiate growth (by 

 cell-division) of the dormant embryo. 



The recent work of Crocker and Harrison {" Catalase and 

 Oxidase Content of Seeds in Relation to their Dormancy, Age, 

 Vitality and Respiration," Joiirn. Agric. Research, xv, 191 8, 

 p. 137) marks an important advance in our knowledge of the 

 physiology of dormant seeds, especially with reference to 

 changes in respiration and in catalase and oxidase activity. 

 Freshly harvested seeds of Johnson grass, if kept in a ger- 

 minator at 20° C, will remain dormant for a year or more. 

 That this condition of dormancy is imposed by the structures 

 enclosing the embryo and that it is not due to a dormant 

 condition of the embryo itself is proved by the fact that their 

 removal leads to prompt germination. A marked reduction 

 in catalase activity takes place while the seeds are kept in the 

 germinator at 20° C, a reduction in catalase activity of 50 per 

 cent, occurring during the first month under these conditions 

 as compared with the catalase activity of similar seeds under 

 ordinary dry-storage conditions. This fall in catalase activity 

 is accompanied by a fall in respiratory intensity, whereas a 

 gradual rise occurs in the respiratory intensity of the seed 

 when dry-stored. If a gradual fall in respiratory activity is a 

 phenomenon of general occurrence in imbibed dormant seeds, 

 and if it is assumed that the death of such seeds depends upon 

 the utilisation and destruction of their food-reserves by respira- 

 tion, then this result will have an important bearing on the lon- 

 gevity of seeds under natural conditions. In this connection the 

 conclusion of Brenchley (" Buried Weed Seeds,'' Journ. Agric. 

 Science, ix, 191 8, p. i) that the seeds of certain plants may 

 survive burial in the soil for at least fifty-eight years is of interest. 



A number of German plant physiologists have directed 

 their attention to the problems associated with the germina- 



